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Acute coronary occlusion: Early changes that induce coronary dilatation and the development of collateral circulation∗
Acute Coronary
Occlusion:
that Induce Coronarv Development
Early Changes
Dilatation
and the
of Collateral Circulation*
ROBERT &I. BERNE, M.D. and RAFAEL RUBIO, PH.D. Charlottesville,
Virginia
Coronary occlusion initiates several chemical changes capable of producing arteriolar dilatation. The most important of these are a reduction in PO,, an increase in pC0, and increases in the concentrations of lactic acid, potassium and adenosine in the interstitial fluid of the myocardium. Of these factors, only adenosine can account for the entire coronary dilatation observed in myocardial ischemia. However, the other factors probably contribute to the relaxation of the vascular smooth muscle. The strongest stimulus for the development of coronary collateral vessels is coronary arterial narrowing, although evidence suggests that hypoxia alone will result in moderate enhancement of collateral circulation. Of the various surgical procedures employed to increase collateral vessel development, the only one that shows promise is implantation of the open internal mammary artery into the myocardium. In experimental coronary arterial constriction the peripheral coronary arterial pressure (pressure distal to the point of constriction) appears to be a good index of collateral vessel development. Intercoronary collateral vessels are insensitive to almost all vasoactive agents, but recent data indicate that the beneficial effects of nitroglycerin in angina pectoris may be due to a selective vasodilator action on well developed collateral arteries. release of potassium from the myocardial cells and (5) enhanced release of adenosine from the myocardium. O2 Tension: Guyton and colleague9 have demonstrated that hypoxemia produces dilatation of the vascular bed of skeletal muscle and of isolated 1 mm. arteries obtained from skeletal muscle. The mechanism whereby a reduction in pOa relaxes vascular smooth muscle is not known but presumably involves some aspect of smooth muscle metabolism. In our studies2 of the effects of hypoxemia on coronary resistance, the degree of dilatation correlated inversely with the oxygen content of the venous blood, and lowering of arterial ~0, was without significant effect on coronary resistance if the myocardium was adequately supplied with oxygen by means of elevating coronary perfusion pressure to overperfuse the
ITH ACUTE 0ccu~sIoN of a coronary artery a number of electrical, mechanical and chemical changes take place in the ischemit area of the myocardium. In the present discussion we will be concerned with the chemical changes, but only those chemical changes that induce relaxation of vascular smooth muscle. In addition, consideration will be given to factors that influence or are thought to influence the development of collateral blood flow to the ischemic cardiac muscle.
W
MYOCARDIAL ISCHEMIA AND VASODILATOR METABOLITES Some of the chemical changes of myocardial kchemia that may elicit vasodilation of the resistance vessels are (1) reduction in oxygen tension per se, (2) increase in carbon dioxide tension, (3) accumulation of lactic acid, (4)
*From the Department of Physiology, University of Virginia School of Medicine, Charlottesville, Va. Address for reprints: Robert M. Beme, M.D., Department of Physiology, University of Virginia, School of Medicine, Charlottesville, Va. 22901. 776
THE
AMERICAN
JOURNAL
OF
CARDIOLOGY
Coronary Dilatation
and Collateral
myocardium. To these observations must be added the facts that in reactive hyperemia the venous blood is almost fully saturated with oxygen and that the duration of reactive hyperemia is roughly proportional to the duration of the arterial occlusion. Together the findings are more in keeping with release of a vasodilator substance from the parenchymal tissue than with the concept that pOz regulates vascular resistance in the intact heart through a direct effect on the vascular smooth muscle. Furthermore, studies by Ross et aL3 have indicated that at equal levels of p0, of the venous blood, the increase in blood flow to skeletal muscle is threefold greater with repetitive contractions (functional hyperemia) than with perfusion with venous blood in the resting state. There is no question that oxygen tension affects the contractile state of vascular smooth muscle. However, there is serious doubt that it plays a direct role in the resistance changes that occur under physiologic conditions, since such adjustments take place continuously in response to changes in metabolic activity of the tissue when arterial blood oxygen saturation remains at normal levels. CO, and Lactic Acid: Concentrations of carbon dioxide and lactic acid in excess of those found in venous effluents from active muscle showing marked vasodilation fail to elicit comparable vasodilation when administered intraarterially.” Release of Potassium Ions: Potassium ions are released from active tissues and in low concentrations can produce a decrease in vascular resistance. In skeletal muscle, the potassium ion, either alone5 or in conjunction with reduced oxygen tension,6 is thought to be the mediator of metabolic vasodilation. We studied this possibility in the heart some years ago7 and were unable to produce coronary vasodilation of the magnitude observed with physiologic stimuli when increasing amounts of potassium were infused into the inflow tubing of a cannulated coronary artery. If high concentrations of potassium were added, the small degree of vasodilation was converted to severe vasoconstriction. Furthermore, procedures which are known to elicit coronary vasodilation, such as increased cardiac pressure work, administration of norepinephrine or myocardial hypoxia failed to release significant amounts of potassium from the myocardium. VOLUME
24,
DECEMBER
1969
Circulation
777
Relea.ie of Adenosine: In early studies$JJ we had reported the presence of degradative products of adenosine in effluents of the isolated and intart hypoxic heart and with the use of an adenosine deaminase inhibitor were able to identify adenosine release from hypoxic isolated hearts, Recently, it has been possible to demonstrate the appearance of adenosine in the coronary sinus blood of hearts subjected to brief periods of ischemia.l” In these esperiments, the left coronary artery was occluded for 30 to 60 seconds, and samples of coronar) sinus and arterial blood were collected simultaneously during the subsequent period of reactive hyperemia. Control arterial and coronary sinus blood samples were obtained during periods of normal coronary blood flow. Only the coronary sinus blood collected during reactive hyperemia contained adenosine as well as significant amounts of its products of degradation, inosine and hypoxanthine. Calculations of the adenosine concentration and myocardial interstitial fluid were based upon (1) the assumption that all of the adenosine washed out of the heart during reactive hyperemia was located in the interstitial fluid (a reasonable assumption in view of the high concentration of adenosine deaminase distributed throughout the myocardial cell) , and (2) the amount of adenosine lost after incubation in blood for 10 seconds (the estimated time of contact before red cell separation and enzymatic inactivation could be accomplished in the in vivo experiments) . The calculated concentration of adenosine that was reached in the interstitial fluid after 30 to 60 seconds of coronary occlusion was greater than that required to elicit maximal coronary dilatation (as determined by intracoronary infusion of adenosine) . More prolonged occlusion (4 to 7 minutes) of the left coronary artery resulted in a fourfold increase in the release of adenosine and a tenfold increase in the release of hypoxanthine and inosine. The adenosine found in the venous effluent of the ischemic heart is probably formed by the action of the enzyme .Y nucleotidase on adenylic acid (AMP) . Histochemical studies employing electron microscopyll indicate the 5’ nucleotidase is located in the T tubule system and along the myocardial cell membrane. We have confirmed these observations and also note high levels of enzymatic activity in the intercalated discs. With myocardial ischemia, the balance between synthesis and breakdown
778
Berne and Rubio
ARTERIOLE
CAPILLARY
Adenosine lhaminase Adenylic Acid Deaminase - 5’ Nucleotidose .-.-. Adenosine Kinose
Figure 1. Diagram of part of a myocardial cell illustrating the formation and distribution of adenosine, the effect of adenosine on an adjacent arteriole, the location of enzymes that degrade AMP (5’nucleotidase and AMP deaminase) and adenosine (adenosine deaminase) and the possible location of the enzyme that synthesizes AMP from adenosine (adenosine kinase) .
of adenosine triphosphate (ATP) is disturbed in favor of degradation, and AMP and inorganic phosphate accumulate within the myocardial cell. The AMP formed reaches the cell membrane where the 5’ nucleotidase is located and is dephosphorylated to adenosine. Some of the adenosine may reenter the myocardial cell where it is either deaminated by adenosine deaminase to inosine or rephosphorylated to AMP by adenosine kinase. Some AMP is undoubtedly deaminated to inosinic acid (IMP) by the adenylic acid deaminase in the cell sap, and the IMP can be dephosphorylated by 5’ nucleotidase to inosine. Adenosine that reaches the interstitial fluid can induce dilatation of the resistance vessels, thereby increasing the blood flow and oxygen supply to the tissue once the circulation is reestablished. With enhanced blood flow through the previously ischemic tissue, the accumulated adenosine is washed out of the interstitial fluid during the period of reactive hyperemia, and coronary blood flow is readjusted to the previous control level. Approximately one half of the adenosine that is carried away by the blood is degraded by the red cells, and the other half is recoverable as adenosine from the plasma
fraction of the coronary sinus blood. The formation, distribution, metabolism and vasodilator effect of adenosine are schematized in Figure 1. The formation of adenosine in the hypoxic myocardial cell is favored by a reduction in the ATP concentration and an increase in the concentration of inorganic phosphate since ATP inhibits 5’ nucleotidase and stimulates adenylic acid deaminase, whereas inorganic phosphate inhibits adenylic acid deaminase. To what extent degradation of adenylic acid is diverted from inosinic acid to adenosine by alterations of the intracellular concentrations of adenosine triphosphate and inorganic phosphate in the intact tissue remains conjectural. CORONARY
COLLATERAL
CIRCULATION
Mechanisms for the Development of Collateral Coronary Vessels: In the normal heart,
the collateral blood flow from extracardiac sources is virtually zero, and flow between coronary arteries is quite small and incapable of sustaining the myocardium in the event of an acute coronary occlusion. Nevertheless, the small interarterial communications that exist in the normal heart can rapidly develop into sizable vessels in response to certain stimuli. The strongest stimulus to the development of coronary collateral vessels is gradual narrowing of a major coronary artery. Whether the trigger to the growth of the minute preexisting collateral vessels is hypoxia or a pressure gradient across the collateral vessels, or both, has still not been determined with any degree of certainty. Studies by Eckstein12 reveal that chronic anemia in dogs leads to the development of collateral vessels, as evidenced by significant increases in retrograde flow (flow from the distal end of an occluded coronary artery corrected for viscosity by acute red cell transfusion) and peripheral coronary pressure (pressure distal to the point of occlusion of a major coronary artery) . Such studies suggest that hypoxia alone can produce growth of collateral vessels, although the magnitude of the retrograde flow and peripheral coronary pressure was small relative to that which develops in response to coronary arterial narrowing. Exercise seems to accelerate collateral vessel development in the presence of coronary arterial narrowing. l3 In these studies one cannot distinguish between myocardial hypoxia and a pressure gradient across potential collateral vessels as the initiating factor. With exercise THE AMERICAN
JOURNAL
OF CARDIOLOGY
Coronary Dilatation
and Collateral
the increased cardiac work in the face of a fixed partial obstruction of a coronary artery leads to myocardial hypoxia, and the arteriolar dilatation in the active myocardium supplied by the constricted artery leads to a reduced pressure distal to the constriction and, therefore, a greater pressure gradient across potential collateral arteries. In keeping with these experimental findings are those of Baroldi et al.14 in human hearts. They found that narrowing is a greater stimulus for the growth of preexisting interarterial vessels than is hypoxemia. Procedures to Enhance Collateral Circulation: A number of procedures have been at-
tempted in an effort to enhance the development of coronary collateral vessels. However, in many of these studies on normal animals the investigators failed to demonstrate that the procedure employed increased the collateral circulation over and above that produced by coronary arterial narrowing alone. To avoid this problem we constricted the left circumflex coronary artery to simulate arteriosclerotic narrowing of a coronary artery in man and compared the effect of constriction alone with constriction plus one of several “revascularizing” procedures on the magnitude of the collateral blood fl0w.l” The procedures studied were (1) application of shredded asbestos to the epicardial surface of the heart, (2) suturing of the mediastinal fat pad (with vascular supply intact) to the abraded surface of the heart in conjunction with application of asbestos, and (3) implantation of the open internal mammary artery into the left ventricular wall (Vineberg procedure) in addition to the application of Ivalon@ sponge to the denuded surface of the left ventricle. With each surgical procedure 1 group of dogs had the left circumflex artery narrowed (tied over 0.9 to 1.2 mm. probes which were then removed) at the time of the procedure and another group had the coronary artery constricted six weeks before the “revascularizing” procedure. Suitable control animals (unoperated upon, coronary arterial narrowing, and coronary arterial narrowing with sham thoracotomy six weeks later) were studied. The results showed that at equal degrees of coronary arterial narrowing the retrograde flow rates and peripheral coronary perfusion pressures were approximately the same’ in the control dogs and in the dogs subjected to the three procedures. Thus, no evidence of increased VOLUME
24.
DECEMBER
1969
Circulation
779
functional collateral development above that produced by narrowing alone was produced by the different revascularizing procedures. In 8 of the last 10 internal mammary artery implantations performed, the vessel remained patent, and in 2 of 5 of these animals in which the internal mammary artery was clamped, the retrograde flow decreased after clamping. At postmortem examination, injection of a barium-gelatin mixture (Schlesinger technic) into the circumflex coronary artery distal to the point of constriction revealed the presence of barium in (1) the fibrous tissue in only 1 of 17 dogs treated with asbestos, (2) the fat pad of 2 of 22 dogs in which the mediastinal fat pad was sutured to the abraded epicardial surface, and (3) the internal mammary artery in 8 of 10 dogs in which the artery remained patent. Thus, of the procedures tested, only the internal mammary artery implantation produced significant vascular communications with extracardiac vessels, and these were of limited functional value. However, in the absence of a source of collateral blood from adjacent coronary arteries, as may occur in man with arteriosclerotic coronary artery disease, collateral development between the ischemic area and an extracardiac source of blood, such as from the implanted internal mammary artery, might be considerably greater than that observed in our experiments. Results similar to ours were obtained by MacLean et al.‘6 They used deuterium oxide as a measure of nutritional collateral blood flow and found that pulmonary artery-left atria1 shunts and poudrage with coronary sinus narrowing, when performed several months before or at the time of occlusion of the left anterior descending artery, failed to increase collateral blood flow to the ischemic area above that produced by occlusion alone. Experimental Coronary Constriction: Elliot and Khouri and their colleaguesl7Js studied the development of collateral vessels in the unanesthetized dog with gradual constriction of the left circumflex coronary artery produced in one series of experiments with ameroid constrictors17 and in the other with an inflatable cuff.ls In the early stages of coronary arterial narrowing the flow was reasonably well maintained, presumably because of progressive dilatation of the resistance vessels as constriction of the parent artery progressed. However, the pressure difference across the constriction increased, and the peak reactive hyperemit response decreased markedly during this early period. With further constriction, circumflex blood
Berne and Rubio
780 KS?TA
VB
C.S
Figure 2. Diagram of a normal artery (A) and vascula? bed (MA) and an adjacent vascular bed (MB) distal to an occluded coronary artery (B). The irregular vessel (C) represents an interarterial collateral artery that carries blood from the normal artery to supply the muscle of the occluded artery. The cannula is
used to measure retrograde flow (indicative of collateral flow) distal to the occlusion. Nitroglycerin increases collateral flow by dilation of vessels C or A or both, whereas dipyridamole dilates the resistance vessels of the normal artery, thereby lowering the pressure in this artery and decreasing the flow through the collateral vessel. VA=coronary vein draining normal myocardium. VB = coronary vein draining ischemic myocardium. CS=coronary sinus. (Reprinted from Fam and McCregorae by permission of the American Heart Association, Inc.) flow decreased, reactive hyperemia disappeared and peripheral coronary pressure began to rise abruptly. When flow in the constricted vessel approached zero, peripheral coronary pressure reached maximal levels as did the pressure difference across the constriction. After complete occlusion, peripheral coronary pressure remained elevated and the pressure difference across the occluded segment decreased, thereby indicating the presence of collateral vessels. The observation that blood flow in the adjacent unoccluded descending coronary artery increased as circumflex blood flow decreased and reached 80 per cent of the sum of the control flow in the two branches of the left coronary artery provides further evidence of collateral development. With the initial reduction in circumflex flow, myocardial contractility was impaired, as evidenced by a reduction of dp/dt, but within 24 hours it returned to control values as descending coronary artery flow increased.
Coronury Vasodilator Drugs: Once collateral vessels are established it would be of considerable value if blood flow through these vessels could be augmented by the use of vasodilator drugs. In the case of the normal dog heart, the small collateral vessels behave as unresponsive passive tubes and only mechanical factors such as increase in perfusion pressure can enhance blood flow.‘g-21 However, in the heart made ischemic by the application of ameroid constrictors to the anterior descending and circumflex branches of the left coronary artery, nitroglycerin increased retrograde flow from the circumflex artery (distal to the amerwhen aortic pressure was oid constrictor) held constant.22 In contrast, dipyridamole did not alter retrograde flow under identical experimental conditions. These findings suggest that nitroglycerin has a selective dilator action on the well developed collateral vessels which may be responsible for its beneficial effects in angina pectoris. The proposed sites of action of nitroglycerin and dipyridamole on collateral coronary vessels are presented diagrammatically in Figure 2. To date, these studies have not been confirmed by other investigators. In fact, Rees and Redding reported that starting 24 hours after coronary occlusion, dipyridamole increased collateral blood flow to the infarcted myocardium. They used rasxenon clearance as a measure of blood flow to the ischemic muscle, and the possibility that increased xenon clearance is due to a large overlap of nutrient vessels from the occluded and adjacent unoccluded arteries at the margins of the ischemic area has not been eliminated. REFERENCES 1. GUYTON, A. C., Ross, J. M., CARRIER, O., JR. and WALKER, J. R. Evidence for tissue oxygen demand as the major factor causing autoregulation. Circulation Res., 15 (Suppl. I) :60, 1964. 2. BERNE, R. M., BLACKMON,J. R. and GARDNER,T. H. Hypoxemia and coronary blood flow. I. CIin. Invest., 36:1101, 1957. 3. Ross, J.. JR., KAISER,G. A. and KLOCKE, F. J. Observations on the role of diminished oxygen tension in the functional hyperemia of skeletal muscle. Circulation Res., 15:473, 1964. 4. H~TON, R. and EICHHOLTZ,F. Influence of chemical factors on the coronary circulation. J. Physiol., 59:413, 1925. 5. KJELLMFR, I. The potassium ion as a vasodiIator during muscular exercise. Acta physiol. scandinav., 63:460, 1965. 6. SKINNER,N. S. and POWELL, W. J., JR. Action of oxygen and potassium on vascular resistance of THE AMERICAN JOURNAL OF CARDIOLOGY
Coronary
7.
8.
9.
10.
11.
12.
13.
14.
15.
Dilatation
and Collateral
dog skeletal muscle. Am. J. Physiol., 212:5X$ 19G7. DRISCOL, T. E. and BERRE, R. M. Role of potassium in regulation of coronary blood flow. Proc. Sot. Exper. Biol. S Med., 96:505, 1957. BERNE, R. M. Cardiac nucleotides in hypoxia: possible role in regulation of coronary blood flow. Am. J. Physiol., 204:317, 1963. KATORI, M. and BERNE, R. M. Release of adenosine from anoxic heart-relationship to coronary flow. Circulation Res., 19:420, 1966. Rumo, R., BERNE, R. M. and KATORI, M. Release of adenosine in reactive hyperemia of the dog heart. Am. J. Physiol., 216:56, 1969. ROSTCAARD, J. and BEHNKE, 0. Fine structure localization of adenine nucleoside phosphatase activity in the sarcoplasmic reticulum and T system of rat myocardium. J. Ultrastruct. Res., 12:579, 1965. Development of interarterial ECKSTEIN, R. W. coronary anastomoses by chronic anemia. Disappearance following correction of anemia. Circulation Res., 3:306, 1955. ECKSTEIN, R. W. Effect of exercise and coronary artery narrowing on coronary collateral circulation. Circulation Res., 5:230, 1957. BAROLDI, G., MANTERO, 0. and SCOMAZZONI, G. The collaterals of the coronary arteries in normal and pathologic hearts. Circulation Res., 4:223, 1956. BERNE, R. M., JONES, R. D. and CROSS, F. S. Evaluation of procedures designed to enhance coronary collateral blood flow. Circulation Res., 10: 142, 1962.
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DECEMBER
1969
Circulation
is1
16. MACLEAN, L. D., HEDENSTRO>~, I’. H. .~ntl R_415E~, R. R. Tissue blood flow to the heart-influence of coronary occlusion and surgical measures. Circulation Res., 10:45, 1962. 17. ELI.IOT, E. C., JONES, E. L., BLOOR, C. M.: LEON, 4. S. and GREGG, D. E. Day-to-day changes in coronary hemodynamics secondary to constriction of circumflex branch of left coronary arter) in conscious dogs. Circulation Res., 22:237, 1068. 18. KHOURI, E. M., GREGG, D. E. and LOWENSOHN. H. S. Flow in the major branches of the left coronary artery during experimental coronary insufficiency in the unanesthetized dog. Circulafion Res., 23: 99, 1968. 19. WIGGERS, C. J. and GREEN, H. D. The ineffectiveness of drugs upon collateral flow after experimental coronary occlusion in dogs. .4?~. HutlIt I., 11:527, 1936. 20. KATNJS, A. A. and GREGG, D. E. Some determinants of coronary collateral blood flow in the open-chest dog. Circulation Res., 7:628. 1959. 21. JOHANSSON, B., LINDER, E. and SEEMAN, 1‘. Effects of heart rate and arterial blood pressure on coronary collateral blood flow in dogs. .4cta plrysio[. srandinav., 68 (Suppl. 272) :33, 1966. 22. F.~M, W. M. and MCGREGOR, M. Effect of coronary vasodilator drugs on retrograde flow in areas of chronic myocardial ischemia. Circlrlation Res., 15:355, 1964. 23. REES, J. R. and REDDINC, V. J. Effects of dipyridamole on anastomotic blood flow in experimental myocardial infarction. Cardiovas. Rrs., 1:li9. 1967.
Occlusion:
that Induce Coronarv Development
Early Changes
Dilatation
and the
of Collateral Circulation*
ROBERT &I. BERNE, M.D. and RAFAEL RUBIO, PH.D. Charlottesville,
Virginia
Coronary occlusion initiates several chemical changes capable of producing arteriolar dilatation. The most important of these are a reduction in PO,, an increase in pC0, and increases in the concentrations of lactic acid, potassium and adenosine in the interstitial fluid of the myocardium. Of these factors, only adenosine can account for the entire coronary dilatation observed in myocardial ischemia. However, the other factors probably contribute to the relaxation of the vascular smooth muscle. The strongest stimulus for the development of coronary collateral vessels is coronary arterial narrowing, although evidence suggests that hypoxia alone will result in moderate enhancement of collateral circulation. Of the various surgical procedures employed to increase collateral vessel development, the only one that shows promise is implantation of the open internal mammary artery into the myocardium. In experimental coronary arterial constriction the peripheral coronary arterial pressure (pressure distal to the point of constriction) appears to be a good index of collateral vessel development. Intercoronary collateral vessels are insensitive to almost all vasoactive agents, but recent data indicate that the beneficial effects of nitroglycerin in angina pectoris may be due to a selective vasodilator action on well developed collateral arteries. release of potassium from the myocardial cells and (5) enhanced release of adenosine from the myocardium. O2 Tension: Guyton and colleague9 have demonstrated that hypoxemia produces dilatation of the vascular bed of skeletal muscle and of isolated 1 mm. arteries obtained from skeletal muscle. The mechanism whereby a reduction in pOa relaxes vascular smooth muscle is not known but presumably involves some aspect of smooth muscle metabolism. In our studies2 of the effects of hypoxemia on coronary resistance, the degree of dilatation correlated inversely with the oxygen content of the venous blood, and lowering of arterial ~0, was without significant effect on coronary resistance if the myocardium was adequately supplied with oxygen by means of elevating coronary perfusion pressure to overperfuse the
ITH ACUTE 0ccu~sIoN of a coronary artery a number of electrical, mechanical and chemical changes take place in the ischemit area of the myocardium. In the present discussion we will be concerned with the chemical changes, but only those chemical changes that induce relaxation of vascular smooth muscle. In addition, consideration will be given to factors that influence or are thought to influence the development of collateral blood flow to the ischemic cardiac muscle.
W
MYOCARDIAL ISCHEMIA AND VASODILATOR METABOLITES Some of the chemical changes of myocardial kchemia that may elicit vasodilation of the resistance vessels are (1) reduction in oxygen tension per se, (2) increase in carbon dioxide tension, (3) accumulation of lactic acid, (4)
*From the Department of Physiology, University of Virginia School of Medicine, Charlottesville, Va. Address for reprints: Robert M. Beme, M.D., Department of Physiology, University of Virginia, School of Medicine, Charlottesville, Va. 22901. 776
THE
AMERICAN
JOURNAL
OF
CARDIOLOGY
Coronary Dilatation
and Collateral
myocardium. To these observations must be added the facts that in reactive hyperemia the venous blood is almost fully saturated with oxygen and that the duration of reactive hyperemia is roughly proportional to the duration of the arterial occlusion. Together the findings are more in keeping with release of a vasodilator substance from the parenchymal tissue than with the concept that pOz regulates vascular resistance in the intact heart through a direct effect on the vascular smooth muscle. Furthermore, studies by Ross et aL3 have indicated that at equal levels of p0, of the venous blood, the increase in blood flow to skeletal muscle is threefold greater with repetitive contractions (functional hyperemia) than with perfusion with venous blood in the resting state. There is no question that oxygen tension affects the contractile state of vascular smooth muscle. However, there is serious doubt that it plays a direct role in the resistance changes that occur under physiologic conditions, since such adjustments take place continuously in response to changes in metabolic activity of the tissue when arterial blood oxygen saturation remains at normal levels. CO, and Lactic Acid: Concentrations of carbon dioxide and lactic acid in excess of those found in venous effluents from active muscle showing marked vasodilation fail to elicit comparable vasodilation when administered intraarterially.” Release of Potassium Ions: Potassium ions are released from active tissues and in low concentrations can produce a decrease in vascular resistance. In skeletal muscle, the potassium ion, either alone5 or in conjunction with reduced oxygen tension,6 is thought to be the mediator of metabolic vasodilation. We studied this possibility in the heart some years ago7 and were unable to produce coronary vasodilation of the magnitude observed with physiologic stimuli when increasing amounts of potassium were infused into the inflow tubing of a cannulated coronary artery. If high concentrations of potassium were added, the small degree of vasodilation was converted to severe vasoconstriction. Furthermore, procedures which are known to elicit coronary vasodilation, such as increased cardiac pressure work, administration of norepinephrine or myocardial hypoxia failed to release significant amounts of potassium from the myocardium. VOLUME
24,
DECEMBER
1969
Circulation
777
Relea.ie of Adenosine: In early studies$JJ we had reported the presence of degradative products of adenosine in effluents of the isolated and intart hypoxic heart and with the use of an adenosine deaminase inhibitor were able to identify adenosine release from hypoxic isolated hearts, Recently, it has been possible to demonstrate the appearance of adenosine in the coronary sinus blood of hearts subjected to brief periods of ischemia.l” In these esperiments, the left coronary artery was occluded for 30 to 60 seconds, and samples of coronar) sinus and arterial blood were collected simultaneously during the subsequent period of reactive hyperemia. Control arterial and coronary sinus blood samples were obtained during periods of normal coronary blood flow. Only the coronary sinus blood collected during reactive hyperemia contained adenosine as well as significant amounts of its products of degradation, inosine and hypoxanthine. Calculations of the adenosine concentration and myocardial interstitial fluid were based upon (1) the assumption that all of the adenosine washed out of the heart during reactive hyperemia was located in the interstitial fluid (a reasonable assumption in view of the high concentration of adenosine deaminase distributed throughout the myocardial cell) , and (2) the amount of adenosine lost after incubation in blood for 10 seconds (the estimated time of contact before red cell separation and enzymatic inactivation could be accomplished in the in vivo experiments) . The calculated concentration of adenosine that was reached in the interstitial fluid after 30 to 60 seconds of coronary occlusion was greater than that required to elicit maximal coronary dilatation (as determined by intracoronary infusion of adenosine) . More prolonged occlusion (4 to 7 minutes) of the left coronary artery resulted in a fourfold increase in the release of adenosine and a tenfold increase in the release of hypoxanthine and inosine. The adenosine found in the venous effluent of the ischemic heart is probably formed by the action of the enzyme .Y nucleotidase on adenylic acid (AMP) . Histochemical studies employing electron microscopyll indicate the 5’ nucleotidase is located in the T tubule system and along the myocardial cell membrane. We have confirmed these observations and also note high levels of enzymatic activity in the intercalated discs. With myocardial ischemia, the balance between synthesis and breakdown
778
Berne and Rubio
ARTERIOLE
CAPILLARY
Adenosine lhaminase Adenylic Acid Deaminase - 5’ Nucleotidose .-.-. Adenosine Kinose
Figure 1. Diagram of part of a myocardial cell illustrating the formation and distribution of adenosine, the effect of adenosine on an adjacent arteriole, the location of enzymes that degrade AMP (5’nucleotidase and AMP deaminase) and adenosine (adenosine deaminase) and the possible location of the enzyme that synthesizes AMP from adenosine (adenosine kinase) .
of adenosine triphosphate (ATP) is disturbed in favor of degradation, and AMP and inorganic phosphate accumulate within the myocardial cell. The AMP formed reaches the cell membrane where the 5’ nucleotidase is located and is dephosphorylated to adenosine. Some of the adenosine may reenter the myocardial cell where it is either deaminated by adenosine deaminase to inosine or rephosphorylated to AMP by adenosine kinase. Some AMP is undoubtedly deaminated to inosinic acid (IMP) by the adenylic acid deaminase in the cell sap, and the IMP can be dephosphorylated by 5’ nucleotidase to inosine. Adenosine that reaches the interstitial fluid can induce dilatation of the resistance vessels, thereby increasing the blood flow and oxygen supply to the tissue once the circulation is reestablished. With enhanced blood flow through the previously ischemic tissue, the accumulated adenosine is washed out of the interstitial fluid during the period of reactive hyperemia, and coronary blood flow is readjusted to the previous control level. Approximately one half of the adenosine that is carried away by the blood is degraded by the red cells, and the other half is recoverable as adenosine from the plasma
fraction of the coronary sinus blood. The formation, distribution, metabolism and vasodilator effect of adenosine are schematized in Figure 1. The formation of adenosine in the hypoxic myocardial cell is favored by a reduction in the ATP concentration and an increase in the concentration of inorganic phosphate since ATP inhibits 5’ nucleotidase and stimulates adenylic acid deaminase, whereas inorganic phosphate inhibits adenylic acid deaminase. To what extent degradation of adenylic acid is diverted from inosinic acid to adenosine by alterations of the intracellular concentrations of adenosine triphosphate and inorganic phosphate in the intact tissue remains conjectural. CORONARY
COLLATERAL
CIRCULATION
Mechanisms for the Development of Collateral Coronary Vessels: In the normal heart,
the collateral blood flow from extracardiac sources is virtually zero, and flow between coronary arteries is quite small and incapable of sustaining the myocardium in the event of an acute coronary occlusion. Nevertheless, the small interarterial communications that exist in the normal heart can rapidly develop into sizable vessels in response to certain stimuli. The strongest stimulus to the development of coronary collateral vessels is gradual narrowing of a major coronary artery. Whether the trigger to the growth of the minute preexisting collateral vessels is hypoxia or a pressure gradient across the collateral vessels, or both, has still not been determined with any degree of certainty. Studies by Eckstein12 reveal that chronic anemia in dogs leads to the development of collateral vessels, as evidenced by significant increases in retrograde flow (flow from the distal end of an occluded coronary artery corrected for viscosity by acute red cell transfusion) and peripheral coronary pressure (pressure distal to the point of occlusion of a major coronary artery) . Such studies suggest that hypoxia alone can produce growth of collateral vessels, although the magnitude of the retrograde flow and peripheral coronary pressure was small relative to that which develops in response to coronary arterial narrowing. Exercise seems to accelerate collateral vessel development in the presence of coronary arterial narrowing. l3 In these studies one cannot distinguish between myocardial hypoxia and a pressure gradient across potential collateral vessels as the initiating factor. With exercise THE AMERICAN
JOURNAL
OF CARDIOLOGY
Coronary Dilatation
and Collateral
the increased cardiac work in the face of a fixed partial obstruction of a coronary artery leads to myocardial hypoxia, and the arteriolar dilatation in the active myocardium supplied by the constricted artery leads to a reduced pressure distal to the constriction and, therefore, a greater pressure gradient across potential collateral arteries. In keeping with these experimental findings are those of Baroldi et al.14 in human hearts. They found that narrowing is a greater stimulus for the growth of preexisting interarterial vessels than is hypoxemia. Procedures to Enhance Collateral Circulation: A number of procedures have been at-
tempted in an effort to enhance the development of coronary collateral vessels. However, in many of these studies on normal animals the investigators failed to demonstrate that the procedure employed increased the collateral circulation over and above that produced by coronary arterial narrowing alone. To avoid this problem we constricted the left circumflex coronary artery to simulate arteriosclerotic narrowing of a coronary artery in man and compared the effect of constriction alone with constriction plus one of several “revascularizing” procedures on the magnitude of the collateral blood fl0w.l” The procedures studied were (1) application of shredded asbestos to the epicardial surface of the heart, (2) suturing of the mediastinal fat pad (with vascular supply intact) to the abraded surface of the heart in conjunction with application of asbestos, and (3) implantation of the open internal mammary artery into the left ventricular wall (Vineberg procedure) in addition to the application of Ivalon@ sponge to the denuded surface of the left ventricle. With each surgical procedure 1 group of dogs had the left circumflex artery narrowed (tied over 0.9 to 1.2 mm. probes which were then removed) at the time of the procedure and another group had the coronary artery constricted six weeks before the “revascularizing” procedure. Suitable control animals (unoperated upon, coronary arterial narrowing, and coronary arterial narrowing with sham thoracotomy six weeks later) were studied. The results showed that at equal degrees of coronary arterial narrowing the retrograde flow rates and peripheral coronary perfusion pressures were approximately the same’ in the control dogs and in the dogs subjected to the three procedures. Thus, no evidence of increased VOLUME
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functional collateral development above that produced by narrowing alone was produced by the different revascularizing procedures. In 8 of the last 10 internal mammary artery implantations performed, the vessel remained patent, and in 2 of 5 of these animals in which the internal mammary artery was clamped, the retrograde flow decreased after clamping. At postmortem examination, injection of a barium-gelatin mixture (Schlesinger technic) into the circumflex coronary artery distal to the point of constriction revealed the presence of barium in (1) the fibrous tissue in only 1 of 17 dogs treated with asbestos, (2) the fat pad of 2 of 22 dogs in which the mediastinal fat pad was sutured to the abraded epicardial surface, and (3) the internal mammary artery in 8 of 10 dogs in which the artery remained patent. Thus, of the procedures tested, only the internal mammary artery implantation produced significant vascular communications with extracardiac vessels, and these were of limited functional value. However, in the absence of a source of collateral blood from adjacent coronary arteries, as may occur in man with arteriosclerotic coronary artery disease, collateral development between the ischemic area and an extracardiac source of blood, such as from the implanted internal mammary artery, might be considerably greater than that observed in our experiments. Results similar to ours were obtained by MacLean et al.‘6 They used deuterium oxide as a measure of nutritional collateral blood flow and found that pulmonary artery-left atria1 shunts and poudrage with coronary sinus narrowing, when performed several months before or at the time of occlusion of the left anterior descending artery, failed to increase collateral blood flow to the ischemic area above that produced by occlusion alone. Experimental Coronary Constriction: Elliot and Khouri and their colleaguesl7Js studied the development of collateral vessels in the unanesthetized dog with gradual constriction of the left circumflex coronary artery produced in one series of experiments with ameroid constrictors17 and in the other with an inflatable cuff.ls In the early stages of coronary arterial narrowing the flow was reasonably well maintained, presumably because of progressive dilatation of the resistance vessels as constriction of the parent artery progressed. However, the pressure difference across the constriction increased, and the peak reactive hyperemit response decreased markedly during this early period. With further constriction, circumflex blood
Berne and Rubio
780 KS?TA
VB
C.S
Figure 2. Diagram of a normal artery (A) and vascula? bed (MA) and an adjacent vascular bed (MB) distal to an occluded coronary artery (B). The irregular vessel (C) represents an interarterial collateral artery that carries blood from the normal artery to supply the muscle of the occluded artery. The cannula is
used to measure retrograde flow (indicative of collateral flow) distal to the occlusion. Nitroglycerin increases collateral flow by dilation of vessels C or A or both, whereas dipyridamole dilates the resistance vessels of the normal artery, thereby lowering the pressure in this artery and decreasing the flow through the collateral vessel. VA=coronary vein draining normal myocardium. VB = coronary vein draining ischemic myocardium. CS=coronary sinus. (Reprinted from Fam and McCregorae by permission of the American Heart Association, Inc.) flow decreased, reactive hyperemia disappeared and peripheral coronary pressure began to rise abruptly. When flow in the constricted vessel approached zero, peripheral coronary pressure reached maximal levels as did the pressure difference across the constriction. After complete occlusion, peripheral coronary pressure remained elevated and the pressure difference across the occluded segment decreased, thereby indicating the presence of collateral vessels. The observation that blood flow in the adjacent unoccluded descending coronary artery increased as circumflex blood flow decreased and reached 80 per cent of the sum of the control flow in the two branches of the left coronary artery provides further evidence of collateral development. With the initial reduction in circumflex flow, myocardial contractility was impaired, as evidenced by a reduction of dp/dt, but within 24 hours it returned to control values as descending coronary artery flow increased.
Coronury Vasodilator Drugs: Once collateral vessels are established it would be of considerable value if blood flow through these vessels could be augmented by the use of vasodilator drugs. In the case of the normal dog heart, the small collateral vessels behave as unresponsive passive tubes and only mechanical factors such as increase in perfusion pressure can enhance blood flow.‘g-21 However, in the heart made ischemic by the application of ameroid constrictors to the anterior descending and circumflex branches of the left coronary artery, nitroglycerin increased retrograde flow from the circumflex artery (distal to the amerwhen aortic pressure was oid constrictor) held constant.22 In contrast, dipyridamole did not alter retrograde flow under identical experimental conditions. These findings suggest that nitroglycerin has a selective dilator action on the well developed collateral vessels which may be responsible for its beneficial effects in angina pectoris. The proposed sites of action of nitroglycerin and dipyridamole on collateral coronary vessels are presented diagrammatically in Figure 2. To date, these studies have not been confirmed by other investigators. In fact, Rees and Redding reported that starting 24 hours after coronary occlusion, dipyridamole increased collateral blood flow to the infarcted myocardium. They used rasxenon clearance as a measure of blood flow to the ischemic muscle, and the possibility that increased xenon clearance is due to a large overlap of nutrient vessels from the occluded and adjacent unoccluded arteries at the margins of the ischemic area has not been eliminated. REFERENCES 1. GUYTON, A. C., Ross, J. M., CARRIER, O., JR. and WALKER, J. R. Evidence for tissue oxygen demand as the major factor causing autoregulation. Circulation Res., 15 (Suppl. I) :60, 1964. 2. BERNE, R. M., BLACKMON,J. R. and GARDNER,T. H. Hypoxemia and coronary blood flow. I. CIin. Invest., 36:1101, 1957. 3. Ross, J.. JR., KAISER,G. A. and KLOCKE, F. J. Observations on the role of diminished oxygen tension in the functional hyperemia of skeletal muscle. Circulation Res., 15:473, 1964. 4. H~TON, R. and EICHHOLTZ,F. Influence of chemical factors on the coronary circulation. J. Physiol., 59:413, 1925. 5. KJELLMFR, I. The potassium ion as a vasodiIator during muscular exercise. Acta physiol. scandinav., 63:460, 1965. 6. SKINNER,N. S. and POWELL, W. J., JR. Action of oxygen and potassium on vascular resistance of THE AMERICAN JOURNAL OF CARDIOLOGY
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dog skeletal muscle. Am. J. Physiol., 212:5X$ 19G7. DRISCOL, T. E. and BERRE, R. M. Role of potassium in regulation of coronary blood flow. Proc. Sot. Exper. Biol. S Med., 96:505, 1957. BERNE, R. M. Cardiac nucleotides in hypoxia: possible role in regulation of coronary blood flow. Am. J. Physiol., 204:317, 1963. KATORI, M. and BERNE, R. M. Release of adenosine from anoxic heart-relationship to coronary flow. Circulation Res., 19:420, 1966. Rumo, R., BERNE, R. M. and KATORI, M. Release of adenosine in reactive hyperemia of the dog heart. Am. J. Physiol., 216:56, 1969. ROSTCAARD, J. and BEHNKE, 0. Fine structure localization of adenine nucleoside phosphatase activity in the sarcoplasmic reticulum and T system of rat myocardium. J. Ultrastruct. Res., 12:579, 1965. Development of interarterial ECKSTEIN, R. W. coronary anastomoses by chronic anemia. Disappearance following correction of anemia. Circulation Res., 3:306, 1955. ECKSTEIN, R. W. Effect of exercise and coronary artery narrowing on coronary collateral circulation. Circulation Res., 5:230, 1957. BAROLDI, G., MANTERO, 0. and SCOMAZZONI, G. The collaterals of the coronary arteries in normal and pathologic hearts. Circulation Res., 4:223, 1956. BERNE, R. M., JONES, R. D. and CROSS, F. S. Evaluation of procedures designed to enhance coronary collateral blood flow. Circulation Res., 10: 142, 1962.
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16. MACLEAN, L. D., HEDENSTRO>~, I’. H. .~ntl R_415E~, R. R. Tissue blood flow to the heart-influence of coronary occlusion and surgical measures. Circulation Res., 10:45, 1962. 17. ELI.IOT, E. C., JONES, E. L., BLOOR, C. M.: LEON, 4. S. and GREGG, D. E. Day-to-day changes in coronary hemodynamics secondary to constriction of circumflex branch of left coronary arter) in conscious dogs. Circulation Res., 22:237, 1068. 18. KHOURI, E. M., GREGG, D. E. and LOWENSOHN. H. S. Flow in the major branches of the left coronary artery during experimental coronary insufficiency in the unanesthetized dog. Circulafion Res., 23: 99, 1968. 19. WIGGERS, C. J. and GREEN, H. D. The ineffectiveness of drugs upon collateral flow after experimental coronary occlusion in dogs. .4?~. HutlIt I., 11:527, 1936. 20. KATNJS, A. A. and GREGG, D. E. Some determinants of coronary collateral blood flow in the open-chest dog. Circulation Res., 7:628. 1959. 21. JOHANSSON, B., LINDER, E. and SEEMAN, 1‘. Effects of heart rate and arterial blood pressure on coronary collateral blood flow in dogs. .4cta plrysio[. srandinav., 68 (Suppl. 272) :33, 1966. 22. F.~M, W. M. and MCGREGOR, M. Effect of coronary vasodilator drugs on retrograde flow in areas of chronic myocardial ischemia. Circlrlation Res., 15:355, 1964. 23. REES, J. R. and REDDINC, V. J. Effects of dipyridamole on anastomotic blood flow in experimental myocardial infarction. Cardiovas. Rrs., 1:li9. 1967.