Ca‐ATPase isozyme expression in sarcoplasmic reticulum is altered by chronic stimulation of skeletal muscle

Abstract
Chronic stimulation of a predominantly fast skeletal muscle enhanced the expression of type I (slow muscle) Ca‐ATPase and suppressed the expression of the type II (fast muscle) Ca‐ATPase. Monoclonal antibodies IID8 and IIH11 against type I (slow) and type II (fast) isozymes respectively, were used to type the Ca‐ATPases of the isolated SR (sarcoplasmic reticulum) by Western blots, and the Ca‐ATPases of the muscle fibers by immunohistochemistry. Of the fibers from control muscles 80% stained for the type II isozyme and 20% for the type I isozyme. Following chronic stimulation all fibers stained for type I isozyme and none stained for type II isozyme. Ca‐ATPase isozyme distribution in isolated SR confirmed this effect of chronic stimulation. The calcium uptake activities of homogenates of stimulated muscles were 22% of the control muscles. The Ca‐ATPase and calcium‐uptake activities of the isolated SR from stimulated muscles were, respectively, 32 and 45% of the control muscles.