Calmidazolium and compound 48/80 inhibit calmodulin-dependent protein phosphorylation and ATP-dependent Ca2+ uptake but not Ca2+-ATPase activity in skeletal muscle sarcoplasmic reticulum.

Two specific calmodulin antagonists, compound 48/80 and calmidazolium , at concentrations of 10-20 micrograms/ml and 10-20 microM, respectively, inhibited Ca2+ uptake in skeletal muscle sarcoplasmic reticulum vesicles without affecting Ca2+-ATPase activity. The drugs also inhibited the calmodulin-dependent phosphorylation of 85,000-, 60,000-, and 20,000-dalton proteins, but not the calmodulin-independent phosphorylation of other sarcoplasmic reticulum proteins. The inhibition of phosphorylation of the 60,000-dalton protein closely paralleled the inhibition of Ca2+ uptake. Neither drug affected the passive permeability of the sarcoplasmic reticulum membrane at concentrations up to 5 times the inhibitory dose, and neither drug inhibited Ca2+ uptake into liposomes reconstituted with the purified Ca2+-ATPase. However, calmodulin-dependent reconstitution of Ca2+ uptake in EGTA-extracted sarcoplasmic reticulum vesicles was inhibited by 48/80. The results of this study suggest that the calmodulin-dependent phosphorylation system plays a functional role in the coupling of ATP hydrolysis and Ca2+ accumulation, perhaps through regulation of Ca2+ release channels in the sarcoplasmic reticulum membrane. Perturbation of phosphorylation by 48/80 and calmidazolium may lead to enhanced Ca2+ release, thereby diminishing Ca2+ accumulation without affecting the Ca2+ uptake mechanism.