QUANTITATIVE MEASUREMENT OF RESTING SKELETAL-MUSCLE [CA2-TERM DOWNHILL RUNNING EXERCISE IN MICE(](I) FOLLOWING ACUTE AND LONG)

Alteration of resting free intracellular [Ca2+] ([Ca2+](i)) homeostasi s has been implicated in the aetiology of skeletal muscle fibre injury following damaging pliometric (lengthening or 'eccentric') contractio ns. Quantitative measurements of resting [Ca2+](i) in skeletal muscles following acute or long-term exercise involving such injurious contra ctions have not been performed. We tested the hypothesis that, followi ng an acute bout of pliometric exercise, the maximum force production (P-o) of isolated skeletal muscles would be significantly reduced and that this deficit in force would be accompanied by an elevation in res ting skeletal muscle [Ca2+](i). Further, we tested whether long-term p liometric exercise training would protect skeletal muscles from contra ction-induced injury. Adult male mice were randomly assigned to either : control, 24-hour, 48-hour, or trained groups. The 24-hour and 48-hou r group animals were subjected to a single acute downhill treadmill ru nning bout (decline 16 degrees, at a rate of 13 m/min, for 60 min) and sacrificed at 24 or 48 h, respectively. Trained animals underwent a 1 4 week endurance training program consisting of a daily (5 days/week) downhill running session, under identical conditions to that of the ac utely exercised groups. The sedentary control animals remained in thei r cages. For each animal, P-o was determined in the fast-twitch EDL an d slow-twitch soleus muscles from one hindlimb and quantitative measur ements of [Ca2+](i) were made in the contralateral muscles using fluor escence digital imaging microscopy in conjunction with Fura-2. P-o was lower in the EDL and soleus muscles from the 48-hour group compared w ith the control group animals. P-o was higher in the EDL muscles of th e trained group compared with the 48-hour group. No significant differ ence in P-o was detected in either muscle from the 24-hour or trained groups compared with muscles from control mice. In EDL mouscles, [Ca2](i) was elevated in the 48-hour group compared with the control and t rained group animals, but was not different between the 24-hour group compared with control mice, [Ca2+](i) was not different in the soleus muscles among the 48-hour, trained or control group mice, but was incr eased in muscles from the 48-hour group compared with the 24-hour grou p. Endurance downhill running training conferred protection to recruit ed skeletal muscles against the effects of an acute bout of repeated p liometric contractions, as evidenced by [Ca2+](i) and P-o values simil ar to muscles from unexercised control mice.