EFFECTS OF ECCENTRIC CONTRACTIONS ON IN VITRO Na super(+)-K super(+)-ATPase ACTIVITY AND SARCOPLASMIC RETICULUM Ca super(2+)-SEQUESTERING IN RAT SKELETAL MUSCLE

The purpose of this study was to examine the effects of eccentric contractions (Ecc) on cation (i.e., K super(+), Na super(+) and Ca super(2+)) regulation in skeletal muscle. The left anterior crural muscles of male Wistar rats were subjected in vivo to either Ecc or isometric contrations (Iso) for 200 cycles. The extensor digitorum longus and tibialis anterior muscles were removed immediately after and 2, 4 and 6 days following contractions and used for measures of force output and biochemical analyses, respectively. Ecc led to a 75% decrease in maximal tetanic force. Decreased force output did not revert to pre-exercise levels during 6 days of recovery. Sarcoplasmic reticulum (SR) Ca super(2+)-ATPase activity was reduced by 52 and 60% 4 and 6 days after Ecc, respectively. The reduction in catalytic activity after 6 days was accompanied by a 63% decrease in SR Ca super(2+)-ATPase protein and an approximately 3.5-fold increase in calpain activity. Na super(+)-K super(+)-ATPase acticity was decreased by 23% immediately after Ecc and restored during 2 days of recovery. These alterations were specific for Ecc and not observed for Iso. These results suggest that disturbances in cation regulation may account, at least partly, for Ecc-induced decreases in force and power which can take a number of days to recover and that the decrease in SR Ca super(2+)-ATPase activity would result from the degradation of the enzyme.