Abstract
1. The effects of the ionophores A23187 and X-537A on various muscle sarcoplasmic reticular and sarcolemmal enzyme activities were studied. 2. Both ionophores (A23187 more potent than X-537A) profoundly stimulated MgCaATPase (not MgATPase or CaATPase) and also stimulated MgCaϱ-nitrophenyl phosphatase and MgCaGTPase. A23187 stimulated sarcolemmal MgCaGTPase in part by increasing the turnover of the phosphoprotein enzyme intermediate. 3. The ionophores (A23187 more effective than X-537A) both blocked calcium influx into membrane vesicles and effected an instantaneous release of bound (uncomplexed) calcium. 4. X-537A inhibited cardiac sarcolemmal (Na +-K +)ATPase. 5. A23187 modestly inhibited adenylate cyclase activity (basal, epi, NaF-stimulated) while X-537A (low concentrations) modestly stimulated guanylate cyclase activity of these membranes.
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