ABSTRACT
Alteration of resting free intracellular [Ca2+] ([Ca2+]i) homeostasis has been implicated in the aetiology of skeletal muscle fibre injury following damaging pliometric (lengthening or 'eccentric') contractions. Quantitative measurements of resting [Ca2+]i in skeletal muscles following acute or long-term exercise involving such injurious contractions have not been performed. We tested the hypothesis that, following an acute bout of pliometric exercise, the maximum force production (Po) of isolated skeletal muscles would be significantly reduced and that this deficit in force would be accompanied by an elevation in resting skeletal muscle [Ca2+]i. Further, we tested whether long-term pliometric exercise training would protect skeletal muscles from contraction-induced injury. Adult male mice were randomly assigned to either, control, 24-hour, 48-hour, or trained groups. The 24-hour and 48-hour group animals were subjected to a single acute downhill treadmill running 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 14 week endurance training program consisting of a daily (5 days/week) downhill running session, under identical conditions to that of the acutely exercised groups. The sedentary control animals remained in their cages. For each animal, Po was determined in the fast-twitch EDL and slow-twitch soleus muscles from one hindlimb and quantitative measurements of [Ca2+]i were made in the contralateral muscles using fluorescence digital imaging microscopy in conjunction with Fura-2. Po was lower in the EDL and soleus muscles from the 48-hour group compared with the control group animals. Po was higher in the EDL muscles of the trained group compared with the 48-hour group. No significant difference in Po was detected in either muscle from the 24-hour or trained groups compared with muscles from control mice. In EDL muscles, [Ca2+]i was elevated in the 48-hour group compared with the control and trained 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 increased in muscles from the 48-hour group compared with the 24-hour group. Endurance downhill running training conferred protection to recruited skeletal muscles against the effects of an acute bout of repeated pliometric contractions, as evidenced by [Ca2+]i and Po values similar to muscles from unexercised control mice.
