Profound Inhibition of Myogenic Tone in Rat Cardiac Allografts Is Due to eNOS- and iNOS-Based Nitric Oxide and an Intrinsic Defect in Vascular Smooth Muscle Contraction

Abstract

Background —The physiological consequences of inducible NO synthase (iNOS) expression were studied in allograft coronary arteries by pressure myography. Methods and Results —Septal coronary arteries (diameter, 200.6±3.3 μm) were harvested from allograft and isograft hearts, and their myogenic properties were measured before and after iNOS and nonselective NOS inhibition with aminoguanidine (AG, 100 μmol/L) and N G -nitro- l -arginine methyl ester (L-NAME) (200 μmol/L). Fura 2 fluorescence microscopy was used to measure [Ca 2+ ] i in isolated endothelial cells. Monoclonal anti-iNOS immunostains demonstrated iNOS protein in day 2, 7, 14, and 28 allograft vessels, but only in day 2 isograft vessels. Myogenic tone was profoundly inhibited in allograft vessels from day 4 onward. In day 4 allograft vessels, these differences were abolished by L-NAME but not AG, suggesting greater basal release of eNOS-based NO from allograft endothelium. Fluorescence measurements confirmed elevation of [Ca 2+ ] i in day 4 allograft endothelium, providing a mechanism for enhanced eNOS activity. For days 7 to 28, AG potentiated myogenic tone in allograft but not isograft vessels, indicating that vasoactive iNOS-based NO was present. In mature vessels, constriction via agonist- and depolarization-mediated mechanisms showed parallel inhibition, suggesting an intrinsic defect in vascular smooth muscle cell contraction. Conclusions —Our data indicate that the profound inhibition of myogenic tone in allograft arteries involves direct vasodilation by eNOS- and iNOS-based NO, as well as an intrinsic defect in vascular smooth muscle contraction. The hemodynamic profile resulting from these changes in allograft resistance vessel function would favor movement of extracellular fluid from the intravascular space into the myocardial interstitium, resulting in edema, increased ventricular stiffness, and poor ventricular performance.