ISCHEMIA-INDUCED ALTERATIONS IN SARCOPLASMIC-RETICULUM CA2-ATPASE ACTIVITY IN RAT SOLEUS AND EDL MUSCLES()

To investigate the time-dependent effects of ischemia, as modified by muscle fiber type composition, on sarcoplasmic reticulum (SR) function , Ca2+-ATPase activity (total minus basal) was measured in homogenates prepared from samples obtained from rat soleus and extensor digitorum longus (EDL) muscle of ischemic and contralateral controls. Ischemia was induced by occlusion of blood flow to one hindlimb for periods of 1, 2, and 3 h (n = 10 per group). In EDL, maximal Ca2+-ATPase activity (expressed in mu mol . g wet wt(-1) min(-1)) was higher (P < 0.05) in ischemic than in control at 1 h (80 +/- 10 vs. 56.5 +/- 5.3) and incr eased progressively with ischemia at both 2 h (88 +/- 4.6 vs. 53.1 +/- 2.8) and 3 h (116 +/- 3.8 vs. 67.8 +/- 3.2). In contrast, in soleus, increases (P < 0.05) in Ca2+-ATPase activity with ischemia were observ ed at 2 h (19.2 +/- 0.86 vs. 14.0 +/- 0.56) and 3 h (19.9 +/- 1.4 vs. 12.4 +/- 0.62) but not at 1 h (10.7 +/- 1.5 vs. 10.0 +/- 0.83). In bot h EDL and soleus, basal Mg2+-ATPase was unchanged with ischemia. On th e basis of these findings, it can be concluded that ischemia results i n an increase in the maximal SR Ca2+-ATPase activity but that the time course of the change is dependent on the fiber type composition of th e muscle.