Inhibition of cardiac sarcolemmal Na+/H+ antiporter by opioids

Abstract

In our routine screening of chemicals that would inhibit cardiac sarcolemmal Na+/H+ antiporter, we discovered that some of the opioids produced inhibition of cardiac sarcolemmal Na+/H+ antiporter in micromolar concentrations. Using U-50,488H, a selective κ-opioid agonist, we characterized the nature of interaction between opioids and the Na+/H+ antiporter. The inhibitory effect of U-50,488H on Na+/H+ antiporter was immediate and reversible, and was not mediated through the interaction with the opioid receptors but due to the direct interaction of U-50.488H with the Na+/H+ antiporter. The kinetic data show that in the presence of U-50,488H the Km for Na+ was increased from 2.5 ± 0.2 to 5.0 ± 0.3 mM, while the Vmax (52.0 ± 5.0 nmol∙mg−1∙min−1) remained the same. These results suggest that U-50,488H and Na+ compete for the same site on the antiporter. When testing the effect of U-50,488H on other transport systems of cardiac sarcolemma, we found that U-50,488H also inhibited Na+/Ca2+ antiporter and Na+/K+ pump but at much higher concentrations suggesting that U-50,488H shows some degree of selectivity for cardiac sarcolemmal Na+/H+ antiporter. When we compared the inhibitory potency of U-50,488H with amiloride and its analog, namely 5-(N,N-hexamethylene)amiloride, we found that U-50,488H (IC50 = 100 ± 15 μM) was threefold more potent than amiloride (IC50 = 300 ± 20 μM) but it was threefold less potent than the amiloride analog (IC50 = 30 ± 10 μM) in inhibiting cardiac sarcolemmal Na+/H+ antiporter. These results show that although U-50,488H is more potent than amiloride, the inhibitory characteristics of U-50,488H on cardiac sarcolemmal Na+/H+ antiporter are similar to amiloride.Key words: Na+/H+ antiporter, Na+ uptake, cardiac sarcolemmal vesicles, opioid agonists, U-50,488H.