952-26 Myogenic Responses and Intrinsic Tone of Coronary Arterioles are Altered by Cardiopulmonary Bypass and Cardioplegia

Abstract

Cardiopulmonary bypass (CPB) and cardioplegia are associated with systemic hypotension and altered vascular responses. Myogenic properties and intrinsic tone of vascular smooth muscle are important mechanisms in the regulation of coronary blood flow and systemic vascular resistance. To examine if CPB and cardioplegic arrest alter myogenic reactivity and the intrinsic tone in the coronary microcirculation, pigs were placed on CPB. Selected hearts (n = 6) were arrested with a cold, hyperkalemic ([K + ] = 25 mM) crystalloid cardioplegic solution for 1 hour. In another group (n = 6), hearts were arrested and then reperfused with warm blood for 1 hour, or pigs were placed on CPB without cardioplegia (n = 6). Coronary arterioles were studied in a pressurized, no-flow state with video-microscopy. Myogenic reactivity was examined to stepwise increases in intraluminal pressure from 10 to 100 mmHg. The vessel diameter was normalized to the diameter at 50 mmHg after application of papaverine (10 -4 M). In vessels from non-instrumented control hearts (n = 6) and vessels in the CPB group, myogenic contraction was observed with pressures >40 mmHg. However, CPB significantly shifted the pressure-diameter relation upward (p < 005 vs control). suggesting a decrease in the intrinsic tone. Cardioplegic arrest, with or without reperfusion, decreased myogenic reactivity with an upward displacement of the pressure-diameter relation (both p < 0.05 vs control). Myogenic reactivity of the control vessel was not altered after mechanical denudation of the endothelium, or following pretreatment with N G -nitro-L-arginine or indomethacin. However, blockade of the ATP-sensitive potassium channel by glybenclamide significantly attenuated the cardioplegia-induced decrease in myogenic reactivity (p < 0.05). These results suggest that coronary microvascular myogenic reactivity and the intrinsic tone are reduced following hyperkalemic cardioplegia, and that CPB alone preserves myogenic reactivity but reduces the intrinsic tone of the vascular smooth muscle.