Tissue contents of the sarcoplasmic-reticulum Ca2+-ATPase (Ca2+ +Mg2+-dependent ATPase), of calsequestrin and of parvalbumin were immunochemically quantified in homogenates of fast- and slow-twitch muscles of embryonic, maturing and adult rabbits. Unlike parvalbumin, Ca2+-ATPase and calsequestrin were expressed in embryonic muscles. Presumptive fast-twitch muscles displayed higher contents of these two proteins than did presumptive slow-twitch muscles. Calsequestrin steeply increased before birth and reached adult values in the two muscle types 4 days after birth. The main increase in Ca2+-ATPase occurred during the first 2 weeks after birth. Denervation of postnatal fast- and slow-twitch muscles decreased calsequestrin to amounts typical of embryonic muscle and suppressed further increases of Ca2+-ATPase. Denervation caused slight decreases in Ca2+-ATPase in adult fast-twitch, but not in slow-twitch, muscles, whereas calsequestrin was greatly decreased in both. Chronic low-frequency stimulation induced a rapid decrease in parvalbumin in fast-twitch muscle, which was preceded by a drastic decrease in the amount of its polyadenylated RNA translatable in vitro. Tissue amounts of Ca2+-ATPase and calsequestrin were essentially unaltered up to periods of 52 days stimulation. These results indicate that in fast- and slow-twitch muscles different basal amounts of Ca2+-ATPase and calsequestrin are expressed independent of innervation, but that neuromuscular activity has a modulatory effect. Conversely, the expression of parvalbumin is greatly enhanced by phasic, and drastically decreased by tonic, motor-neuron activity.

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