K-Cl cotransport in rabbit red cells: further evidence for regulation by protein phosphatase type 1.

We have examined inhibition of swelling-induced K-Cl cotransport in rabbit red blood cells by calyculin A, a potent serine-threonine protein phosphatase inhibitor, to determine whether transport is regulated by phosphatase type 1 or type 2A. Calyculin A blocks K(Rb) influx [half-maximal inhibitory concentration (IC50) = 3-6 nM] 10 times more potently than a second phosphatase inhibitor, okadaic acid (IC50 = 40 nM), consistent with earlier pharmacological studies showing that calyculin A inhibits phosphatase type 1 10 times more effectively than does okadaic acid. Calyculin A always inhibits Rb influx when added either before or after cell swelling, indicating that the phosphatase must operate continually to first activate and then maintain high transport rates in swollen cells. Similarly, N-ethylmaleimide (NEM) fails to stimulate K-Cl cotransport only when added to cells pretreated with calyculin A. Therefore, like cell swelling, activation of K-Cl cotransport by NEM involves a phosphatase sensitive to calyculin A. We conclude that cell swelling and NEM activate K-Cl cotransport via a net dephosphorylation that appears to involve protein phosphatase type 1.