MECHANISM OF ACTIVATION OF THE NA+/H+ EXCHANGER BY ARGININE-VASOPRESSIN IN HEPATOCYTES

Arginine vasopressin has been shown to activate the Na+/H+ exchanger in hepatocytes by calcium/calmodulin-dependent processes. Whether this activation also involves protein kinase C and is associated with changes in the intracellular pH setpoint was investigated in this study. Changes in pH(i) and intracellular Ca++ concentration were measured with the fluorescent probes BCECF and quin-2, respectively. Intracellular pH recovery rate was calculated from time-dependent changes in intracellular pH in hepatocytes acid-loaded with sodium propionate. Arginine vasopressin, phorbol myristate acetate and thapsigargin stimulated intracellular pH recovery but did not increase basal intracellular pH. Arginine vasopressin and thapsigargin, but not phorbol myristol acetate, increased intracellular Ca++ concentration. The protein kinase C inhibitors staurosporine and calphostin C inhibited arginine vasopressin- and phorbol myristol acetate-induced, but not thapsigargin-induced, intracellular pH recovery. Neither staurosporine nor calphostin C affected arginine vasopressin- and thapsigargin-induced increases in intracellular Ca++ concentration, and no inhibitor affected basal intracellular pH recovery. Arginine vasopressin, phorbol myristol acetate and thapsigargin increased intracellular pH dependency of intracellular pH recovery without affecting intracellular pH setpoint. These results indicate that the activation of the Na+/H+ exchanger by arginine vasopressin is mediated both by Ca++/calmodulin and protein kinase C and may be due to enhanced interaction of H+ with the internal modifier site of the exchanger.