Role of protein phosphatase in activation of KCl cotransport in human erythrocytes.

We investigated the effects of okadaic acid, a novel highly specific inhibitor of protein phosphatases on swelling-activated transport in human erythrocytes. Nanomolar concentrations of this compound inhibited swelling-activated K transport. Complete inhibition of this transport was observed at 1 microM. Analysis of the time course of activation of K transport upon swelling revealed that okadaic acid not only decreased the final steady-state flux but also markedly increased the time lag for activation. These results suggest that okadaic acid decreases the rate constant for the conversion of KCl transporters from the resting to the active form. Okadaic acid also inhibited N-ethylmaleimide (NEM)-stimulated K transport, and this inhibition was also observed when cells were first treated with NEM before exposure to okadaic acid. The latter finding suggests that NEM activation of KCl transport is reversible and that a later step for this activation may involve the net dephosphorylation of the KCl transport protein. These results provide the first evidence that activation of KCl cotransport in human erythrocytes is regulated by phosphoprotein phosphatase.