Effect of Na+/H+ exchange inhibitors on agonist-induced contraction of rat aorta

A number of vasoconstrictor agonists activate the Na+/H+ antiport system in vascular smooth muscle, leading to alkalinization of the cytosol and influx of Na+. It is believed that agonist-induced Na+/H+ exchange may play an important role in contraction. We have evaluated this hypothesis by determining the effect of inhibition of Na+/H+ exchange on phenylephrine (PE)-induced contraction of rat aorta. Preincubation of rat aorta with the Na+/H+ exchange inhibitor amiloride (0.5 mM) inhibited subsequent PE-induced contraction. However, the more potent and specific Na+/H+ exchange inhibitor hexamethylene amiloride (10 microM) did not inhibit PE-induced contraction. Inhibition of Na+/H+ exchange by removal of extracellular Na+ and substitution with N-methyl-D-glucamine only moderately reduced PE-induced contraction. Hexamethylene amiloride (10 microM) and methyl-isobutyl amiloride (30 microM), another potent and specific inhibitor of Na+/H+ exchange, caused a slow, sustained and reversible contraction of rat aorta which was 114.7 and 86.6%, respectively, of maximal PE-induced contraction. Hexamethylene amiloride-induced contraction was dose-dependent, dependent on extracellular calcium and inhibited by nifedipine. We conclude that the vasorelaxant effects of amiloride are unrelated to inhibition of Na+/H+ exchange, and may be mediated by inhibition of kinases involved in contraction. These results demonstrate that inhibition of Na+/H+ exchange has no major effect on agonist-induced contraction of rat aorta, and suggest that vasoconstrictor activation of Na+/H+ exchange does not play a major role in agonist-induced contraction of rat aorta.