Reversible inhibition of sarcoplasmic reticulum Ca‐ATPase by altered neuromuscular activity in rabbit fast‐twitch muscle

Abstract

A 50% decrease in both the initial rate and the total capacity of Ca2+ uptake by the sarcoplasmic reticulum (SR) occurred 2 days after the onset of chronic (10 Hz) nerve stimulation in rabbit fast‐twitch muscle. Prolonged stimulation (up to 28 days) did not lead to further decreases. This reduction, which was detected in muscle homogenates using a Ca2+‐sensitive electrode, was reversible after 6 days cessation of stimulation and was not accompanied by changes in the immunochemically (ELISA) determined tissue level or isozyme characteristics of the SR Ca2+‐ATPase protein. However, as measured in isolated SR, it correlated with a reduced specific activity of the Ca2+‐ATPase. Kinetic analyses demonstrated that affinities of the SR Ca2+‐ATPase towards Ca2+ and ATP were unaltered. Positive cooperativity for Ca2+ binding (h= 1.5) was maintained. However, a 50% decrease in Ca2+‐dependent phosphoprotein formation indicated the presence of inactive forms of Ca2+‐ATPase in stimulated muscle. The reduced phosphorylation of the enzyme was accompanied by an approximately 50% lowered binding of fluorescein isothiocyanate, a competitor at the ATP‐binding site. In view of the unaltered affinity for ATP, this finding suggests that active Ca2+‐ATPase molecules coexist in stimulated muscle with inactive enzyme molecules, the latter displaying altered properties at the nucleotide‐binding site. Copyright © 1987, Wiley Blackwell. All rights reserved