α₁-Adrenoceptor and purinoceptor agonists modulate Na-H antiport in single cardiac cells

We investigated the effects of an α₁-adrenoceptor (phenylephrine) and a purinoceptor agonist (ATP), both of which accelerate the phosphoinositide turnover, on the Na-H antiport activity of rat single cardiac cells using the pH-sensitive fluorescent indicator seminaphthorhodafluor-1 (SNARF-1). Both phenylephrine, in the presence of a β-adrenoceptor blocker, and ATP enhanced the ability of the cell to regulate its intracellular pH (pH(i)) after an imposed acid load. This effect was observed in HCO₃-free N-2- hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) and prevented by Na- H antiport inhibitors ethylisopropylamiloride (EIPA) or amiloride. Similar results were obtained when cells were bathed in an acidic extracellular medium. Hence, the α₁-adrenoceptor and purinoceptor agonists activate the Na-H antiport even when it is partially inhibited by extracellular protons. To further evaluate the effects of the two neurohormones, the rate of proton efflux was estimated as a function of the magnitude of the imposed acid load. The results indicate that the agonist-induced modulation of the Na-H antiport is caused by an acceleration of its exchange activity and by a shift of its dependence on pH(i) toward more alkaline pH values. The agonist-mediated stimulation of the antiport was also observed in partially depolarized cells and was not dependent on intracellular Ca. Phorbol 12-myristate 13-acetate was not able to reproduce the effects of the agonists on the Na-H antiport. Conversely, the inhibitors of protein kinase C did not prevent the activation of the antiport by the neurohormones. Thus our data suggest that neither a Ca-calmodulin-dependent kinase nor protein kinase C is responsible for the α₁-adrenoceptor- and purinoceptor-mediated stimulation of the antiport.