Variations in Ca(2+)-mediated activation of Ca(2+)-ATPase and its associated inhibitor in erythrocyte membrane.

1. Two distinct patterns of Ca(2+)-mediated activation of Ca(2+)-ATPase were identified in calmodulin-depleted membranes. 2. In membranes showing no activation (type A), preincubation with micromolar concentration of cyclic AMP and ATP made possible stimulation of the enzyme while in membranes already exhibiting activation (type B), preincubation with cyclic AMP and ATP abolished the activation. 3. ATPase stimulation in type A membranes was suppressible by leupeptin. 4. Triton extractable inhibitor isolated from type A membranes was as active as that derived from type B membranes only after preincubating the membranes with cyclic AMP and ATP. 5. The inhibitor could be inactivated by alkaline phosphatase.