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Systemically administered adenosine increases caudate blood flow in rabbits
Neuroscience Letters, 80 (1987) 224 -228 Elsevier Scientific Publishers Ireland Ltd.
224
NSL 04821
Systemically administered adenosine increases caudate blood flow in rabbits Serge Puiroud, Elisabeth Pinard, Marie-Christine Miller and Jacques Seylaz Laboratoire de Physiologic et Physiopathologie C~rebrovasculaire, U. 182 INSERM, U.A. 641 C.N.R.S.. Universitb Paris VII, Paris (France) (Received 23 March 1987; Revised version received and accepted 2 June 1987) Key words.
Adenosine; Cerebral blood flow; Rabbit; Caudate nucleus
Adenosine has been proposed to be a chemical link between cerebral metabolism and blood flow. In the present study, we investigated whether the intravenous or intracarotid administration of adenosine could influence regional cerebral blood flow in anesthetized rabbits. The study was performed with the [~4C]ethanol tissue sampling technique which enables quantitative, instantaneous, multiregional blood flow measurements. With either mode of adenosine administration, no change in cerebral blood flow was observed, except in the caudate nucleus in which a significant vasodilation took place. These data indicate that, in rabbits exogenous adenosine increases blood flow in highly specific brain areas, by mechanisms that are discussed.
Adenosine is a purine nucleoside which is synthesized by the dephosphorylation of AMP. Adenosine is a potent vasodilator in various organs and it has been suggested that this adenine nucleoside plays a major role in the regulation of coronary blood flow [2]. For many years now, adenosine has been proposed as a chemical link that ensures the coupling of cerebral metabolism to blood flow [11]. This hypothesis is supported by several findings: during critical conditions of reactive cerebral vasodilation, such as hypoxia, hypoglycemia and epilepsy, brain adenosine concentrations are markedly increased, as they are during ischaemia (see ref. I 1 for a review). Furthermore, adenosine induces the dilatation of pial vessels both in vitro [6] and in situ [10]. In spite of the poor penetrability of adenosine across the blood-brain barrier, several studies have been devoted to the cerebrovascutar effects of systemically injected adenosine in order to determine in vivo the reactivity of cerebral vessels to adenosine; their results do not lead to a general consensus. These discrepancies might depend on the species studied, since in rabbits [8] and baboons [7], adenosine induces a cerebral vasodilatation, whereas adenosine fails to change cerebral blood flow in cats [8] and dogs [3, 8]. Moreover, much of the previous work in the investigation Correspondence: E. Pinard, U. 182 INSERM, U.A. 641 C.N.R.S., 10, avenue de Verdun, F-75010 Paris, France. 0304-3940/87/$ 03.50 O 1987 Elsevier Scientific Publishers Ireland Ltd.
225
of the cerebrovascular effects of exogenous adenosine has been performed by using a global evaluation of cerebral blood flow. In the present study, we further investigated the regional influence of adenosine on cerebral blood flow in anesthetized rabbits, during intravenous perfusion or during infusion into one internal carotid artery, in order to determine the local reactivity of the vessels to increased adenosine concentrations. The study was performed in 15 male New Zealand rabbits weighing 2.0-2.5 kg by means of the [14C]ethanol tissue sampling technique [9]. The animals were anesthetized with Althesin (Glaxo; 24 mg/kg/h), tracheotomized, paralyzed with gallamine triethiodide (Flaxedil, 5 mg/kg/h) and artificially ventilated (Braun 50 Respirator) with room air and supplemental oxygen to maintain normal blood gases and pH. Polyethylene catheters were inserted into both saphenous arteries (one serving for blood pressure recording: P 23 ID Statham transducer; the other one for the serial arterial blood sampling every 3 s, required to measure cerebral blood flow) and one into the femoral vein for the tracer injection. The rectal temperature was also monitored and adjusted with a heating pad. The animals were heparizined with 400 IU/kg of lyophilized heparin solution (Choay). Arterial blood was regularly sampled to control the physiological state of the animals by means of a blood gas analyzer (Radiometer ABL 2). When the variables were stable and in the range of physiological values, the rabbits underwent either an intravenous adenosine perfusion (Sigma; 0.2 mg/kg/min: n = 9) during 3 min or a saline injection at the same rate (1 ml/min; n = 6 ) . The pH of the adenosine solution was adjusted to physiological values (7.3 7.4). The [14C]ethanol (Amersham) used as tracer was administered during the last 30 s of the injection. The animals were then sacrificed and their heads frozen in liquid nitrogen. Samples of 10 structures in each hemisphere were performed in order to determine their blood flow values. These blood flow values were calculated by computer for each structure from the equation 7"
Ci(T) = 2Ki ~ C a ( t ) ' e - r~cr - t ) d t 0
where Ci(T) is the final tissue tracer concentration at time T ( = 30 s), C a ( t ) the arterial concentration of the tracer, 2 the tissue/blood partition coefficient, and Ki is related to flow fi by the equation fi : • K i
Results are given as mean _+S.E.M. Statistical analysis was performed with Student's t-test. Prior to adenosine administration, all rabbits were in comparable physiological conditions (Table I). During the saline perfusion, the systemic parameters did not change. During the adenosine intravenous perfusion, none of these variables was modified, except the arterial blood pressure which decreased significantly from 91.9+4.1 to 77.3+2.2 mmHg, i.e. a 15.1 +2.9% decrease (P<0.001). In none of the animals did the blood pressure fall below the lower limit of autoregulation.
226 TABLE I PHYSIOLOGICAL VARIABLES Mean values of the systemic variables of rabbits before receiving either (1) an i.v. saline perfusion, or (2) an i.v. adenosine perfusion (0.2 mg/kg/min), or (3) an adenosine perfusion (25/Jg/kg/min) into one carotid artery (i.c.). MABP, mean arterial blood pressure; PaO2, arterial partial pressure of oxygen; paCO2, arterial partial pressure of carbon dioxide; pH, arterial pH; Temp, rectal temperature.
(l) Saline-injected group (n = 6) (i,v.) (2) Adenosine-injected group (n = 9) (i.v.) (3) Adenosine-injected group (n = 9) (i.c.)
MABP (mmHg)
p~O2 (mmHg)
paCO2 (mmHg)
pH
Temp ('C)
88.5_+5:0
123.3_+6.8
35.2+1.6
7.37_+0.03
39.4_+0.2
91.9_+4.1
132.0_+6.2
36.1_+1.1
7.38+0.02
39.6_+0.2
82.8+2.9
127.6-+8.1
34.2+0.2
7.39-+0.01
39.2_+0.2
The mean cerebrovascular effects of the intravenous adenosine administration are presented in Table II. Adenosine induced no significant change in blood flow in all the cerebral structures studied with the exception of both caudate nuclei in which a significant vasodilation was observed. The mean increase in caudate blood flow was 23.1%. In an additional study, the cerebrovascular consequences of the intracarotid infusion of adenosine were investigated in 9 rabbits. The experimental protocol was similar, except that the left common carotid artery was carefully exposed, the external TABLE II CEREBROVASCULAR VARIABLES Mean quantitative values of blood flow (ml/100 g/min) in 10 cerebral structures during i.v. perfusion of (1) saline and (2) adenosine (0.2 mg/kg/min). (Front. Ctx, frontal cortex; Pariet. Ctx, parietal cortex; Occ. Ctx, occipital cortex; C.N., caudate nucleus; Hipp., hippocampus; Thai., thalamus; Hypoth., Hypothalamus; R.F., reticular formation; Sup. Coll., superior colliculi; Cereb., cerebellum.
Front. Ctx Pariet. Ctx Occ. Ctx C.N. Hipp. Thai. Hypoth. R.F. Sup. Coll. Cereb.
(1) Saline-injected
(2) Adenosine-injected
group (n = 6)
group (n = 9)
77.6_+ 5.4 97.4 + 10.7 71.2+ 6.1 53.7+ 2.7 49.6_+ 3.2 61.1 + 4.6 51.0_+ 3.2 50.3_+ 2.3 84.5_+ 8.5 60.4_+ 2.8
70.5+2.5 86.7 + 3.0 72.9+2.3 66.1___2.1 48.7+2.0 63.4___2.4 54.6_+ 1.9 50.30-1.7 75.2_+3.4 60.7_+ 1.9
Significance
n.s. n.s. n.s. P<0.01 n.s. n.s. n.s. n.s. n.s. n.s.
227 ligated, and adenosine was infused at a dose of 25/zg/kg/min at a rate of 80/A/min over 3 min. Cerebral blood flow measurements were made during the last 30 s of this infusion. The lateralization of the drug effects was estimated by side-to-side comparison of paired structures. Statistical comparisons were performed with paired Student's t-test. Blood flow was found to be 16.7_+3.3% ( P < 0 . 0 1 ) higher in the caudate nucleus of the adenosine-perfused hemisphere than in that of the contralateral hemisphere. Except for this specific side-to-side difference, no significant cerebrovascular effect was observed in any of the hemispheric structures studied. These results demonstrate that, in anesthetized, paralyzed and ventilated rabbits, there is no homogeneous increase in cerebral blood flow associated with the systemic administration of adenosine. Our findings show that, a m o n g the cerebral structures studied, the caudate nucleus is the only one to undergo a significant vasodilatation when adenosine is administered by the intravenous route, i.e. concomitant with a slight arterial hypotension. In the case of intracarotid adenosine infusion, i.e. without inducing any change in arterial blood pressure, a significant side-to-side flow difference was evidenced only in the caudate nucleus, which speaks in favor of a local effect of adenosine on caudate blood vessels. A very tentative explanation of the heterogeneity of the response would be that endothelial cells possess different intrinsic properties as a function of the territory irrigated. It has been demonstrated that there exists a nucleoside transport system associated with the blood brain barrier, which is a high-affinity uptake system [1, 5, 12]; the affinity of such a system could vary according to the cerebral structures. These barrier mechanisms could also vary with species, enabling, or not, the access of exogenous adenosine to the cerebrovascular smooth muscle cells. Another speculative hypothesis would be that vascular adenosine receptors are not homogeneously distributed throughout the brain. This is supported by the fact that [3H]5-N-ethylcarboxamide-adenosine (NECA) binding to high affinity Az-adenosine receptors has been found to be highest in the striatum but to be detectable at much lower levels in other brain areas in the rat [4]. I f one were to consider the results that indicate that the receptor site mediating relaxation of cerebral vessels is of the A2-subtype [6], it is tempting to extrapolate these observations in the literature to our present findings. In conclusion, the present study suggests that the discrepancies in the reported cerebrovascular effects of exogenous adenosine are related to mechanisms much more complicated than simple species differences and that the heterogeneity of the adenosine-induced blood flow changes could be explored locally, at the capillary level.
This work was supported by grants from the Direction des Recherches, Etudes et Techniques (contract no. 2645). 1 Beck, D.W., Vinters, H.V., Hart, M.N., Henn, F.A. and Cancilla, P.A., Uptake of adenosine into cultured cerebral endothelium, Brain Res., 27 (1983) 18(L183. 2 Berne, R.M., Cardiac nucleotides in hypoxia: possible role in regulation of coronary blood flow, Am. J. Physiol., 204 (1963) 317 322.
228 3 Boarini, D.J., Kassel, N.F., Sprowell, J.A. and Olin, J., Intravertebral artery adenosine fails to alter cerebral blood flow in the dog, Stroke, 15 (1984) 1057-1060. 4 Bruns, R.F., Lu, G.H. and Pugsley, T.A., Characterization of the A2 adenosine receptor labeled by (3H)NECA in rat striatal membranes, Mol. Pharmacol., 29 (1986) 331-346. 5 Cornford, E.M. and Oldendorf, W.H., Independent blood-brain barrier transport systems for nucleic acid precursors, Biochem. Biophys. Acta, 394 (1975) 211 219. 6 Edvinsson, L. and Fredholm, B.B., Characterization of adenosine receptors in isolated cerebral arteries of cat, Br. J. Pharmacol., 80 (1983)631~537. 7 Forrester, T., Harper, A.M., MacKenzie, E.T. and Thomson, E.M., Effect of adenosin e triphosphate and some derivatives on cerebral blood flow and metabolism, J. Physiol. (London), 296 (1979) 343 355. 8 Heistad, D.D., Marcus, M.L., Gourley, J.K. and Busija, D.W., Effect of adenosine and dipyridamole on cerebral blood flow, Am. J. Physiol., 240 (1981) H775-H780. 9 Lacombe, P., Meric, P., Reynier-Rebuffel, A.M. and Seylaz, J., Critical evaluation of cerebral blood flow measurements made with 14C-ethanol, Med. Biol. Eng. Comput., 17 (1979) 126-142. 10 Wahl, M. and Kuschinsky, W., The dilatory action of adenosine in pial arteries of cats and its inhibition by theophylline, Pttiigers Arch., 362 (1976) 55-59. I 1 Winn, H.R., Rubio, R. and Berne, R.M., The role of adenosine in the regulation of cerebral blood flow, J. Cereb. Blood Flow Metab., 1 (1981) 239-244. 12 Wu, P.H. and Phillis, J.W., Uptake of adenosine by isolated rat brain capillaries, J. Neurochem., 38 (1982) 687~590.
224
NSL 04821
Systemically administered adenosine increases caudate blood flow in rabbits Serge Puiroud, Elisabeth Pinard, Marie-Christine Miller and Jacques Seylaz Laboratoire de Physiologic et Physiopathologie C~rebrovasculaire, U. 182 INSERM, U.A. 641 C.N.R.S.. Universitb Paris VII, Paris (France) (Received 23 March 1987; Revised version received and accepted 2 June 1987) Key words.
Adenosine; Cerebral blood flow; Rabbit; Caudate nucleus
Adenosine has been proposed to be a chemical link between cerebral metabolism and blood flow. In the present study, we investigated whether the intravenous or intracarotid administration of adenosine could influence regional cerebral blood flow in anesthetized rabbits. The study was performed with the [~4C]ethanol tissue sampling technique which enables quantitative, instantaneous, multiregional blood flow measurements. With either mode of adenosine administration, no change in cerebral blood flow was observed, except in the caudate nucleus in which a significant vasodilation took place. These data indicate that, in rabbits exogenous adenosine increases blood flow in highly specific brain areas, by mechanisms that are discussed.
Adenosine is a purine nucleoside which is synthesized by the dephosphorylation of AMP. Adenosine is a potent vasodilator in various organs and it has been suggested that this adenine nucleoside plays a major role in the regulation of coronary blood flow [2]. For many years now, adenosine has been proposed as a chemical link that ensures the coupling of cerebral metabolism to blood flow [11]. This hypothesis is supported by several findings: during critical conditions of reactive cerebral vasodilation, such as hypoxia, hypoglycemia and epilepsy, brain adenosine concentrations are markedly increased, as they are during ischaemia (see ref. I 1 for a review). Furthermore, adenosine induces the dilatation of pial vessels both in vitro [6] and in situ [10]. In spite of the poor penetrability of adenosine across the blood-brain barrier, several studies have been devoted to the cerebrovascutar effects of systemically injected adenosine in order to determine in vivo the reactivity of cerebral vessels to adenosine; their results do not lead to a general consensus. These discrepancies might depend on the species studied, since in rabbits [8] and baboons [7], adenosine induces a cerebral vasodilatation, whereas adenosine fails to change cerebral blood flow in cats [8] and dogs [3, 8]. Moreover, much of the previous work in the investigation Correspondence: E. Pinard, U. 182 INSERM, U.A. 641 C.N.R.S., 10, avenue de Verdun, F-75010 Paris, France. 0304-3940/87/$ 03.50 O 1987 Elsevier Scientific Publishers Ireland Ltd.
225
of the cerebrovascular effects of exogenous adenosine has been performed by using a global evaluation of cerebral blood flow. In the present study, we further investigated the regional influence of adenosine on cerebral blood flow in anesthetized rabbits, during intravenous perfusion or during infusion into one internal carotid artery, in order to determine the local reactivity of the vessels to increased adenosine concentrations. The study was performed in 15 male New Zealand rabbits weighing 2.0-2.5 kg by means of the [14C]ethanol tissue sampling technique [9]. The animals were anesthetized with Althesin (Glaxo; 24 mg/kg/h), tracheotomized, paralyzed with gallamine triethiodide (Flaxedil, 5 mg/kg/h) and artificially ventilated (Braun 50 Respirator) with room air and supplemental oxygen to maintain normal blood gases and pH. Polyethylene catheters were inserted into both saphenous arteries (one serving for blood pressure recording: P 23 ID Statham transducer; the other one for the serial arterial blood sampling every 3 s, required to measure cerebral blood flow) and one into the femoral vein for the tracer injection. The rectal temperature was also monitored and adjusted with a heating pad. The animals were heparizined with 400 IU/kg of lyophilized heparin solution (Choay). Arterial blood was regularly sampled to control the physiological state of the animals by means of a blood gas analyzer (Radiometer ABL 2). When the variables were stable and in the range of physiological values, the rabbits underwent either an intravenous adenosine perfusion (Sigma; 0.2 mg/kg/min: n = 9) during 3 min or a saline injection at the same rate (1 ml/min; n = 6 ) . The pH of the adenosine solution was adjusted to physiological values (7.3 7.4). The [14C]ethanol (Amersham) used as tracer was administered during the last 30 s of the injection. The animals were then sacrificed and their heads frozen in liquid nitrogen. Samples of 10 structures in each hemisphere were performed in order to determine their blood flow values. These blood flow values were calculated by computer for each structure from the equation 7"
Ci(T) = 2Ki ~ C a ( t ) ' e - r~cr - t ) d t 0
where Ci(T) is the final tissue tracer concentration at time T ( = 30 s), C a ( t ) the arterial concentration of the tracer, 2 the tissue/blood partition coefficient, and Ki is related to flow fi by the equation fi : • K i
Results are given as mean _+S.E.M. Statistical analysis was performed with Student's t-test. Prior to adenosine administration, all rabbits were in comparable physiological conditions (Table I). During the saline perfusion, the systemic parameters did not change. During the adenosine intravenous perfusion, none of these variables was modified, except the arterial blood pressure which decreased significantly from 91.9+4.1 to 77.3+2.2 mmHg, i.e. a 15.1 +2.9% decrease (P<0.001). In none of the animals did the blood pressure fall below the lower limit of autoregulation.
226 TABLE I PHYSIOLOGICAL VARIABLES Mean values of the systemic variables of rabbits before receiving either (1) an i.v. saline perfusion, or (2) an i.v. adenosine perfusion (0.2 mg/kg/min), or (3) an adenosine perfusion (25/Jg/kg/min) into one carotid artery (i.c.). MABP, mean arterial blood pressure; PaO2, arterial partial pressure of oxygen; paCO2, arterial partial pressure of carbon dioxide; pH, arterial pH; Temp, rectal temperature.
(l) Saline-injected group (n = 6) (i,v.) (2) Adenosine-injected group (n = 9) (i.v.) (3) Adenosine-injected group (n = 9) (i.c.)
MABP (mmHg)
p~O2 (mmHg)
paCO2 (mmHg)
pH
Temp ('C)
88.5_+5:0
123.3_+6.8
35.2+1.6
7.37_+0.03
39.4_+0.2
91.9_+4.1
132.0_+6.2
36.1_+1.1
7.38+0.02
39.6_+0.2
82.8+2.9
127.6-+8.1
34.2+0.2
7.39-+0.01
39.2_+0.2
The mean cerebrovascular effects of the intravenous adenosine administration are presented in Table II. Adenosine induced no significant change in blood flow in all the cerebral structures studied with the exception of both caudate nuclei in which a significant vasodilation was observed. The mean increase in caudate blood flow was 23.1%. In an additional study, the cerebrovascular consequences of the intracarotid infusion of adenosine were investigated in 9 rabbits. The experimental protocol was similar, except that the left common carotid artery was carefully exposed, the external TABLE II CEREBROVASCULAR VARIABLES Mean quantitative values of blood flow (ml/100 g/min) in 10 cerebral structures during i.v. perfusion of (1) saline and (2) adenosine (0.2 mg/kg/min). (Front. Ctx, frontal cortex; Pariet. Ctx, parietal cortex; Occ. Ctx, occipital cortex; C.N., caudate nucleus; Hipp., hippocampus; Thai., thalamus; Hypoth., Hypothalamus; R.F., reticular formation; Sup. Coll., superior colliculi; Cereb., cerebellum.
Front. Ctx Pariet. Ctx Occ. Ctx C.N. Hipp. Thai. Hypoth. R.F. Sup. Coll. Cereb.
(1) Saline-injected
(2) Adenosine-injected
group (n = 6)
group (n = 9)
77.6_+ 5.4 97.4 + 10.7 71.2+ 6.1 53.7+ 2.7 49.6_+ 3.2 61.1 + 4.6 51.0_+ 3.2 50.3_+ 2.3 84.5_+ 8.5 60.4_+ 2.8
70.5+2.5 86.7 + 3.0 72.9+2.3 66.1___2.1 48.7+2.0 63.4___2.4 54.6_+ 1.9 50.30-1.7 75.2_+3.4 60.7_+ 1.9
Significance
n.s. n.s. n.s. P<0.01 n.s. n.s. n.s. n.s. n.s. n.s.
227 ligated, and adenosine was infused at a dose of 25/zg/kg/min at a rate of 80/A/min over 3 min. Cerebral blood flow measurements were made during the last 30 s of this infusion. The lateralization of the drug effects was estimated by side-to-side comparison of paired structures. Statistical comparisons were performed with paired Student's t-test. Blood flow was found to be 16.7_+3.3% ( P < 0 . 0 1 ) higher in the caudate nucleus of the adenosine-perfused hemisphere than in that of the contralateral hemisphere. Except for this specific side-to-side difference, no significant cerebrovascular effect was observed in any of the hemispheric structures studied. These results demonstrate that, in anesthetized, paralyzed and ventilated rabbits, there is no homogeneous increase in cerebral blood flow associated with the systemic administration of adenosine. Our findings show that, a m o n g the cerebral structures studied, the caudate nucleus is the only one to undergo a significant vasodilatation when adenosine is administered by the intravenous route, i.e. concomitant with a slight arterial hypotension. In the case of intracarotid adenosine infusion, i.e. without inducing any change in arterial blood pressure, a significant side-to-side flow difference was evidenced only in the caudate nucleus, which speaks in favor of a local effect of adenosine on caudate blood vessels. A very tentative explanation of the heterogeneity of the response would be that endothelial cells possess different intrinsic properties as a function of the territory irrigated. It has been demonstrated that there exists a nucleoside transport system associated with the blood brain barrier, which is a high-affinity uptake system [1, 5, 12]; the affinity of such a system could vary according to the cerebral structures. These barrier mechanisms could also vary with species, enabling, or not, the access of exogenous adenosine to the cerebrovascular smooth muscle cells. Another speculative hypothesis would be that vascular adenosine receptors are not homogeneously distributed throughout the brain. This is supported by the fact that [3H]5-N-ethylcarboxamide-adenosine (NECA) binding to high affinity Az-adenosine receptors has been found to be highest in the striatum but to be detectable at much lower levels in other brain areas in the rat [4]. I f one were to consider the results that indicate that the receptor site mediating relaxation of cerebral vessels is of the A2-subtype [6], it is tempting to extrapolate these observations in the literature to our present findings. In conclusion, the present study suggests that the discrepancies in the reported cerebrovascular effects of exogenous adenosine are related to mechanisms much more complicated than simple species differences and that the heterogeneity of the adenosine-induced blood flow changes could be explored locally, at the capillary level.
This work was supported by grants from the Direction des Recherches, Etudes et Techniques (contract no. 2645). 1 Beck, D.W., Vinters, H.V., Hart, M.N., Henn, F.A. and Cancilla, P.A., Uptake of adenosine into cultured cerebral endothelium, Brain Res., 27 (1983) 18(L183. 2 Berne, R.M., Cardiac nucleotides in hypoxia: possible role in regulation of coronary blood flow, Am. J. Physiol., 204 (1963) 317 322.
228 3 Boarini, D.J., Kassel, N.F., Sprowell, J.A. and Olin, J., Intravertebral artery adenosine fails to alter cerebral blood flow in the dog, Stroke, 15 (1984) 1057-1060. 4 Bruns, R.F., Lu, G.H. and Pugsley, T.A., Characterization of the A2 adenosine receptor labeled by (3H)NECA in rat striatal membranes, Mol. Pharmacol., 29 (1986) 331-346. 5 Cornford, E.M. and Oldendorf, W.H., Independent blood-brain barrier transport systems for nucleic acid precursors, Biochem. Biophys. Acta, 394 (1975) 211 219. 6 Edvinsson, L. and Fredholm, B.B., Characterization of adenosine receptors in isolated cerebral arteries of cat, Br. J. Pharmacol., 80 (1983)631~537. 7 Forrester, T., Harper, A.M., MacKenzie, E.T. and Thomson, E.M., Effect of adenosin e triphosphate and some derivatives on cerebral blood flow and metabolism, J. Physiol. (London), 296 (1979) 343 355. 8 Heistad, D.D., Marcus, M.L., Gourley, J.K. and Busija, D.W., Effect of adenosine and dipyridamole on cerebral blood flow, Am. J. Physiol., 240 (1981) H775-H780. 9 Lacombe, P., Meric, P., Reynier-Rebuffel, A.M. and Seylaz, J., Critical evaluation of cerebral blood flow measurements made with 14C-ethanol, Med. Biol. Eng. Comput., 17 (1979) 126-142. 10 Wahl, M. and Kuschinsky, W., The dilatory action of adenosine in pial arteries of cats and its inhibition by theophylline, Pttiigers Arch., 362 (1976) 55-59. I 1 Winn, H.R., Rubio, R. and Berne, R.M., The role of adenosine in the regulation of cerebral blood flow, J. Cereb. Blood Flow Metab., 1 (1981) 239-244. 12 Wu, P.H. and Phillis, J.W., Uptake of adenosine by isolated rat brain capillaries, J. Neurochem., 38 (1982) 687~590.