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

Arginine vasopressin has been shown to activate the Na+/H+ exchanger i n hepatocytes by calcium/calmodulin-dependent processes. Whether this activation also involves protein kinase C and is associated with chang es in the intracellular pH setpoint was investigated in this study. Ch anges 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 intrace llular pH in hepatocytes acid-loaded with sodium propionate. Arginine vasopressin, phorbol myristate acetate and thapsigargin stimulated int racellular pH recovery but did not increase basal intracellular pH. Ar ginine vasopressin and thapsigargin, but not phorbol myristol acetate, increased intracellular Ca++ concentration. The protein kinase C inhi bitors staurosporine and calphostin C inhibited arginine vasopressin- and phorbol myristol acetate-induced, but not thapsigargin-induced, in tracellular pH recovery. Neither staurosporine nor calphostin C affect ed arginine vasopressin- and thapsigargin-induced increases in intrace llular Ca++ concentration, and no inhibitor affected basal intracellul ar pH recovery. Arginine vasopressin, phorbol myristol acetate and tha psigargin increased intracellular pH dependency of intracellular pH re covery without affecting intracellular pH setpoint. These results indi cate that the activation of the Na+/H+ exchanger by arginine vasopress in 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.