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; V₂, coupled to adenylate cyclase, and V₁, 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⁺ (Ca²⁺_i) and total inositol trisphosphate. The increase in I_eq was dependent on the rise in (Ca²⁺_i), 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 V₁ receptor and a Ca²⁺- dependent mechanism.