Ca-ATPase activation and NaK-ATPase inhibition as a function of calcium concentration in human red cell membranes

Abstract

In vitro ATPase activities of human red cell membranes were examined as a function of [Ca ++] from 10 −7 to 10 −3 M. The ouabain-sensitive, Na +,K +-activated ATPase (NaK-ATPase) and the ouabain-insensitive, Ca ++-activated ATPase (Ca-ATPase) were both influenced by [Ca ++]. Ca-ATPase activation was half-maximal at approximately 6 × 10 −6 M [Ca ++]. Ca inhibited the NaK-ATPase, with half-maximal inhibition at approximately 10 −4 M. Over a wide range of [Ca ++], activation of Ca-ATPase was greater than inhibition of NaK-ATPase. Assuming that Ca inhibition of NaK-ATPase is related to the [Ca ++] at the inner membrane surface, the results are compatible with the suggestion that low intracellular [Ca ++] is a necessary condition for active NaK-transport in red cells.