Effect of amiloride on regulatory mechanisms of vascular smooth muscle contraction

Experiments were conducted to characterize the effects of amiloride on the regulatory mechanisms of vascular smooth muscle contraction. Intact, saponin-skinned and A23187-treated strips of rabbit aorta were used for these studies. Amiloride significantly (P less than 0.05) reduced the norepinephrine bitartrate (NE)-stimulated increase in intracellular Ca2+ in intact arteries. In saponin-skinned arteries, amiloride depressed both stress and concomitant levels of myosin light chain phosphorylation. This inhibition of stress appeared to be competitive with MgATP. In A23187-treated preparations, where the effects of amiloride were studied at physiological [MgATP] in the absence of functional membrane Ca2+-channels, amiloride caused a reduction in both stress and myosin light chain phosphorylation. In other experiments on intact arteries, the contractile response to phorbol 12,13-dibutyrate, an activator of protein kinase C, was reduced by amiloride. We conclude that the vasorelaxant effects of amiloride are mediated via inhibition of myosin light chain kinase and protein kinase C, in addition to the inhibition of Ca2+ influx.