Synergistic alpha-1 and alpha-2 adrenergic stimulation of rat proximal nephron Na+/H+ exchange

Abstract

Both alpha-1 and alpha-2 adrenoceptors have been localized to the renal cortex, with the majority of binding sites on the proximal tubule. Because the major regulator of Na+ uptake into the proximal tubule is the Na+/H+ exchanger, and because alpha-1 and alpha-2 adrenoceptors stimulate it in other tissues, we tested the hypothesis that both alpha adrenoceptor subtypes can increase Na+ uptake into the proximal nephron by stimulating the Na+/H+ antiporter. Enhancement of Na+ transport by agonists was studied in isolated rat proximal tubules by determining the uptake of 22Na that was suppressible by the Na+/H+ inhibitor, 5-(N-ethyl-N-isopropyl)amiloride (EIPA). The phorbol ester, phorbol-12-myristate-13-acetate, (0.1 microM), directly stimulated the antiporter through protein kinase C and increased EIPA-suppressible 22Na uptake 250% above control. The alpha-1 adrenoceptor agonists, cirazoline and phenylephrine, in addition to the mixed agonist, norepinephrine, maximally stimulated uptake by 226 to 232% at 1 microM concentrations. alpha-2 agonists produced a range of maximal stimulations at 1 microM from 65% with guanabenz to 251% with B-HT 933. Increases in 22Na uptake by agonists were inhibited by selective adrenergic antagonists and by EIPA. The drugs did not change the EIPA-resistant component of 22Na uptake. Inasmuch as the adrenoceptor subtypes likely stimulated Na+/H+ exchange by differing intracellular pathways impinging upon common transport steps, we examined whether simultaneous stimulation of both pathways was additive. Submaximal concentrations (5 nM each) of alpha-1 and alpha-2 adrenoceptor agonists in combination synergistically enhanced 22Na uptake to a level similar to 1 microM concentrations of adrenoceptor agonists alone or in combination.(ABSTRACT TRUNCATED AT 250 WORDS)