The accumulation of Ca++ by microsomal (sarcotubular) preparations of rabbit skeletal muscle in the presence of oxalate, and the concurrent splitting of nucleoside triphosphate, displayed moderate nucleotide specificity in the sequence ATP > GTP, CTP, ITP > UTP > (ADP) > ATetraP for the former, ATP > (ADP) > ITP > GTP > CTP > UTP > ATetraP for the latter process. The "calcium pump" was weakly inhibited by caffeine, and was inhibited together with the ATPase by pyridoxalphosphate. Carnosine had no effect as such nor in the presence of pyridoxalphosphate except at high concentration; thiourea and p-chloromercuribenzoate were inhibiting while iodoacetate was inactive. Ca++ accumulation and ATPase were inhibited by atabrine (not tested on ATPase), dinitrophenol, and amytal. High concentrations of oligomycin and rutamycin inhibited Ca++ uptake while slightly stimulating ATPase. Antimycin A stimulated the Ca++ uptake. These results are discussed in the light of their possible relation to partial reactions in oxidative phosphorylation. The Ca++ uptake and relaxing factor activities did not behave identically throughout. This is in part ascribed to changes in reactivity of actomyosin in the relaxation test, in part to the participation of relaxing substances other than the calcium pump.

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