Inhibition of cardiac sarcolemmal Na+/H+ antiporter by opioids.
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 kappa-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 microM) was threefold more potent than amiloride (IC50 = 300 +/- 20 microM) but it was three-fold less potent than the amiloride analog (IC50 = 30 +/- 10 microM) 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.
Periyasamy
S M
SM
Department of Pharmacology, Medical College of Ohio, Toledo 43699.
D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; D000584:Amiloride / Q000494:pharmacology; D000700:Analgesics / Q000494:pharmacology; D000818:Animals; D001575:Benzomorphans / Q000494:pharmacology; D002352:Carrier Proteins / Q000037:antagonists & inhibitors*; D002417:Cattle; D004723:Endorphins / Q000494:pharmacology*; D006321:Heart / Q000187:drug effects*; D066298:In Vitro Techniques; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D011759:Pyrrolidines / Q000494:pharmacology; D012508:Sarcolemma / Q000187:drug effects / Q000378:metabolism*; D017923:Sodium-Hydrogen Exchangers
D000700:Analgesics; D001575:Benzomorphans; D002352:Carrier Proteins; D004723:Endorphins; D009292:Narcotic Antagonists; D011759:Pyrrolidines; D017923:Sodium-Hydrogen Exchangers; D009270:Naloxone; C027247:MR 2266; D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; D000584:Amiloride