title: 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)
authors.affiliation: Department of Physiology & Pharmacology, Wake Forest University Medical Center, Winston-Salem, North Carolina.
authors.forename: F A
authors.initials: FA
authors.lastname: Gesek
authors.forename: E J
authors.initials: EJ
authors.lastname: Cragoe
authors.forename: J W
authors.initials: JW
authors.lastname: Strandhoy
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D004357:Drug Synergism; D066298:In Vitro Techniques; D007474:Ion Exchange / Q000187:drug effects; D007687:Kidney Tubules, Proximal / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*; D012979:Sodium Radioisotopes; D017923:Sodium-Hydrogen Exchangers
chemical_list: D000316:Adrenergic alpha-Agonists; D002352:Carrier Proteins; D011942:Receptors, Adrenergic, alpha; D012979:Sodium Radioisotopes; D017923:Sodium-Hydrogen Exchangers; C039465:5-dimethylamiloride; D000584:Amiloride