Isolated human red blood cell membrane fragments (RBCMF) were found to take up Ca++ in the presence of ATP.1 This ATP-dependent Ca++ uptake by RBCMF appears to be the manifestation of an active Ca++ transport mechanism in the red cell membrane reported previously (Schatzmann, 1966; Lee and Shin, 1969). The influences of altering experimental conditions on Ca++-stimulated Mg++ ATPase (Ca++ ATPase) and Ca++ uptake of RBCMF were studied. It was found that pretreatment of RBCMF at 50°C abolished both Ca++ ATPase and Ca++ uptake. Pretreatment of RBCMF with phospholipases A and C decreased both Ca++ ATPase and Ca++ uptake, whereas pretreatment with phospholipase D did not significantly alter either Ca++ ATPase or Ca++ uptake. Both Ca++ ATPase and Ca++ uptake had ATP specificity, similar optimum pH's, and optimum incubation temperatures. From these results, it was concluded that Ca++ uptake is intimately linked to Ca++ ATPase.

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