The interplay between platelet and vessel-wall mediators in coronary artery occlusion

Platelet-induced vasomotion of isolated canine coronary artery in the presence of halothane or isoflurane

To determine the effect of 1.5 MAC of two volatile anesthetics (halothane and isoflurane) on platelet-induced contraction of canine coronary artery, isolated, denuded coronary rings were suspended between two stirrups, placed into organ chambers filled with an oxygenated Krebs-Ringer solution, and connected to an isometric force transducer. Human platelets were obtained from healthy donors and introduced into the organ chambers in increasing amounts to reach 20,50 and, 70 x 10(9) platelets/L. The tension generated in both the control and anesthetic-treated rings was recorded; only halothane reduced the tension induced by platelet activation in the organ chambers. In some experiments, aliquots of Krebs-Ringer solution were taken to determine the amount of 5-HT and TB2 released by 70 x 10(9) human platelets in the presence and absence of the anesthetics. Only halothane reduced TA2 production by the activated platelets. Finally, the contractile response of isolated denuded canine coronary artery rings to U46619, a thromboxane analog, was measured in the presence and absence of the anesthetics. Neither halothane nor isoflurane attenuated the response. In another series of experiments, in vitro platelet aggregation was induced by epinephrine, collagen, ADP, or arachidonic acid in the presence or absence of 1.5 MAC isoflurane or halothane. Both anesthetics significantly reduced the aggregation.

Do deficiencies of endothelial derived relaxing factor contribute to myocardial stunning?

Myocardial stunning is a phenomenon in which ventricular function is depressed by prior ischemia and remains suboptimal when blood flow is resumed. It is also characterized by coronary vascular endothelial dysfunction that is related to the duration of ischemia and can result in protein leakage, myocardial hemorrhage, or increased coronary vascular resistance. It is thought that local vascular tone is regulated by endothelium derived relaxing factor (EDRF), a compound commonly believed to be identical to nitric oxide (NO). EDRF increases the effects of other vasodilators through the formation of cyclic guanosine monophosphate (cGMP) by guanylate cyclase. Patients with various cardiovascular disorders have been found to have dysfunction in the EDRF system, and EDRF paradoxically produces vasoconstriction in atherosclerotic arteries. Similarly, EDRF stimulation of vasodilation due to acetylcholine, calcium ionophores, or platelets appears to be reduced in coronary arteries that have been damaged by ischemia and reperfusion. By increasing EDRF production or inhibiting its breakdown, EDRF precursors such as L-arginine, the superoxide radical scavenger superoxide dismutase, nitroglycerin, and nitroprusside all cause vasodilation by increasing NO levels in the setting of myocardial ischemia. These therapies may also improve local "microvascular" function, thereby improving ventricular function.

Inhibition of platelet accumulation by beta 1-adrenoceptor blockade in the thoracic aorta of rabbits subjected to experimental sympathetic activation

Arterial platelet adhesion is an initiating event in the thrombo-embolic complications of atherosclerosis and may also accelerate the development rate of atherosclerotic lesions. Psychosocial stress has been shown to accelerate atherogenesis in animals, an effect probably mediated via beta-adrenoceptor activation. In view of the postulated roles of platelets and beta-adrenoceptor activation in atherosclerosis development, we decided to test whether beta blockade affects arterial platelet accumulation. We studied the accumulation of radioactivity from 111In-labelled platelets on the wall of the thoracic aorta of rabbits as a measure of platelet accumulation. During the exposure to the labelled platelets, the animals were also exposed to 3 hours of chloralose anesthesia. This is a reproducible model of experimental sympathetic activation, including beta-adrenoceptor activation, which we used to amplify possible effects of beta-blockade on platelet-vessel wall interaction. The effectiveness of the anesthesia in increasing sympathetic activity was verified by significant rises in mean arterial blood pressure (from 77 to 88 mmHg), heart rate (190 to 290 bpm), and plasma levels of norepinephrine (1.0 to 3.3 nM) and epinephrine (0.13 to 0.83 nM). In chloralose anesthetized rabbits, approximately 30 x 10(-9)% of the injected 111In accumulated in each square millimeter of intima at unbranched thoracic aorta. Platelet accumulation was significantly higher at arterial branching points, 70% higher at intercostal artery bifurcations, and 150% higher at coronary artery bifurcations than in unbranched aortic intima. Pretreatment with metoprolol in a dose resulting in "therapeutic" plasma levels significantly reduced platelet accumulation by 48% in unbranched aorta, 65% at intercostal, and 53% at coronary artery bifurcations.(ABSTRACT TRUNCATED AT 250 WORDS)