Time course of changes in in vitro sarcoplasmic reticulum Ca-handling and Na-K-ATPase activity during repetitive contractions Running head: Time-course changes in SR Ca-handling and Na-K-ATPase activity

Time-dependent changes in sarcoplasmic reticulum (SR) Ca-handling and Na-K-ATPase activity, as assessed in vitro, were investigated in the superficial (GS) and deep regions (GD) of rat gastrocnemius muscles undergoing short-term (up to 30 min) electrical stimulation. There was a rapid and progressive loss of force output during the first 5 min of stimulation. For GS, significant depressions (P < 0.05) in SR Ca-uptake rate and Ca-ATPase activity were observed during only the first 1 min. No further reductions occurred with stimulation time. SR Ca-release rate was significantly (P < 0.05) decreased at 3 min. For GD, significant reductions (P <0.05) in Ca-uptake rate, Ca-release rate and Ca-ATPase activity were manifested after 3, 5 and 5 min, respectively. A decay in Na-K-ATPase activity was found only in 1-min stimulated GD and 30-min stimulated GS. After 30 min, the depressed functions reverted to resting levels in GD but not in GS. The alterations in any variables examined were not parallel with changes in force output. These results suggest that, at least under the conditions used in this study, in vivo disruptions in cation regulation mediated by vigorous contractile activity would be attributable primarily to events other than structural alterations to the respective proteins.