Effects of arginine vasotocin and vasopressin receptor antagonists on Na⁺ and Cl^- transport in the isolated skin of two frog species, Hyla japonica and Rana nigromaculata

Physiological function of arginine vasotocin (AVT) and effect of receptor antagonists of vasopressin were electrophysiologically investigated on transepithelial transport of ions in the abdominal skin of Hyla japonica and Rana nigromaculata by means of the Ussing chamber system. Administrations of AVT and forskolin (adenylate cyclase activator) in the serosal side of normal Ringer's solution significantly increased transepithelial potential difference (PD) and short-circuit current (Isc) accounting for Na⁺ influx, mucosal to serosal direction, across the skin of H. japonica. In contrast, AVT administrations significantly decreased PD but not Isc on the skin of R. nigromaculata in a concentration-dependent manner ranging from 10^-11 to 10^-8 M. Administration of 10^-5 M forskolin also significantly decreased PD in normal and low Na⁺ Ringer's solution and in the presence of amiloride (Na⁺ channel blocker) on the mucosal side of normal Ringer's solution. On the other hand, forskolin significantly increased PD and Isc in the Cl^- free Ringer's solution. These results suggested that AVT and forskolin stimulated mainly Cl^- influx across the skin of R. nigromaculata. In two frog species, the AVT actions on ion transports were inhibited by pretreatment of OPC-31260 (a vasopressin V₂ receptor antagonist) but not OPC-21268 (a vasopressin V₁ receptor antagonist). These results suggested that AVT activates adenylate cyclase via V₂-like receptor and stimulates actively net Na⁺ and net Cl^- transports in the abdominal skin of H. japonica and R. nigromaculata, respectively.