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We sought to compare the sensitivity of endocardial and epicardial microvessels to several important neurohumoral substances. Porcine endocardial microvessels (86-200 microns diam) from the anterior papillary muscle and epicardial microvessels of similar size from the left anterior descending distribution were studied in a pressurized state using an in vitro microvessel-imaging apparatus. Endothelium-dependent relaxations to bradykinin, ADP, A23187, and to the endothelium-independent vasodilator nitroprusside were identical between endocardial and epicardial microvessels. In contrast, the sensitivity of endocardial microvessels to adenosine was substantially greater than that of epicardial microvessels (ED50s of -6.59 +/- 0.05 vs. -5.66 +/- 0.11, P less than 0.001, endocardial vs. epicardial, respectively), although adenosine caused 100% relaxation of both groups of vessels at the highest concentrations. Adenosine vasorelaxation was not affected by inhibition of cyclooxygenase by indomethacin (1 microM) or depletion of guanosine 3',5'-cyclic monophosphate by LY 83583 (1 microM). Forskolin dilated both endocardial and epicardial vessels completely but was more potent in endocardial vessels. These data show that endocardial and epicardial microvessels exhibit similar sensitivity to most vasodilator agents. Endocardial microvessels, however, are more sensitive to both adenosine and forskolin. The enhanced responsiveness to adenosine may be related to adenosine 3',5'-cyclic monophosphate-mediated mechanisms and may have important implications regarding regulation of myocardial perfusion in deeper subendocardial layers.

Previous in vivo studies have shown that vasopressin, which releases the endothelium-derived relaxing factor and constricts coronary smooth muscle, produces augmented constriction of coronary microvessels perfused by mature collaterals. We hypothesized that chronic perfusion through collaterals produces endothelial dysfunction in the recipient vasculature. Mature collaterals were stimulated in mongrel dogs by the ameroid constrictor technique. After 3-6 months, rings of conduit vessels (obtuse marginals) were studied in organ chambers, and coronary microvessels (100-220 microns) were studied in a pressurized, no-flow state with a microvessel imaging apparatus. Eleven dogs were used as controls. Large vessels were preconstricted with prostaglandin F2 alpha to 30-70% of the maximum potassium chloride tension, and microvessels were preconstricted to 20-60% of the baseline diameter with the thromboxane mimetic U46619. Relaxations to the receptor-mediated agents acetylcholine and ADP were markedly impaired in collateral-dependent coronary microvessels, whereas relaxations to nitroglycerin were enhanced compared with microvessels from control dogs. Relaxation to the calcium ionophore A23187, which releases the endothelium-derived relaxing factor through nonreceptor-mediated mechanisms, were similar in control and ameroid microvessels. Constriction to vasopressin was augmented in collateral-dependent microvessels compared with controls. Responses to all agonists were similar between control and collateral-dependent large vascular rings. In conclusion, chronic perfusion through collateral vessels selectively impairs receptor-mediated endothelium-dependent relaxations and augments constriction to vasopressin in the coronary microcirculation. These findings may have important implications regarding neurohumoral regulation of perfusion to collateral-dependent myocardium.

Ischemia and reperfusion alter the reactivity of large coronary arteries, but the effect of ischemia and reperfusion on the coronary microcirculation has been less well defined. Elevated circulating levels of vasopressin are associated with cardiopulmonary bypass and numerous other clinical states in which vascular ischemia and reperfusion may occur. We examined the effects of ischemia with and without reperfusion on the responses to vasopressin of both large coronary arteries and coronary arterial microvessels. Studies were performed on vessels from control dogs (n = 8), dogs undergoing 1 hour of ischemia only (n = 8), and dogs undergoing 1 hour of ischemia followed with 1 hour of reperfusion (n = 9). Rings of proximal obtuse marginal coronary arteries distal to the site of circumflex coronary artery occlusion were studied in isolated organ chambers. Coronary microvessels (110 to 220 microns in diameter) were studied in a pressurized (20 mm Hg), no-flow state with a microvessel imaging apparatus and electronic dimension analyzer. Microvessels were preconstricted with the thromboxane A2 analog U46619. Responses of large vessel rings were studied in the nonpreconstricted state and after preconstriction with prostaglandin F2 alpha. Large vessel response to vasopressin was minimal and not altered by ischemia with or without reperfusion. In contrast, ischemia markedly affected the coronary microvascular response to vasopressin (10 to 1000 microU/ml). Control coronary microvessels constricted minimally to vasopressin (4% +/- 2% of the baseline diameter), while microvessels after either ischemia alone or ischemia followed by reperfusion constricted 22% +/- 5% and 21% +/- 3%, respectively (p less than 0.05 versus control for both). Hemoglobin, which inactivates the endothelium-derived relaxing factor, augmented microvascular constrictions to vasopressin in all groups to a similar extent. Relaxations to the endothelium-independent agent nitroglycerin were not altered by ischemia. Constrictions of the coronary microcirculation to vasopressin in conditions such as cardiopulmonary bypass or myocardial ischemia, in which circulating levels of vasopressin are increased, may predispose to persistent myocardial ischemia in the perioperative setting.

Hypercholesterolemia impairs endothelium-dependent relaxations to several platelet-derived substances in large vessels. The effect of hypercholesterolemia on the response of coronary microvessels to platelet products and thrombin is less well defined. Three groups of cynomolgus monkeys were studied: normal (n = 6), short-term hypercholesterolemic (8-11 weeks, n = 5), and long-term hypercholesterolemic (18-80 months, n = 6). Responses of coronary microvessels, 100-200 microns in diameter, to thrombin (0.1-10 units/ml) and the platelet products ADP (1 nM-100 microM), serotonin (1 nM-100 microM), and the thromboxane A2 analogue U46619 were studied using an in vitro microvessel imaging apparatus. Vessels were studied after preconstriction with thromboxane A2 analogue U46619 to evaluate both relaxations and constrictions to each agent. Concentrations of U46619 to attain preconstriction were much lower for the long-term group (16 +/- 19 nM) as compared with the control and short-term hypercholesterolemic groups (689 +/- 48 and 664 +/- 63 nM, respectively, p less than 0.01). Relaxations of long-term hypercholesterolemic vessels to ADP tended to be less than those of either control or short-term hypercholesterolemic vessels. Thrombin, when added to normal or short-term hypercholesterolemic vessels, caused identical relaxations but paradoxically caused constrictions in microvessels of long-term hypercholesterolemic monkeys (55 +/- 17% of KCl contraction, p less than 0.001 vs. other groups). Peak constrictions to serotonin were markedly enhanced in the long-term hypercholesterolemic group (59 +/- 7% of maximal KCl responses) compared with control and short-term hypercholesterolemic responses (28 +/- 8% and 32 +/- 5%, respectively, both p less than 0.05 vs. long-term hypercholesterolemia.(ABSTRACT TRUNCATED AT 250 WORDS)

This study evaluates the role of leukocyte depletion during initial reoxygenation with normal blood and blood cardioplegic reperfusates in limiting reperfusion damage.Twenty-eight dogs underwent 2 hours of ligation of the left anterior descending coronary artery. The initial reperfusate (37 degrees C) was delivered on total vented bypass to the left anterior descending artery by a calibrated pump via an internal mammary artery graft at 50 mm Hg for 20 minutes. Eight dogs received normal (normokalemic, nonenriched) blood reperfusion (leukocyte count 8000/mm3) and six were reperfused with leukocyte-depleted normal blood (leukocyte count less than 100/mm3). Of 14 dogs reperfused with substrate-enriched (hyperkalemic) blood cardioplegic solution, six received a cardioplegic solution with a leukocyte count less than 100/mm3.Leukocyte depletion of normal blood reduced reperfusion-induced arrhythmias from 63% to 17% (p less than 0.05). Coronary vascular resistance at initial reperfusion was low and remained low during substrate-enriched blood cardioplegic reperfusion with both normal and reduced leukocyte counts. In contrast, coronary vascular resistance rose 63% with normal blood reperfusion, and this increase was avoided by leukocyte depletion (2.6 versus 4.0 mm Hg x ml/min, p less than 0.05). Coronary vascular resistance after 20 minutes was, however, higher than that with blood cardioplegia with normal or decreased leukocyte counts. Negligible functional recovery followed reperfusion with normal blood and leukocyte-depleted blood (12% and 6% of control systolic shortening). In contrast, substantial segmental recovery followed blood cardioplegic reperfusion (73% systolic shortening, p less than 0.05) but was not improved by leukopheresis (81% systolic shortening). Leukocyte depletion of normal blood reperfusate reduced histochemical damage from 53% to 38% (p less than 0.05), but the least histochemical damage followed blood cardioplegic reperfusion with a normal or reduced leukocyte count (8% or 11%, p less than 0.05).These findings suggest an important role for leukocytes in reperfusion damage, but reperfusate leukocyte filtration alone is inferior to blood cardioplegic reperfusion. Leukocyte depletion of blood cardioplegic solutions seems unnecessary after only 2 hours of ischemia.

As a result of gradual coronary occlusion, coronary collaterals are stimulated to develop. This maturation process involves not only dilatation of the vessel, but the development of new vascular smooth muscle. Experiments have been performed to examine vasomotor characteristics of mature coronary collaterals from dogs 3 to 6 months following ameroid constrictor placement. Studies in Langendorff blood-perfused hearts have shown that transcollateral resistance does not change during either the administration of α1-or α2-adrenergic agonists. Isolated collateral vessels studied as rings in organ chambers do not constrict to either α1- or α2-adrenergic agonists. These studies show that mature collateral vessels are not likely to possess functioning α-adrenergic receptors. Subsequent experiments using a cover slip autoradiographic ligand-binding approach have demonstrated a population of β-adrenergic receptors on mature coronary collaterals. Studies of isolated collaterals have demonstrated β-adrenoceptor-mediated relaxation that appears due to a population of mixed β1- and β2-adrenergic receptors.

We examined the effects of ischemia with and without reperfusion on endothelium-dependent and -independent vascular relaxation in both conduit and resistance coronary arteries. Studies were performed on dogs under control conditions (n = 13) or after 1 hour of circumflex coronary artery occlusion with (n = 10) or without (n = 8) 1 hour of reperfusion. Rings of obtuse marginal branches of the left circumflex coronary artery (conduit arteries) were studied in organ chambers. Coronary microvessels (110-220-microns diameter) were studied in a pressurized state with an in vitro microvessel imaging apparatus. Relaxation was evaluated after preconstriction with prostaglandin F2 alpha and U46619 (a thromboxane A2 analogue) in conduit and resistance vessels, respectively. Conduit vessel function was not altered by ischemia with or without reperfusion. Endothelium-dependent microvascular relaxation was depressed in response to acetylcholine, ADP, and calcium ionophore A23187 after ischemia with reperfusion compared with control relaxation (ED50 as -log[M]: 6.0 +/- 0.2 [p less than 0.05], 5.1 +/- 0.4 [p less than 0.05], and 5.8 +/- 0.1 versus 6.8 +/- 0.2, 6.8 +/- 0.2, and 6.6 +/- 0.2, respectively). Ischemia without reperfusion modestly altered microvascular endothelium-dependent relaxation. Microvascular relaxation to nitroglycerin was not altered by ischemia with reperfusion. We conclude that 1) endothelium-dependent relaxation in large epicardial coronary arteries is relatively refractory to ischemia with or without reperfusion, 2) ischemia alone produces mild alterations of coronary microvascular reactivity, 3) ischemia followed by reperfusion produces a marked and selective impairment of endothelium-dependent responses in the coronary microcirculation.

As a result of gradual coronary occlusion, coronary collaterals are stimulated to develop. This maturation process involves not only dilatation of the vessel, but the development of new vascular smooth muscle. Experiments have been performed to examine vasomotor characteristics of mature coronary collaterals from dogs 3 to 6 months following ameroid constrictor placement. Studies in Langendorff blood-perfused hearts have shown that transcollateral resistance does not change during either the administration of alpha 1- or alpha 2-adrenergic agonists. Isolated collateral vessels studied as rings in organ chambers do not constrict to either alpha 1- or alpha 2-adrenergic agonists. These studies show that mature collateral vessels are not likely to possess functioning alpha-adrenergic receptors. Subsequent experiments using a cover slip autoradiographic ligand-binding approach have demonstrated a population of beta-adrenergic receptors on mature coronary collaterals. Studies of isolated collaterals have demonstrated beta-adrenoceptor-mediated relaxation that appears due to a population of mixed beta 1- and beta 2-adrenergic receptors. Subsequent studies have demonstrated that mature collateral vessels are hyperresponsive to the vasoconstrictor effects of vasopressin and that concentrations of vasopressin which may be encountered in pathophysiologic conditions can markedly attenuate coronary collateral perfusion. Finally, the microcirculation of the collateral-dependent myocardium develops endothelial cell dysfunction. This results in impaired endothelium-dependent relaxations to adenosine diphosphate and acetylcholine and enhanced vasoconstriction to vasopressin. These alterations of the coronary circulation may have important implications regarding neurohumoral regulation of myocardial perfusion in collateral-dependent myocardium.