Calcium modulates osmosensitive taurine efflux in HeLa cells.

The role of Ca2+ in the signaling transduction pathway involved in osmosensitive taurine efflux in HeLa cells was studied using radiotracer efflux techniques. Taurine efflux induced by extracellular hypotonicity was decreased by 85% by removal of extracellular Ca2+ and simultaneous depletion of intracellular Ca2+ stores with thapsigargin. Extracellular Ca2+ removal, thapsigargin treatment, or addition of Gd3+ all decreased taurine efflux by approximately 50%. To explore the putative signal transduction pathways involved in swelling-induced taurine efflux, HeLa cells were exposed to PP1, an inhibitor of the Src family of tyrosine kinases, the phospholipase C inhibitor U73122, the IP3 receptor antagonist 2-APB, and the generic protein kinase C inhibitor chelerythrine. All of these treatments caused approximately 50% inhibition of taurine release in Ca2+-rich extracellular medium and approximately 85%-90% in Ca2+-free conditions. The inhibitors of the conventional protein kinase C isoforms BIM-1 and Gö6976 reduced taurine efflux to a lesser extent. Acute (10-min) exposure to the phorbol ester tetradecanoyl phorbol acetate (TPA) increased taurine efflux in 25%, whilst overnight exposure had an inhibitory effect decreasing efflux by 22%. A working model for activation of osmosensitive taurine efflux in HeLa cells involving different Ca2+ signaling pathways is presented.