VASOPRESSIN-STIMULATED ELECTROGENIC SODIUM-TRANSPORT IN A6 CELLS IS LINKED TO A CA2+-MOBILIZING SIGNAL MECHANISM

Vasopressin is known to activate two types of cell surface receptors; V-2, coupled to adenylate cyclase, and V-1, linked to a Ca2+-dependent transduction system. We investigated whether arginine vasopressin (AVP) stimulation of electrogenic sodium transport in A6 cells, derived from Xenopus laevis, is mediated by activation of either one or both types of AVP-specific receptors. AVP caused a rapid increase in electrogenic sodium transport, reflected by the transepithelial potential difference (V-T) and equivalent short circuit current (I-eq) measurements. AVP also rapidly increased intracellular Ca2+ (Ca-i(2+)) and total inositol trisphosphate. The increase in I-eq was dependent on the rise in (Ca-i(2+)), because 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) dose-dependently inhibited the I-eq response. There was no evidence, however, that activation of adenylate cyclase mediated AVP-stimulated I-eq; transport was not inhibited after AVP-induced activation of adenylate cyclase was abolished by 2',5'-dideoxyadenosine or when cAMP-dependent protein kinase (PKA) activity was abolished by the specific PKA inhibitor IP20. Further studies showed that although both forskolin and 8-(4-chlorophenylthio)-cAMP stimulated I-eq, this occurred by mechanisms independent of PKA activation. These results indicate that AVP-stimulated Na+ transport is mediated by a V-1 receptor and a Ca2+-dependent mechanism.