Hormone-stimulated Ca2+transport in rabbit kidney: multiple sites of inhibition by exogenous ATP

Abstract

Exogenous ATP markedly reduced 1-desamino-8-d-arginine vasopressin (dDAVP)-stimulated Ca²⁺transport and cAMP accumulation in primary cultures of rabbit connecting tubule and cortical collecting duct cells. Similarly, ATP inhibited the stimulatory effect of 8-bromo-cAMP. At first sight, this is in agreement with the “classic” concept that dDAVP exerts its stimulatory effect via cAMP. However, dDAVP-stimulated Ca²⁺transport was markedly reduced by the protein kinase C (PKC) inhibitor chelerythrine, reported previously to inhibit the cAMP-independent pathway responsible for parathyroid hormone-, [Arg⁸]vasopressin-, PGE₂-, and adenosine-stimulated Ca²⁺transport. Chelerythrine also inhibited the increase in Ca²⁺transport evoked by the cAMP-independent A₁receptor agonist N⁶-cyclopentyladenosine (CPA). Downregulation of phorbol ester-sensitive PKC isoforms by chronic phorbol ester treatment has been shown before to be without effect on hormone-stimulated Ca²⁺transport, indicating that the chelerythrine-inhibitable pathway consists of a phorbol ester-insensitive PKC isoform. Here, this maneuver did not affect ATP inhibition of dDAVP-stimulated Ca²⁺transport and cAMP formation, while abolishing ATP inhibition of CPA-stimulated Ca²⁺transport. These findings show that ATP acts via 1) a phorbol ester-sensitive PKC isoform to inhibit hormonal stimulation of Ca²⁺transport at the level of the chelerythrine-inhibitable pathway involving a phorbol ester-insensitive PKC isoform and 2) a phorbol ester-insensitive mechanism to inhibit V₂receptor-mediated concomitant activation of this pathway and adenylyl cyclase.