Phorbol ester TPA inhibits the stimulation of bumetanide‐sensitive Na+/K+/Cl– transporter by different mitogens in quiescent BALB/c 3T3 mouse fibroblasts

In this study we examined the effect of the phorbol ester 12‐O‐tetradecanoyl‐phorbol‐13‐acetate (TPA) on the bumetanide‐sensitive Na+/K+/Cl– transporter in quiescent BALB/c 3T3 cells. We have shown that exposure of quiescent BALB/c 3T3 cultures to phorbol ester did not inhibit the basal bumetanide‐sensitive Rb+ influx or efflux. In fact, at high concentration (100 ng/ml), TPA slightly stimulated the bumetanide‐sensitive Rb+ influx and efflux. However, when the quiescent cultures were stimulated by serum or by defined growth factors, the stimulated fraction of the bumetanide‐sensitive Rb+ influx was drastically inhibited by exposure of the cells to the phorbol ester TPA. Based on the above findings, we propose that activation of protein kinase C by the phorbol ester TPA does not inhibit the Na+/K+/Cl– cotransport activity; however it does suppress only the growth‐factors‐stimulated fraction of the cotransport in quiescent BALB/c 3T3 cells. These data propose that activation of kinase C has a regulatory feedback effect on the stimulation of the Na+/K+/Cl– cotransport activity by growth factors.