Serine/threonine protein phosphatases and regulation of K-Cl cotransport in human erythrocytes.

Activation of K-Cl cotransport is associated with activation of membrane-bound serine/threonine protein phosphatases (S/T-PPases). We characterize red blood cell S/T-PPases and K-Cl cotransport activity regarding protein phosphatase inhibitors and response to changes in ionic strength and cell size. Protein phosphatase type 1 (PP1) activity is highly sensitive to calyculin A (CalA) but not to okadaic acid (OA). PP2A activity is highly sensitive to CalA and OA. CalA completely inhibits K-Cl cotransport activity, whereas OA partially inhibits K-Cl cotransport. Membrane PP1 and membrane PP2A activities are elevated in cells suspended in hypotonic solutions, where K-Cl cotransport is elevated. Increases in membrane PP1 activity (62 +/- 10% per 100 meq/l) result from decreases in intracellular ionic strength and correlate with increases in K-Cl cotransport activity (54 +/- 10% per 100 meq/l). Increases in membrane PP2A activity (270 +/- 77% per 100 mosM) result from volume increases and also correlate with increases in K-Cl cotransport activity (420 +/- 47% per 100 mosM). The characteristics of membrane-associated PP1 and PP2A are consistent with a role for both phosphatases in K-Cl cotransport activation in human erythrocytes.