- Email: [email protected]
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[11] ter Laan M, van Dijk JM, Elting JW, Staal MJ, Absalom AR. Sympathetic regulation of cerebral blood flow in humans: a review. Br J Anaesth 2013;111:361–7. [12] Toth P, Tucsek Z, Sosnowska D, Gautam T, Mitschelen M, Tarantini S, et al. Age-related autoregulatory dysfunction and cerebromicrovascular injury in mice with angiotensin II-induced hypertension. J Cereb Blood Flow Metab 2013;33:1732–42.
[13] van Beek AH, Claassen JA, Rikkert MG, Jansen RW. Cerebral autoregulation: an overview of current concepts and methodology with special focus on the elderly. J Cereb Blood Flow Metab 2008;28:1071–85. [14] Willie CK, Tzeng YC, Fisher JA, Ainslie PN. Integrative regulation of human brain blood flow. J Physiol 2014;592:841–59.
C H A P T E R
11 Cerebral Blood Flow Regulation (Carbon Dioxide, Oxygen, and Nitric Oxide) R.J. Traystman University of Colorado Denver, Aurora, CO, United States
INTRODUCTION The effects of carbon dioxide (CO2), oxygen (O2), and nitric oxide (NO) on the cerebrovasculature are the most pronounced, easily demonstrated, and reproduced phenomena observed in the cerebral circulation. Studies in man and animals, using many different techniques, have shown that CO2, O2, and NO exert a profound influence on cerebral blood flow (CBF). Cerebral vasodilation to hypercapnia, hypoxia, and NO, and vasoconstriction to hypocapnia, hyperoxia, and NO inhibitors are universal findings in mammals, regardless of age or sex. Thus, these gases are considered to be fundamental regulators of CBF. Here I briefly review the effects of CO2, O2, and NO on the cerebral vasculature and the potential mechanisms of action which account for these effects.
PHYSIOLOGICAL RESPONSES OF CO2 An increase in arterial CO2 tension (PaCO2) produces perhaps the most marked and consistent cerebral vasodilation of any known agent. In man, 5% CO2 inhalation increases CBF by about 50%, and 7% CO2 by 100% [1]. It had been proposed that the CBF response to alterations in PaCO2 was a threshold phenomenon; however, Primer on Cerebrovascular Diseases, Second Edition http://dx.doi.org/10.1016/B978-0-12-803058-5.00011-4
it was subsequently shown that this response is a continuous one [2] (Fig. 11.1). Furthermore, the CBF/ PaCO2 relationship can be described by an S-shaped curve. There also appears to be a maximal increase in CBF with hypercapnia. When PaCO2 is altered from 4 to over 400 mmHg, a maximal increase in CBF occurs at about 150 mmHg. On the other hand, reducing PaCO2 from about 45 to 25 mmHg reduces CBF by about 35%. These alterations in CBF with hyper- or hypocapnia are reversible. Despite all cerebral vessels respond to changes in CO2, hypercapnia dilates smaller cerebral arterioles more than larger ones, but the hypocapnic vasoconstriction effect is size independent. While the effects of hypercapnia are reversed when PaCO2 is reduced, animals exposed to prolonged increases in PaCO2 increase CBF initially; however, after many hours or days of exposure, CBF returns toward baseline despite continued elevated PaCO2. Men exposed to high altitude for 3–5 days have higher CBF than those at sea level, and prolonged hypercapnia reduces CBF responsiveness to acute alterations in PaCO2. Prolonged hypocapnia also alters CBF responsiveness to acute changes in PaCO2. During prolonged hypocapnia CBF initially decreases but later increases toward baseline despite the continued lowered PaCO2.
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80 100 120 140 160 200 240 280 320 360 400 440 ARTERIAL PCO2(mm Hg)
FIGURE 11.1 Curve and its equation describing relationship between cerebral blood flow (CBF) and arterial PCO2, individual data points for each of eight monkeys. From Reivich M. Arterial PCO2 and cerebral hemodynamics. Am J Physiol 1964;206:25–35.
Anesthetized animals exposed to concentrations of CO2 respond to a lesser degree than conscious animals. This may, at least in part, be explained by the depressive effect of anesthetics on brain metabolism and CO2 production. This effect may be due to effects of metabolism on tissue levels of PCO2 because a reduction in O2 consumption would likely decrease the amount of CO2 that is generated and diffuses from brain tissue to the vessel wall. The question of whether alterations in PaCO2 change CBF equally in all brain regions is controversial. Some investigators show no differences in CO2 reactivity among brain regions and that blood flow to the hemispheres, brainstem, cerebellum, and medulla is altered by the same percentage per mmHg change in PaCO2. Others found that gray and white matter blood flow increased with hypercapnia; however, the white matter increase was less than the gray matter increase. Other brain regional areas such as the posterior pituitary and choroid plexus demonstrate minimal increases in flow with hypercapnia. There appears to be a developmental difference in the CBF response to CO2, although in all age groups, CBF increases with increasing PaCO2. In both fetus and newborn, gray matter blood flow increases at PaCO2 greater than 40 mmHg, but changes little at lower PaCO2 levels. It has also been demonstrated that the change in CBF/ PaCO2 is higher in the newborn than in the fetus, and this suggests that the cerebrovascular response to CO2 may not be completely developed at birth. This depressed CO2 response in the fetus may be correlated with a difference in cerebral O2 consumption (cerebral metabolic rate of O2, CMRO2). However, when CBF responses are normalized for CMRO2, the increase in CBF is greatest in newborns, smaller in adults, and even smaller in fetuses. The reactivity of cerebral vessels in mid-gestational fetuses (sheep, 93 days) versus near-term fetuses (sheep, 133 days) is interesting. CBF and CMRO2 increase threefold between
93 and 133 days of gestation. The CBF response to hypercapnia is greater at 133 days in mL/min/100 g of flow, but not as a percentage of baselines or as a ratio of CBF/ CMRO2. Thus, CO2 reactivity appears normal relative to metabolism by 93 days gestation. Old age may also affect the responses to CO2, and a decreased CBF responsiveness has been observed with increasing age in humans.
MECHANISMS OF ACTION OF CO2 Several mechanisms have been proposed to account for the effects of CO2 on the cerebrovasculature: extracellular fluid [H+], prostaglandins, NO, and neural pathways.
Extracellular Fluid [H+] pH Hypothesis The main mechanism of the potent effect of CO2 on CBF is a local action on cerebral arteries mediated by extracellular fluid [H+] [3,4]. Marked changes in PaCO2 and bicarbonate ion concentration of cerebrospinal fluid (CSF) do not affect pial arteriolar caliber unless a change in pH occurs. This demonstrates that molecular CO2 and bicarbonate ion have no vasoactivity and that it is the [H+] which is the important vasoactive agent. The cerebral vasodilation produced by hypercapnia can be completely counteracted by a change in extravascular PaCO2 of the same magnitude but in the opposite direction. This indicates that local effects of CO2 can explain the alterations in vascular caliber produced by PaCO2 changes. The pH hypothesis regulation of the cerebrovasculature was originally described more than 50 years ago [5] and states that the actions of CO2 are mediated by direct effects of [H+] on cerebrovascular smooth muscle. The [H+] in the area of vascular muscle depends on bicarbonate concentration and PCO2 of the extracellular fluid
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at that site. In turn, extracellular fluid PCO2 depends on both PaCO2 and PCO2 in CSF. Since the blood–brain barrier (BBB) is impermeable to bicarbonate and [H+], but freely permeable to CO2, when PaCO2 increases, molecular CO2 diffuses across the barrier to increase local PCO2 of vascular muscle, reduces extracellular fluid pH, and produces vasodilation. The reverse occurs when PaCO2 is decreased. This local nature of CO2 control by [H+] has been verified using ventriculocisternal perfusion techniques. Alteration of bicarbonate concentration in one lateral ventricle lowered caudate nucleus blood flow when bicarbonate increased and suppressed the increased flow when PaCO2 was elevated compared to contralateral caudate blood flow [6].
Prostaglandins Prostaglandins may be mediators of the CBF CO2 response. Vasodilator prostanoids are important in vasodilation to hypercapnia in some species (gerbil, mice, rat, and baboon), but not in others (rabbits and cats). That prostanoids are important in hypercapnia comes from the observation that indomethacin, a cyclooxygenase inhibitor, decreases the CBF response to CO2 inhalation in baboons [7]. Others have shown a complete abolition of the CBF response to hypercapnia with indomethacin and with no alteration in CMRO2. In premature infants, indomethacin blunts the cerebrovascular response to hypercapnia, but while indomethacin affects CBF in man, administration of aspirin and indomethacin does not decrease control of CBF or attenuate the increase in CBF with hypercapnia. Other more specific cyclooxygenase inhibitors (AHR-5850-sodium amfenac) do not alter diameter of pial arterioles during normo- or hypercapnia. There may be interaction between the prostanoid system and NO production, with prostacyclin facilitating the release of NO. Thus, in species in which prostanoids act as mediators of hypercapnic vasodilation, inhibition of NO synthase may impair the cerebrovascular response to hypercapnia. The cerebral circulatory response to CO2 may be gender specific, and it has been demonstrated that this response is altered more by indomethacin in women than in men. The response of cerebral vessels to CO2 is universal among species; thus, it is curious that the prostaglandin mechanism of hypercapnic vasodilation is species dependent. For a response so prevalent, the mechanism of action is likely to be similar across species.
Nitric Oxide NO is an important messenger involved in a wide variety of biological processes including regulation of the cerebral circulation. It plays a role in the maintenance of resting cerebrovascular tone and perhaps in evoked vasodilation. Since NO is a diffusible, short lived, highly reactive
molecule, its effects have usually been inferred from studies of NO synthase (NOS) activity or inhibitors of this enzyme. Therefore, the importance of NO in the mechanism of hypercapnic cerebrovasodilation is somewhat unclear. However, a large number of studies have found that NOS inhibitors attenuate the increase in CBF with hypercapnia [8] by 35–95%. Because cerebral vessels remain responsive to other vasodilator stimuli (papaverine, nitroprusside, hypotension, and hypoxia) after NOS inhibition, the absent or reduced CBF response to hypercapnia is not due to nonspecific reduction of cerebral vascular responsivity. However, the studies are limited because cerebral vascular resistance was not calculated and to determine whether cerebral vessels truly vasodilated this must be known. This is important because blood pressure increases considerably following administration of NOS inhibitors, and CBF is decreased. On the other hand, other investigators have found little or no attenuation of cerebral vasodilation to hypercapnia following NOS inhibition [9]. Other data indicate that NO may play a small role in cerebral vasodilation to hypercapnia at moderate PaCO2 levels (∼50 mmHg) but not at higher levels (70 mmHg). Recent data in early gestation (93 days) and near-term gestation (133 days) sheep fetuses demonstrate that NOS inhibition does not alter cerebrovascular reactivity to CO2. The precise factors which account for these discrepant findings may involve species differences, methodological differences, dose of NOS inhibitor and consequent inhibition, timing of NOS inhibition relative to hypercapnia onset, anesthetic, degree of hypercapnia, and the failure to calculate cerebrovascular resistance which truly defines vasodilation or vasoconstriction. The fact is that in all species studied, hypercapnia leads to cerebral vasodilation and an increase in CBF, and NOS inhibition does not completely ablate CO2 reactivity. Considering that there are region-specific responses to CO2 within the brain, this likely means that there is more than one mechanism that accounts for the CO2-mediated vasodilation. At best it would appear that the role of NO in the mechanism of the cerebrovascular response to CO2 is as a modulator. There is no doubt that the major mechanism is increased perivascular [H+] during hypercapnia which reduces extracellular fluid pH and relaxes cerebral vascular smooth muscle. Other additional overlapping mechanisms involving NO or prostanoids are likely to involve reduced extracellular fluid pH. It is possible that increased extracellular fluid [H+] increases NOS activity or prostanoid production and/or release. It is also possible that there are multiple mechanisms accounting for the effects of CO2 on the cerebrovasculature.
Neural Pathways While this mechanism is understudied, the available literature is conflicting. Years ago it was suggested
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PHYSIOLOGICAL RESPONSES OF O2
that hypercapnia stimulates cholinergic vasodilator reflex pathways [10]. Possibly, CO2 exerts its effects on cerebral vessels via remote neural sites such as arterial chemoreceptors or brainstem vasomotor centers. In fact, the CBF response to CO2 may be abolished or attenuated by a number of interventions: atropine administration, α-adrenergic blockade, arterial chemoreceptor denervation, vagal section, section of the seventh nerve, and certain brainstem lesions. There is also impressive evidence arguing against a neural role in the regulation of cerebral vessels by CO2. Atropine, α-adrenergic blockade, section of the seventh, ninth, and tenth nerves, and arterial chemoreceptor denervation do not alter the CBF response to CO2 [11,12]. This potential mechanism of action is controversial, and there is no convincing evidence of neural involvement in cerebrovascular responses to CO2.
CONCLUSION Hypercapnia profoundly increases CBF, and hypocapnia decreases CBF. Although there may be several factors that can influence hyper- and hypocapnic CBF responses, the major mechanism is related to the [H+] of extracellular fluid. This mechanism appears to occur across species but could work in conjunction with other mechanisms such as prostanoids, NO, and neurogenic components.
PHYSIOLOGICAL RESPONSES OF O2 A tremendous amount of information concerning the effects of alteration in arterial oxygen tension (PaO2) on the cerebral circulation has been reported. The relatively
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sparse capillarity and high CMRO2 of brain indicate that the brain relies on a continuous supply of O2. It is generally agreed that if PaO2 is lowered sufficiently, CBF will increase (Fig. 11.2) [13]. The increase in CBF during hypoxemia has been observed in different animal species including man, with different anesthetics and different CBF techniques, regardless of accompanying alterations in PCO2. Inhalation of low O2 mixtures results in an increase in pial vessel diameter whether or not PaCO2 is controlled. Many investigators have dealt with whether there is a threshold PaO2 for alterations in CBF. A general threshold number has been determined to be around 50 mmHg below which CBF increases markedly (Fig. 11.2) [13]. In these experiments, the animals (dogs) were ventilated and normocapnic. CBF began to increase as PaO2 approached 50 mmHg and, at 30 mmHg, CBF increased to 220%. Others have reported an increase in CBF at PaO2 as high as 85 mmHg. Thus, it is a most consistent finding that if PaO2 is reduced, an increase in CBF occurs. The increased CBF during hypoxemia maintains a normal CMRO2 up to a limit. CMRO2 is maintained constant even when PaO2 is reduced to 30–40 mmHg (an O2 content of around 8–10 vol % or less). Brain tissue concentrations of ATP, ADP, and AMP have also been shown to remain unchanged at lower levels of PaO2 [14]. The consistent findings of a maintenance of cerebral energy production with severe hypoxemia have led to the conclusion that the functional symptoms accompanying hypoxemia are not due to energy failure but depend on other metabolic perturbations, and that powerful homeostatic mechanisms come into play to prevent energy failure. This compensatory response is predominantly the increase in CBF.
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FIGURE 11.2 Effect of alterations in arterial PO2 on cortical blood flow in dogs. From MacDowell DG. Interrelationships between blood oxygen tensions and cerebral blood flow. In: Payne JP, Hill DW, editors. Oxygen measurements in blood and tissues. London: Churchill ltd; 1966. p. 205–19.
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MECHANISMS OF ACTION OF O2 Direct Effects of O2 While it is clear that hypoxemia produces cerebral vasodilation and increased CBF, the precise mechanism by which hypoxemia produces this vasodilation is not. Hypotheses to explain this mechanism include direct effects of O2, chemical or metabolic mediators, and neurogenic and NO theories. Little evidence exists concerning the direct effects of O2 on cerebral vessels. However, there is some evidence that O2 may act directly on the smooth muscle of cerebral vessels with high PaO2 resulting in vasoconstriction and low PaO2 leading to vasodilation. The dependence of the contractile response to PO2 is explained if one assumes that O2 plays a metabolic role within the mitochondria of smooth muscle cells. It has also been suggested that receptors exist in smooth muscle which are sensitive to PO2 and work as chemoreceptors.
Chemical or Metabolic Mechanism The mechanism of cerebral vasodilation with hypoxemia may be mediated chemically by extracellular acidosis secondary to cerebral lactate production. Reducing PaO2 to less than 50 mmHg increases CBF and the concentration of intracellular and extracellular cerebral lactate. Thus, cerebral metabolic acidosis could affect cerebral vascular smooth muscle by altering pH within the cell. Cerebral vasodilation correlates well with cerebral cortical acidosis and it is possible that hypoxemia exerts its effects on cerebral vessels secondary to the formation of parenchymal lactate from anaerobic glycolysis. However, others refute this finding and have demonstrated that during the initial, rapid, nonsteady state increases in CBF during hypoxemia, there is only a slight increase in lactate or none at all. Also, this increase in CBF leads to a reduction in tissue PCO2 and a subsequent increase in pH. Thus, the increased CBF must be related to some aspect of cellular metabolism less sluggish than lactate formation. The relationship between organ blood flow and the metabolism of that organ is a very old physiological issue (dating back to 1870s), and a close relationship between CBF and the concentration of metabolic byproducts in the interstitial fluid was proposed in 1880s. One such metabolic by-product, adenosine, has been proposed to be the mechanism by which metabolic demands of the brain are transformed into the stimulus to increase CBF in hypoxemia [15]. Hypoxemia increases brain adenosine levels rapidly (within 2–3 s) and to extremely high levels. Coupled with the fact that adenosine is a strong dilator of pial arterioles when applied to the perivascular space and that adenosine causes cerebral vasodilation, this supports the
potential role of adenosine as a chemical link between metabolism and CBF during hypoxemia. Other metabolic substrates such as oxygenases may also play a role in hypoxic vasodilation since oxygenase inhibitors attenuate cerebral vasodilation with hypoxemia [16]. The precise nature and location of these oxygenases is unclear, but it has been suggested that these “receptors” for hypoxemia are located close to the CSF. The idea of an O2 sensor is not new and it has been proposed to exist in cerebral parenchymal tissue, or in CSF areas. These O2 receptors could participate in a neural feedback loop originating within cerebral tissue to produce vasodilation with hypoxemia. In addition to the already mentioned adenosine and oxygenases, other vasoactive mediators of blood flow are bradykinin, histamine, prostaglandin, and serotonin.
Neurogenic Mechanism During mid-1960s it was thought that neurogenic mechanisms were responsible for the vasodilator response to hypoxemia. It was suggested that the carotid chemoreceptors acting through neurogenic mechanisms were responsible for virtually all of the cerebral vasodilation with hypoxemia. It was shown that carotid chemoreceptor and baroreceptor denervation abolished the cerebral vascular response to hypoxemia; however, this was subsequently shown not to be the case [17]. In addition it was demonstrated that the cerebral vasodilation to hypoxemia was not different from that induced by elevating carboxyhemoglobin concentration, so that the arterial O2 content was reduced equally with both types of hypoxemia. With carbon monoxide hypoxia, O2 content is decreased but PaO2 is unchanged, thus providing no stimulus to the chemoreceptors. The aortic chemoreceptors, glossopharyngeal and vagus nerves, are also not involved in the mechanism for the increased CBF with hypoxemia. Finally, it is possible that central brainstem mechanisms are involved with cerebral hypoxic vasodilation and the importance of the pons has been demonstrated.
Nitric Oxide As opposed to the moderate role of NO in hypercapnic vasodilation, its role in hypoxic vasodilation is much less clear, and if there is a role for NO it is much less robust. Initial studies suggested that vasodilation and increased CBF with hypoxemia were not dependent on NO, however, more recent studies indicate that NO may play a larger role in severe hypoxia and in newborn animals. The determination of whether NO is or is not involved with the CBF response to hypoxemia has been made using NO inhibitors and comparing the hypoxemic CBF response before and after the
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inhibitor. Most of these studies indicate that the effect of NO is minimal, although there are studies that show a mild effect. The reasons for the discrepancy in these data, as with hypercapnia, may involve species differences, methodological differences, age of animals, dose of NOS inhibitor and consequent inhibition, timing of NOS inhibition relative to hypoxemic onset, anesthetic, degree of hypoxemia, and the failure to calculate cerebrovascular resistance.
CONCLUSION Hypoxemia profoundly increases CBF at PaO2 of 50 mmHg or less, and hyperoxia slightly decreases CBF. Although there may be several factors that can influence both hypoxemic and hyperoxic CBF responses, it is likely that the major mechanism is related to the alterations in brain metabolism, or chemical aspects such as lactacidosis and adenosine. This mechanism appears to occur across species but could work in conjunction with other mechanisms such as prostanoids, NO, and neurogenic components.
PHYSIOLOGICAL RESPONSES OF NITRIC OXIDE Biology of NO More than 35 years ago, it was recognized that there was a factor released by endothelium that relaxed vascular smooth muscle and resulted in vasodilation [18]. This factor, “endothelium-derived relaxing factor,” EDRF, was subsequently identified as NO [19]. Since that time there have been volumes written concerning the physiology and pharmacology of NO in mediating many physiological functions, and its role in the pathophysiology of a variety of disorders particularly those dealing with the regulation of blood flow and inflammation [20]. NO is an inorganic, uncharged gas that easily crosses biological membranes, including the BBB. NO is synthesized together with l-citrulline by NOS from the precursor l-arginine in the presence of O2 and cofactors, including NADPH, tetrahydrobiopterin (BH4), heme, FAD, FMN, and calmodulin. It is deactivated rapidly via oxidative pathways to nitrite or nitrate and has a short half-life of only a few seconds. It is also scavenged by superoxide-generating agents such as pyrogallol, hydroquinone, oxyhemoglobin, and others. There are also conditions in which superoxide and NO are generated to form peroxynitrite, another free radical that has biological actions. Thus, overproduction of NO can be neurotoxic via the production of peroxynitrite and superoxide which can bind directly
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to DNA, changing its structure and causing cell injury and enhancement of apoptosis.
Isoforms of NOS There are at least three isoforms of NOS: type I NOS from neurons (nNOS constitutive), which is Ca2+ dependent; type II NOS from macrophages, astrocytes, and glia (iNOS inducible), which is inducible by inflammatory substances such as cytokines and endotoxins and generates high levels of NO; and type III NOS from endothelium (eNOS-constitutive), which is Ca2+ dependent, and under physiological conditions generates low levels of NO. The three forms of NOS have been cloned and appear to be products of different genes that have about 55% amino acid identity. NOS in brain and endothelium are quite similar, with the major difference being that NOS in brain is cytosolic and NOS in endothelium is mainly a membrane-associated protein. Two major strategies are used to study the normal and pathophysiological function of these NOS isoforms. One is pharmacological and the other is genetic. The pharmacological strategy is based on several NOS inhibitors which have been used to demonstrate the functional roles of endogenous NO and are as follows: NG-monomethyll-arginine (L-NMMA), NG-nitro-l-arginine (L-NA), L-NA methyl ester (l-NAME), and asymmetric dimethylarginine (ADMA) as nonselective inhibitors; 7-nitroindazol (7-NI), as a relatively selective inhibitor of nNOS; and aminoguanidine, N6-iminoethyl-l-lysine, and Wl1400 as selective inhibitors of iNOS. The genetic approach is based on the development of mutant mice lacking expression of the genes for nNOS, iNOS, and eNOS. There exists knockout (KO) mice for each of the NOS isoforms and much work has been performed with each of these NOS-KO mice to determine the physiological and pathophysiological characteristics in these animals and their involvement with a variety of disease processes.
NO and Cerebrovascular Physiology Endothelial NOS eNOS plays an important role in the regulation of CBF and in the regulation of the vasculature throughout the body. eNOS results in cerebral vasodilation, a reduction in vascular resistance, a reduced blood pressure, platelet aggregation and adhesion inhibition, leukocyte adhesion and migration inhibition, and a reduction of smooth muscle proliferation. It also has an important role in vascular remodeling and angiogenesis. Using pharmacological inhibitors of eNOS and eNOS KO mice, many investigators have demonstrated the role of eNOS in the control of cerebral circulation
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under a variety of conditions, even to control basal CBF levels. NOS inhibitors lead to vasoconstriction and decreased CBF and NO release leads to vasodilation and increased CBF. Vasodilation occurs when NO, released from endothelium, stimulates soluble guanylate cyclase in smooth muscle which results in vasorelaxation. Endothelial NO production is important for other aspects of vascular function such as intimal proliferation which is a response to arterial injury. eNOS KO mice develop more neointimal proliferation after vascular injury than do wild-type mice. What is curious and interesting is that in both KO and wild-type mice there is less intimal proliferation in female animals compared with male animals, indicating a possible role for estrogen in this response. The role of NO in the cerebral vascular response to CO2 and O2 was previously discussed. Cerebral autoregulation, the maintenance of CBF despite changes in perfusion pressure, is another basic physiological characteristic of the cerebral circulation. Since autoregulation involves cerebral vasodilation and vasoconstriction, NO is thought to be involved with this mechanism, and most studies have shown that NO is involved with the mechanism of cerebral autoregulation. While much work has been performed in this area, as with the role of NO in the mechanism of CO2 and O2, the precise mechanism involving NO in autoregulation is unclear and somewhat controversial. Inducible NOS The inducible form of NOS (iNOS) has been described in many cell types: macrophages, astrocytes, microglia, leukocytes, and endothelial cells. iNOS is upregulated as a response to immunological stimuli such as endotoxin and cytokines. Many cytokines induce iNOS: interferon gamma, tumor necrosis factor, interleukin-1, 1B, 6, and so on. Lipopolysaccharide also induces iNOS. These agents can produce cerebral vasodilation which is dependent on the formation of NO and may be mediated by NOS. iNOS produces large amounts of NO which can damage or even destroy cells. Cerebral ischemia and other types of brain injury (cardiac arrest and traumatic brain injury) result in a marked inflammatory reaction. This inflammation activates a variety of cells; iNOS is upregulated and NO is produced in high, even toxic amounts, can then combine with superoxide to produce peroxynitrite, and results in injury. While this mechanistic view prevails, it is a simplified view, and it is likely that iNOS generated NO, like eNOS NO has several roles to play, and the balance between neurotoxicity and neuroprotection depends on many factors. iNOS-derived NO also promotes lipid peroxidation, DNA damage, and BBB breakdown, all of which have prominent roles in brain ischemic injury.
Neuronal NOS nNOS is a constitutive enzyme expressed in brain, peripheral nervous system, and skeletal muscles. nNOS is expressed in neuronal cell bodies, and NO derived from nNOS is an important neurotransmitter associated with neuronal plasticity, memory formation, regulation of CBF, transmission of pain signals, and neurotransmitter release. The excitotoxic transmitter glutamate increases cellular Ca2+ with ischemia and Ca2+ is required for nNOS activity. Ca2+ mediates calmodulin binding to nNOS. Since nNOS is produced from excitotoxicity, nNOS plays a crucial role in ischemic injury. Thus nNOS NO mediates synaptic plasticity and neuronal signaling, but promotes neurotoxicity following ischemic damage, whereas NO produced by eNOS is protective. Neurogenesis also appears to be decreased by nNOS, while iNOS seems to stimulate it.
CONCLUSION NO is an endogenous signaling agent involved in many physiological and pathophysiological processes. It is an important mediator in the regulation of CBF in the resting state, acting as a key mediator on the pathways responsible for maintaining resting CBF and perfusion in physiological situations, such as hypoxia, hypercapnia, and autoregulation. Disruption of NO synthesis and metabolism underlies many pathophysiological processes that occurs following brain injury. NO is also involved as a neuroprotective and neurotoxic agent following cerebral ischemia. This contradiction can be explained by the type of NOS, the cell type producing the NOS, the amount produced, and whether NO undergoes further oxidation. NO has wide-ranging, complex effects in brain and it is important to consider both protective and detrimental effects of NO while inhibiting the particular NOS for therapeutic effect against cerebral ischemia.
References [1] Kety SS, Schmidt CF. The effects of altered arterial tensions of carbon dioxide and oxygen on cerebral blood flow and cerebral oxygen consumption of normal young men. J Clin Invest 1948;27:484–92. [2] Reivich M. Arterial PCO2 and cerebral hemodynamics. Am J Physiol 1964;206:25–35. [3] Kontos HA, Raper AJ, Patterson JL. Analysis of vasoactivity of local pH1. PC02 and bicarbonate on pial vessels. Stroke 1977;8:358–60. [4] Kontos HA, Wei EP, Raper AJ, Patterson JL. Local mechanisms of CO2 action on cat pial arterioles. Stroke 1977;8:226–9. [5] Gotoh F, Tazaki Y, Meyer JS. Transport of gases through brain and their extravascular vasomotor action. Exp Neural 1961;4:48–58. [6] Koehler RC, Traystman RJ. Bicarbonate ion modulation of cerebral blood flow during hypoxia and hypercapnia. Am J Physiol 1982;243:H33–40.
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REFERENCES
[7] Pickard JD, MacKenzie ET. Inhibition of prostaglandin synthesis and the response of baboon cerebral circulation to carbon dioxide. Nature 1973;245:187–8. [8] Iadecola C. Does nitric oxide mediate the increases in cerebral blood flow elicited by hypercapnia? Proc Natl Acad Sci USA 1992;89:3913–6. [9] McPherson RW, Kirsch JR, Ghaly RF, Traystman RJ. Effect of nitric oxide synthase inhibition on the cerebral vascular response to hypercapnia in primates. Stroke 1995;26:682–7. [10] Shalit MN, Reinmuth OM, Shimojyo S, Scheinberg P. Carbon dioxide and cerebral circulatory control. III. The effects of brain stem lesions. Arch Neurol 1967;17:342–53. [11] Bates DB, Chir B, Sundt Jr TM. The relevance of peripheral baroreceptors and chemoreceptors to regulation of cerebral blood flow in the cat. Circ Res 1976;38:488–93. [12] Hoff JT, MacKenzie ET, Harper AM. Responses of the cerebral circulation to hypercapnia and hypoxia after seventh cranial nerve transection in baboons. Circ Res 1977;40:258–62. [13] MacDowell DG. Interrelationships between blood oxygen tensions and cerebral blood flow. In: Payne JP, Hill DW, editors. Oxygen Measurements in Blood and Tissues. London: Churchill ltd; 1966. p. 205–19.
[14] Siesjo BK. Hypoxia. In: Siesjo BK, editor. Brain Energy Metabolism. New York: John Wiley and Sons; 1978. p. 398–452. [15] Berne RM, Rubio R, Curnish RR. Release of adenosine from ischemic brain. Effect on cerebral vascular resistance and incorporation into cerebral adenine nucleotides. Circ Res 1974;35:262–71. [16] Traystman RJ. Regulation of Cerebral Blood Flow. In: Vanhoutte P, Leusen I, editors. Vasodilation. New York: Raven; 1981. p. 39–48. [17] Traystman RJ, Fitzgerald RS. Cerebrovascular response to hypoxia in baroreceptor and chemoreceptor-denervated dogs, 1981 baroreceptor and chemoreceptor-denervated dogs. Am J Physiol 1981;241:H724–31. [18] Furchgott RF, Zawadzki JV. The obligatory role of endothelial cells in the relaxation of arterial smooth muscle by acetylcholine. Nature 1980;288:373–6. [19] Ignarro LJ, Buga GM, Wood KS, Byrns RE, Chadhuri G. Endothelium-derived relaxing factor produced and released from artery and vein is nitric oxide. Proc Natl Acad Sci USA 1987;84:9265–9. [20] Iadecola C. Bright and dark sides of nitric oxide in ischemic brain injury. Trends Neurosci 1997;20:132–9.
C H A P T E R
12 CBF–Metabolism Coupling P. Venkat1,2, M. Chopp1,2, J. Chen1 1Henry
Ford Hospital, Detroit, MI, United States; 2Oakland University, Rochester, MI, United States
INTRODUCTION The brain requires oxygen and glucose to meet its metabolic demands, and cerebral blood flow (CBF) is its supply channel. The brain has high energy requirements but limited storage capacity, which means persistent CBF is critical for its proper functioning and prevention of damage and death. Therefore, in spite of constituting only about 2% of total body weight, the brain is easily the most perfused organ with almost 15–20% of the total cardiac output directed as CBF. Moreover, a process called cerebral autoregulation thrives to maintain adequate CBF at a constant rate. The arteries supplying the brain namely the internal carotid arteries and vertebral arteries that merge
Primer on Cerebrovascular Diseases, Second Edition http://dx.doi.org/10.1016/B978-0-12-803058-5.00012-6
to form the basilar artery arrange themselves into the “circle of Willis” creating collaterals in the cerebral circulation. This is a defense mechanism against CBF drop such that if an artery supplying the circle is blocked, blood flow from the other blood vessels is able to sustain cerebral circulation. The demand–supply relationship between CBF and cerebral metabolism is tightly coupled and brain regions are either hypoperfused or hyperperfused depending on metabolic needs. A sudden decrease in CBF (either temporary or permanent) due to the occlusion of a cerebral artery is called cerebral ischemia and leads to ischemic stroke, neurological deficits, tissue damage, and even death. An excess of blood flow results in hyperemia in which the intracranial pressure may increase and evoke
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10. CEREBRAL AUTOREGULATION
[11] ter Laan M, van Dijk JM, Elting JW, Staal MJ, Absalom AR. Sympathetic regulation of cerebral blood flow in humans: a review. Br J Anaesth 2013;111:361–7. [12] Toth P, Tucsek Z, Sosnowska D, Gautam T, Mitschelen M, Tarantini S, et al. Age-related autoregulatory dysfunction and cerebromicrovascular injury in mice with angiotensin II-induced hypertension. J Cereb Blood Flow Metab 2013;33:1732–42.
[13] van Beek AH, Claassen JA, Rikkert MG, Jansen RW. Cerebral autoregulation: an overview of current concepts and methodology with special focus on the elderly. J Cereb Blood Flow Metab 2008;28:1071–85. [14] Willie CK, Tzeng YC, Fisher JA, Ainslie PN. Integrative regulation of human brain blood flow. J Physiol 2014;592:841–59.
C H A P T E R
11 Cerebral Blood Flow Regulation (Carbon Dioxide, Oxygen, and Nitric Oxide) R.J. Traystman University of Colorado Denver, Aurora, CO, United States
INTRODUCTION The effects of carbon dioxide (CO2), oxygen (O2), and nitric oxide (NO) on the cerebrovasculature are the most pronounced, easily demonstrated, and reproduced phenomena observed in the cerebral circulation. Studies in man and animals, using many different techniques, have shown that CO2, O2, and NO exert a profound influence on cerebral blood flow (CBF). Cerebral vasodilation to hypercapnia, hypoxia, and NO, and vasoconstriction to hypocapnia, hyperoxia, and NO inhibitors are universal findings in mammals, regardless of age or sex. Thus, these gases are considered to be fundamental regulators of CBF. Here I briefly review the effects of CO2, O2, and NO on the cerebral vasculature and the potential mechanisms of action which account for these effects.
PHYSIOLOGICAL RESPONSES OF CO2 An increase in arterial CO2 tension (PaCO2) produces perhaps the most marked and consistent cerebral vasodilation of any known agent. In man, 5% CO2 inhalation increases CBF by about 50%, and 7% CO2 by 100% [1]. It had been proposed that the CBF response to alterations in PaCO2 was a threshold phenomenon; however, Primer on Cerebrovascular Diseases, Second Edition http://dx.doi.org/10.1016/B978-0-12-803058-5.00011-4
it was subsequently shown that this response is a continuous one [2] (Fig. 11.1). Furthermore, the CBF/ PaCO2 relationship can be described by an S-shaped curve. There also appears to be a maximal increase in CBF with hypercapnia. When PaCO2 is altered from 4 to over 400 mmHg, a maximal increase in CBF occurs at about 150 mmHg. On the other hand, reducing PaCO2 from about 45 to 25 mmHg reduces CBF by about 35%. These alterations in CBF with hyper- or hypocapnia are reversible. Despite all cerebral vessels respond to changes in CO2, hypercapnia dilates smaller cerebral arterioles more than larger ones, but the hypocapnic vasoconstriction effect is size independent. While the effects of hypercapnia are reversed when PaCO2 is reduced, animals exposed to prolonged increases in PaCO2 increase CBF initially; however, after many hours or days of exposure, CBF returns toward baseline despite continued elevated PaCO2. Men exposed to high altitude for 3–5 days have higher CBF than those at sea level, and prolonged hypercapnia reduces CBF responsiveness to acute alterations in PaCO2. Prolonged hypocapnia also alters CBF responsiveness to acute changes in PaCO2. During prolonged hypocapnia CBF initially decreases but later increases toward baseline despite the continued lowered PaCO2.
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MECHANISMS OF ACTION OF CO2
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92.8 Y=20.9 + ───────────── 1+10570e–5.251 log X
40 20 0
0
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80 100 120 140 160 200 240 280 320 360 400 440 ARTERIAL PCO2(mm Hg)
FIGURE 11.1 Curve and its equation describing relationship between cerebral blood flow (CBF) and arterial PCO2, individual data points for each of eight monkeys. From Reivich M. Arterial PCO2 and cerebral hemodynamics. Am J Physiol 1964;206:25–35.
Anesthetized animals exposed to concentrations of CO2 respond to a lesser degree than conscious animals. This may, at least in part, be explained by the depressive effect of anesthetics on brain metabolism and CO2 production. This effect may be due to effects of metabolism on tissue levels of PCO2 because a reduction in O2 consumption would likely decrease the amount of CO2 that is generated and diffuses from brain tissue to the vessel wall. The question of whether alterations in PaCO2 change CBF equally in all brain regions is controversial. Some investigators show no differences in CO2 reactivity among brain regions and that blood flow to the hemispheres, brainstem, cerebellum, and medulla is altered by the same percentage per mmHg change in PaCO2. Others found that gray and white matter blood flow increased with hypercapnia; however, the white matter increase was less than the gray matter increase. Other brain regional areas such as the posterior pituitary and choroid plexus demonstrate minimal increases in flow with hypercapnia. There appears to be a developmental difference in the CBF response to CO2, although in all age groups, CBF increases with increasing PaCO2. In both fetus and newborn, gray matter blood flow increases at PaCO2 greater than 40 mmHg, but changes little at lower PaCO2 levels. It has also been demonstrated that the change in CBF/ PaCO2 is higher in the newborn than in the fetus, and this suggests that the cerebrovascular response to CO2 may not be completely developed at birth. This depressed CO2 response in the fetus may be correlated with a difference in cerebral O2 consumption (cerebral metabolic rate of O2, CMRO2). However, when CBF responses are normalized for CMRO2, the increase in CBF is greatest in newborns, smaller in adults, and even smaller in fetuses. The reactivity of cerebral vessels in mid-gestational fetuses (sheep, 93 days) versus near-term fetuses (sheep, 133 days) is interesting. CBF and CMRO2 increase threefold between
93 and 133 days of gestation. The CBF response to hypercapnia is greater at 133 days in mL/min/100 g of flow, but not as a percentage of baselines or as a ratio of CBF/ CMRO2. Thus, CO2 reactivity appears normal relative to metabolism by 93 days gestation. Old age may also affect the responses to CO2, and a decreased CBF responsiveness has been observed with increasing age in humans.
MECHANISMS OF ACTION OF CO2 Several mechanisms have been proposed to account for the effects of CO2 on the cerebrovasculature: extracellular fluid [H+], prostaglandins, NO, and neural pathways.
Extracellular Fluid [H+] pH Hypothesis The main mechanism of the potent effect of CO2 on CBF is a local action on cerebral arteries mediated by extracellular fluid [H+] [3,4]. Marked changes in PaCO2 and bicarbonate ion concentration of cerebrospinal fluid (CSF) do not affect pial arteriolar caliber unless a change in pH occurs. This demonstrates that molecular CO2 and bicarbonate ion have no vasoactivity and that it is the [H+] which is the important vasoactive agent. The cerebral vasodilation produced by hypercapnia can be completely counteracted by a change in extravascular PaCO2 of the same magnitude but in the opposite direction. This indicates that local effects of CO2 can explain the alterations in vascular caliber produced by PaCO2 changes. The pH hypothesis regulation of the cerebrovasculature was originally described more than 50 years ago [5] and states that the actions of CO2 are mediated by direct effects of [H+] on cerebrovascular smooth muscle. The [H+] in the area of vascular muscle depends on bicarbonate concentration and PCO2 of the extracellular fluid
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at that site. In turn, extracellular fluid PCO2 depends on both PaCO2 and PCO2 in CSF. Since the blood–brain barrier (BBB) is impermeable to bicarbonate and [H+], but freely permeable to CO2, when PaCO2 increases, molecular CO2 diffuses across the barrier to increase local PCO2 of vascular muscle, reduces extracellular fluid pH, and produces vasodilation. The reverse occurs when PaCO2 is decreased. This local nature of CO2 control by [H+] has been verified using ventriculocisternal perfusion techniques. Alteration of bicarbonate concentration in one lateral ventricle lowered caudate nucleus blood flow when bicarbonate increased and suppressed the increased flow when PaCO2 was elevated compared to contralateral caudate blood flow [6].
Prostaglandins Prostaglandins may be mediators of the CBF CO2 response. Vasodilator prostanoids are important in vasodilation to hypercapnia in some species (gerbil, mice, rat, and baboon), but not in others (rabbits and cats). That prostanoids are important in hypercapnia comes from the observation that indomethacin, a cyclooxygenase inhibitor, decreases the CBF response to CO2 inhalation in baboons [7]. Others have shown a complete abolition of the CBF response to hypercapnia with indomethacin and with no alteration in CMRO2. In premature infants, indomethacin blunts the cerebrovascular response to hypercapnia, but while indomethacin affects CBF in man, administration of aspirin and indomethacin does not decrease control of CBF or attenuate the increase in CBF with hypercapnia. Other more specific cyclooxygenase inhibitors (AHR-5850-sodium amfenac) do not alter diameter of pial arterioles during normo- or hypercapnia. There may be interaction between the prostanoid system and NO production, with prostacyclin facilitating the release of NO. Thus, in species in which prostanoids act as mediators of hypercapnic vasodilation, inhibition of NO synthase may impair the cerebrovascular response to hypercapnia. The cerebral circulatory response to CO2 may be gender specific, and it has been demonstrated that this response is altered more by indomethacin in women than in men. The response of cerebral vessels to CO2 is universal among species; thus, it is curious that the prostaglandin mechanism of hypercapnic vasodilation is species dependent. For a response so prevalent, the mechanism of action is likely to be similar across species.
Nitric Oxide NO is an important messenger involved in a wide variety of biological processes including regulation of the cerebral circulation. It plays a role in the maintenance of resting cerebrovascular tone and perhaps in evoked vasodilation. Since NO is a diffusible, short lived, highly reactive
molecule, its effects have usually been inferred from studies of NO synthase (NOS) activity or inhibitors of this enzyme. Therefore, the importance of NO in the mechanism of hypercapnic cerebrovasodilation is somewhat unclear. However, a large number of studies have found that NOS inhibitors attenuate the increase in CBF with hypercapnia [8] by 35–95%. Because cerebral vessels remain responsive to other vasodilator stimuli (papaverine, nitroprusside, hypotension, and hypoxia) after NOS inhibition, the absent or reduced CBF response to hypercapnia is not due to nonspecific reduction of cerebral vascular responsivity. However, the studies are limited because cerebral vascular resistance was not calculated and to determine whether cerebral vessels truly vasodilated this must be known. This is important because blood pressure increases considerably following administration of NOS inhibitors, and CBF is decreased. On the other hand, other investigators have found little or no attenuation of cerebral vasodilation to hypercapnia following NOS inhibition [9]. Other data indicate that NO may play a small role in cerebral vasodilation to hypercapnia at moderate PaCO2 levels (∼50 mmHg) but not at higher levels (70 mmHg). Recent data in early gestation (93 days) and near-term gestation (133 days) sheep fetuses demonstrate that NOS inhibition does not alter cerebrovascular reactivity to CO2. The precise factors which account for these discrepant findings may involve species differences, methodological differences, dose of NOS inhibitor and consequent inhibition, timing of NOS inhibition relative to hypercapnia onset, anesthetic, degree of hypercapnia, and the failure to calculate cerebrovascular resistance which truly defines vasodilation or vasoconstriction. The fact is that in all species studied, hypercapnia leads to cerebral vasodilation and an increase in CBF, and NOS inhibition does not completely ablate CO2 reactivity. Considering that there are region-specific responses to CO2 within the brain, this likely means that there is more than one mechanism that accounts for the CO2-mediated vasodilation. At best it would appear that the role of NO in the mechanism of the cerebrovascular response to CO2 is as a modulator. There is no doubt that the major mechanism is increased perivascular [H+] during hypercapnia which reduces extracellular fluid pH and relaxes cerebral vascular smooth muscle. Other additional overlapping mechanisms involving NO or prostanoids are likely to involve reduced extracellular fluid pH. It is possible that increased extracellular fluid [H+] increases NOS activity or prostanoid production and/or release. It is also possible that there are multiple mechanisms accounting for the effects of CO2 on the cerebrovasculature.
Neural Pathways While this mechanism is understudied, the available literature is conflicting. Years ago it was suggested
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PHYSIOLOGICAL RESPONSES OF O2
that hypercapnia stimulates cholinergic vasodilator reflex pathways [10]. Possibly, CO2 exerts its effects on cerebral vessels via remote neural sites such as arterial chemoreceptors or brainstem vasomotor centers. In fact, the CBF response to CO2 may be abolished or attenuated by a number of interventions: atropine administration, α-adrenergic blockade, arterial chemoreceptor denervation, vagal section, section of the seventh nerve, and certain brainstem lesions. There is also impressive evidence arguing against a neural role in the regulation of cerebral vessels by CO2. Atropine, α-adrenergic blockade, section of the seventh, ninth, and tenth nerves, and arterial chemoreceptor denervation do not alter the CBF response to CO2 [11,12]. This potential mechanism of action is controversial, and there is no convincing evidence of neural involvement in cerebrovascular responses to CO2.
CONCLUSION Hypercapnia profoundly increases CBF, and hypocapnia decreases CBF. Although there may be several factors that can influence hyper- and hypocapnic CBF responses, the major mechanism is related to the [H+] of extracellular fluid. This mechanism appears to occur across species but could work in conjunction with other mechanisms such as prostanoids, NO, and neurogenic components.
PHYSIOLOGICAL RESPONSES OF O2 A tremendous amount of information concerning the effects of alteration in arterial oxygen tension (PaO2) on the cerebral circulation has been reported. The relatively
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sparse capillarity and high CMRO2 of brain indicate that the brain relies on a continuous supply of O2. It is generally agreed that if PaO2 is lowered sufficiently, CBF will increase (Fig. 11.2) [13]. The increase in CBF during hypoxemia has been observed in different animal species including man, with different anesthetics and different CBF techniques, regardless of accompanying alterations in PCO2. Inhalation of low O2 mixtures results in an increase in pial vessel diameter whether or not PaCO2 is controlled. Many investigators have dealt with whether there is a threshold PaO2 for alterations in CBF. A general threshold number has been determined to be around 50 mmHg below which CBF increases markedly (Fig. 11.2) [13]. In these experiments, the animals (dogs) were ventilated and normocapnic. CBF began to increase as PaO2 approached 50 mmHg and, at 30 mmHg, CBF increased to 220%. Others have reported an increase in CBF at PaO2 as high as 85 mmHg. Thus, it is a most consistent finding that if PaO2 is reduced, an increase in CBF occurs. The increased CBF during hypoxemia maintains a normal CMRO2 up to a limit. CMRO2 is maintained constant even when PaO2 is reduced to 30–40 mmHg (an O2 content of around 8–10 vol % or less). Brain tissue concentrations of ATP, ADP, and AMP have also been shown to remain unchanged at lower levels of PaO2 [14]. The consistent findings of a maintenance of cerebral energy production with severe hypoxemia have led to the conclusion that the functional symptoms accompanying hypoxemia are not due to energy failure but depend on other metabolic perturbations, and that powerful homeostatic mechanisms come into play to prevent energy failure. This compensatory response is predominantly the increase in CBF.
&+$1*(,1%/22')/2:
±
$57(5,$/32 PP+J
FIGURE 11.2 Effect of alterations in arterial PO2 on cortical blood flow in dogs. From MacDowell DG. Interrelationships between blood oxygen tensions and cerebral blood flow. In: Payne JP, Hill DW, editors. Oxygen measurements in blood and tissues. London: Churchill ltd; 1966. p. 205–19.
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MECHANISMS OF ACTION OF O2 Direct Effects of O2 While it is clear that hypoxemia produces cerebral vasodilation and increased CBF, the precise mechanism by which hypoxemia produces this vasodilation is not. Hypotheses to explain this mechanism include direct effects of O2, chemical or metabolic mediators, and neurogenic and NO theories. Little evidence exists concerning the direct effects of O2 on cerebral vessels. However, there is some evidence that O2 may act directly on the smooth muscle of cerebral vessels with high PaO2 resulting in vasoconstriction and low PaO2 leading to vasodilation. The dependence of the contractile response to PO2 is explained if one assumes that O2 plays a metabolic role within the mitochondria of smooth muscle cells. It has also been suggested that receptors exist in smooth muscle which are sensitive to PO2 and work as chemoreceptors.
Chemical or Metabolic Mechanism The mechanism of cerebral vasodilation with hypoxemia may be mediated chemically by extracellular acidosis secondary to cerebral lactate production. Reducing PaO2 to less than 50 mmHg increases CBF and the concentration of intracellular and extracellular cerebral lactate. Thus, cerebral metabolic acidosis could affect cerebral vascular smooth muscle by altering pH within the cell. Cerebral vasodilation correlates well with cerebral cortical acidosis and it is possible that hypoxemia exerts its effects on cerebral vessels secondary to the formation of parenchymal lactate from anaerobic glycolysis. However, others refute this finding and have demonstrated that during the initial, rapid, nonsteady state increases in CBF during hypoxemia, there is only a slight increase in lactate or none at all. Also, this increase in CBF leads to a reduction in tissue PCO2 and a subsequent increase in pH. Thus, the increased CBF must be related to some aspect of cellular metabolism less sluggish than lactate formation. The relationship between organ blood flow and the metabolism of that organ is a very old physiological issue (dating back to 1870s), and a close relationship between CBF and the concentration of metabolic byproducts in the interstitial fluid was proposed in 1880s. One such metabolic by-product, adenosine, has been proposed to be the mechanism by which metabolic demands of the brain are transformed into the stimulus to increase CBF in hypoxemia [15]. Hypoxemia increases brain adenosine levels rapidly (within 2–3 s) and to extremely high levels. Coupled with the fact that adenosine is a strong dilator of pial arterioles when applied to the perivascular space and that adenosine causes cerebral vasodilation, this supports the
potential role of adenosine as a chemical link between metabolism and CBF during hypoxemia. Other metabolic substrates such as oxygenases may also play a role in hypoxic vasodilation since oxygenase inhibitors attenuate cerebral vasodilation with hypoxemia [16]. The precise nature and location of these oxygenases is unclear, but it has been suggested that these “receptors” for hypoxemia are located close to the CSF. The idea of an O2 sensor is not new and it has been proposed to exist in cerebral parenchymal tissue, or in CSF areas. These O2 receptors could participate in a neural feedback loop originating within cerebral tissue to produce vasodilation with hypoxemia. In addition to the already mentioned adenosine and oxygenases, other vasoactive mediators of blood flow are bradykinin, histamine, prostaglandin, and serotonin.
Neurogenic Mechanism During mid-1960s it was thought that neurogenic mechanisms were responsible for the vasodilator response to hypoxemia. It was suggested that the carotid chemoreceptors acting through neurogenic mechanisms were responsible for virtually all of the cerebral vasodilation with hypoxemia. It was shown that carotid chemoreceptor and baroreceptor denervation abolished the cerebral vascular response to hypoxemia; however, this was subsequently shown not to be the case [17]. In addition it was demonstrated that the cerebral vasodilation to hypoxemia was not different from that induced by elevating carboxyhemoglobin concentration, so that the arterial O2 content was reduced equally with both types of hypoxemia. With carbon monoxide hypoxia, O2 content is decreased but PaO2 is unchanged, thus providing no stimulus to the chemoreceptors. The aortic chemoreceptors, glossopharyngeal and vagus nerves, are also not involved in the mechanism for the increased CBF with hypoxemia. Finally, it is possible that central brainstem mechanisms are involved with cerebral hypoxic vasodilation and the importance of the pons has been demonstrated.
Nitric Oxide As opposed to the moderate role of NO in hypercapnic vasodilation, its role in hypoxic vasodilation is much less clear, and if there is a role for NO it is much less robust. Initial studies suggested that vasodilation and increased CBF with hypoxemia were not dependent on NO, however, more recent studies indicate that NO may play a larger role in severe hypoxia and in newborn animals. The determination of whether NO is or is not involved with the CBF response to hypoxemia has been made using NO inhibitors and comparing the hypoxemic CBF response before and after the
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Physiological Responses of Nitric Oxide
inhibitor. Most of these studies indicate that the effect of NO is minimal, although there are studies that show a mild effect. The reasons for the discrepancy in these data, as with hypercapnia, may involve species differences, methodological differences, age of animals, dose of NOS inhibitor and consequent inhibition, timing of NOS inhibition relative to hypoxemic onset, anesthetic, degree of hypoxemia, and the failure to calculate cerebrovascular resistance.
CONCLUSION Hypoxemia profoundly increases CBF at PaO2 of 50 mmHg or less, and hyperoxia slightly decreases CBF. Although there may be several factors that can influence both hypoxemic and hyperoxic CBF responses, it is likely that the major mechanism is related to the alterations in brain metabolism, or chemical aspects such as lactacidosis and adenosine. This mechanism appears to occur across species but could work in conjunction with other mechanisms such as prostanoids, NO, and neurogenic components.
PHYSIOLOGICAL RESPONSES OF NITRIC OXIDE Biology of NO More than 35 years ago, it was recognized that there was a factor released by endothelium that relaxed vascular smooth muscle and resulted in vasodilation [18]. This factor, “endothelium-derived relaxing factor,” EDRF, was subsequently identified as NO [19]. Since that time there have been volumes written concerning the physiology and pharmacology of NO in mediating many physiological functions, and its role in the pathophysiology of a variety of disorders particularly those dealing with the regulation of blood flow and inflammation [20]. NO is an inorganic, uncharged gas that easily crosses biological membranes, including the BBB. NO is synthesized together with l-citrulline by NOS from the precursor l-arginine in the presence of O2 and cofactors, including NADPH, tetrahydrobiopterin (BH4), heme, FAD, FMN, and calmodulin. It is deactivated rapidly via oxidative pathways to nitrite or nitrate and has a short half-life of only a few seconds. It is also scavenged by superoxide-generating agents such as pyrogallol, hydroquinone, oxyhemoglobin, and others. There are also conditions in which superoxide and NO are generated to form peroxynitrite, another free radical that has biological actions. Thus, overproduction of NO can be neurotoxic via the production of peroxynitrite and superoxide which can bind directly
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to DNA, changing its structure and causing cell injury and enhancement of apoptosis.
Isoforms of NOS There are at least three isoforms of NOS: type I NOS from neurons (nNOS constitutive), which is Ca2+ dependent; type II NOS from macrophages, astrocytes, and glia (iNOS inducible), which is inducible by inflammatory substances such as cytokines and endotoxins and generates high levels of NO; and type III NOS from endothelium (eNOS-constitutive), which is Ca2+ dependent, and under physiological conditions generates low levels of NO. The three forms of NOS have been cloned and appear to be products of different genes that have about 55% amino acid identity. NOS in brain and endothelium are quite similar, with the major difference being that NOS in brain is cytosolic and NOS in endothelium is mainly a membrane-associated protein. Two major strategies are used to study the normal and pathophysiological function of these NOS isoforms. One is pharmacological and the other is genetic. The pharmacological strategy is based on several NOS inhibitors which have been used to demonstrate the functional roles of endogenous NO and are as follows: NG-monomethyll-arginine (L-NMMA), NG-nitro-l-arginine (L-NA), L-NA methyl ester (l-NAME), and asymmetric dimethylarginine (ADMA) as nonselective inhibitors; 7-nitroindazol (7-NI), as a relatively selective inhibitor of nNOS; and aminoguanidine, N6-iminoethyl-l-lysine, and Wl1400 as selective inhibitors of iNOS. The genetic approach is based on the development of mutant mice lacking expression of the genes for nNOS, iNOS, and eNOS. There exists knockout (KO) mice for each of the NOS isoforms and much work has been performed with each of these NOS-KO mice to determine the physiological and pathophysiological characteristics in these animals and their involvement with a variety of disease processes.
NO and Cerebrovascular Physiology Endothelial NOS eNOS plays an important role in the regulation of CBF and in the regulation of the vasculature throughout the body. eNOS results in cerebral vasodilation, a reduction in vascular resistance, a reduced blood pressure, platelet aggregation and adhesion inhibition, leukocyte adhesion and migration inhibition, and a reduction of smooth muscle proliferation. It also has an important role in vascular remodeling and angiogenesis. Using pharmacological inhibitors of eNOS and eNOS KO mice, many investigators have demonstrated the role of eNOS in the control of cerebral circulation
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under a variety of conditions, even to control basal CBF levels. NOS inhibitors lead to vasoconstriction and decreased CBF and NO release leads to vasodilation and increased CBF. Vasodilation occurs when NO, released from endothelium, stimulates soluble guanylate cyclase in smooth muscle which results in vasorelaxation. Endothelial NO production is important for other aspects of vascular function such as intimal proliferation which is a response to arterial injury. eNOS KO mice develop more neointimal proliferation after vascular injury than do wild-type mice. What is curious and interesting is that in both KO and wild-type mice there is less intimal proliferation in female animals compared with male animals, indicating a possible role for estrogen in this response. The role of NO in the cerebral vascular response to CO2 and O2 was previously discussed. Cerebral autoregulation, the maintenance of CBF despite changes in perfusion pressure, is another basic physiological characteristic of the cerebral circulation. Since autoregulation involves cerebral vasodilation and vasoconstriction, NO is thought to be involved with this mechanism, and most studies have shown that NO is involved with the mechanism of cerebral autoregulation. While much work has been performed in this area, as with the role of NO in the mechanism of CO2 and O2, the precise mechanism involving NO in autoregulation is unclear and somewhat controversial. Inducible NOS The inducible form of NOS (iNOS) has been described in many cell types: macrophages, astrocytes, microglia, leukocytes, and endothelial cells. iNOS is upregulated as a response to immunological stimuli such as endotoxin and cytokines. Many cytokines induce iNOS: interferon gamma, tumor necrosis factor, interleukin-1, 1B, 6, and so on. Lipopolysaccharide also induces iNOS. These agents can produce cerebral vasodilation which is dependent on the formation of NO and may be mediated by NOS. iNOS produces large amounts of NO which can damage or even destroy cells. Cerebral ischemia and other types of brain injury (cardiac arrest and traumatic brain injury) result in a marked inflammatory reaction. This inflammation activates a variety of cells; iNOS is upregulated and NO is produced in high, even toxic amounts, can then combine with superoxide to produce peroxynitrite, and results in injury. While this mechanistic view prevails, it is a simplified view, and it is likely that iNOS generated NO, like eNOS NO has several roles to play, and the balance between neurotoxicity and neuroprotection depends on many factors. iNOS-derived NO also promotes lipid peroxidation, DNA damage, and BBB breakdown, all of which have prominent roles in brain ischemic injury.
Neuronal NOS nNOS is a constitutive enzyme expressed in brain, peripheral nervous system, and skeletal muscles. nNOS is expressed in neuronal cell bodies, and NO derived from nNOS is an important neurotransmitter associated with neuronal plasticity, memory formation, regulation of CBF, transmission of pain signals, and neurotransmitter release. The excitotoxic transmitter glutamate increases cellular Ca2+ with ischemia and Ca2+ is required for nNOS activity. Ca2+ mediates calmodulin binding to nNOS. Since nNOS is produced from excitotoxicity, nNOS plays a crucial role in ischemic injury. Thus nNOS NO mediates synaptic plasticity and neuronal signaling, but promotes neurotoxicity following ischemic damage, whereas NO produced by eNOS is protective. Neurogenesis also appears to be decreased by nNOS, while iNOS seems to stimulate it.
CONCLUSION NO is an endogenous signaling agent involved in many physiological and pathophysiological processes. It is an important mediator in the regulation of CBF in the resting state, acting as a key mediator on the pathways responsible for maintaining resting CBF and perfusion in physiological situations, such as hypoxia, hypercapnia, and autoregulation. Disruption of NO synthesis and metabolism underlies many pathophysiological processes that occurs following brain injury. NO is also involved as a neuroprotective and neurotoxic agent following cerebral ischemia. This contradiction can be explained by the type of NOS, the cell type producing the NOS, the amount produced, and whether NO undergoes further oxidation. NO has wide-ranging, complex effects in brain and it is important to consider both protective and detrimental effects of NO while inhibiting the particular NOS for therapeutic effect against cerebral ischemia.
References [1] Kety SS, Schmidt CF. The effects of altered arterial tensions of carbon dioxide and oxygen on cerebral blood flow and cerebral oxygen consumption of normal young men. J Clin Invest 1948;27:484–92. [2] Reivich M. Arterial PCO2 and cerebral hemodynamics. Am J Physiol 1964;206:25–35. [3] Kontos HA, Raper AJ, Patterson JL. Analysis of vasoactivity of local pH1. PC02 and bicarbonate on pial vessels. Stroke 1977;8:358–60. [4] Kontos HA, Wei EP, Raper AJ, Patterson JL. Local mechanisms of CO2 action on cat pial arterioles. Stroke 1977;8:226–9. [5] Gotoh F, Tazaki Y, Meyer JS. Transport of gases through brain and their extravascular vasomotor action. Exp Neural 1961;4:48–58. [6] Koehler RC, Traystman RJ. Bicarbonate ion modulation of cerebral blood flow during hypoxia and hypercapnia. Am J Physiol 1982;243:H33–40.
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12 CBF–Metabolism Coupling P. Venkat1,2, M. Chopp1,2, J. Chen1 1Henry
Ford Hospital, Detroit, MI, United States; 2Oakland University, Rochester, MI, United States
INTRODUCTION The brain requires oxygen and glucose to meet its metabolic demands, and cerebral blood flow (CBF) is its supply channel. The brain has high energy requirements but limited storage capacity, which means persistent CBF is critical for its proper functioning and prevention of damage and death. Therefore, in spite of constituting only about 2% of total body weight, the brain is easily the most perfused organ with almost 15–20% of the total cardiac output directed as CBF. Moreover, a process called cerebral autoregulation thrives to maintain adequate CBF at a constant rate. The arteries supplying the brain namely the internal carotid arteries and vertebral arteries that merge
Primer on Cerebrovascular Diseases, Second Edition http://dx.doi.org/10.1016/B978-0-12-803058-5.00012-6
to form the basilar artery arrange themselves into the “circle of Willis” creating collaterals in the cerebral circulation. This is a defense mechanism against CBF drop such that if an artery supplying the circle is blocked, blood flow from the other blood vessels is able to sustain cerebral circulation. The demand–supply relationship between CBF and cerebral metabolism is tightly coupled and brain regions are either hypoperfused or hyperperfused depending on metabolic needs. A sudden decrease in CBF (either temporary or permanent) due to the occlusion of a cerebral artery is called cerebral ischemia and leads to ischemic stroke, neurological deficits, tissue damage, and even death. An excess of blood flow results in hyperemia in which the intracranial pressure may increase and evoke
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