Reductions in sarcoplasmic reticulum Ca<SUP>2+</SUP> ATPase activity in rat skeletal muscles of different fibre composition with ischemia and reperfusion
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To investigate the significance of fibre type and the duration of ischemia on changes in sarcoplasmic reticulum Ca2+ ATPase activity (SR Ca2+ ATPase), blood flow was occluded to the rat hind limb for 1, 2, or 3 h (n = 10 per group) and the soleus and extensor digitorum longus (EDL) muscles were examined following 2 h of reperfusion. When compared with the contralateral control muscles, calcium-dependent (total tau basal) SR Ca2+ ATPase activity in soleus was reduced (p < 0.05) to 75.9% by 1 h of ischemia and 2 h of reperfusion (13.1 +/- 0.6 vs. 9.95 +/- 0.85 mumol.mg-1 wet weight.min-1; X +/- SE) with no further reduction (p > 0.05) observed at either 2 h (9.75 +/- 0.57) or 3 h (9.40 +/- 0.64) of ischemia and 2 h of reperfusion. For the EDL muscles, SR Ca2+ ATPase activity with 2 h reperfusion was not reduced (p > 0.05) with 1 h of ischemia (80.4 +/- 3.0 vs. 70.7 +/- 2.9 mumol.mg-1 wet weight.min-1) but was reduced (66.7 +/- 2.3 mumol.mg-1 wet weight.min-1; p < 0.05) in the 2-h ischemia group, with further reductions (53.2 +/- 3.4 mumol.mg-1 wet weight.min-1; p < 0.05) in the 3-h ischemia group. No changes (p > 0.05) in basal or SR Mg2+ ATPase were found for either muscle group with ischemia and reperfusion, regardless of the duration of ischemia. When these results are interpreted in the context of the increases in SR Ca2+ ATPase activity that occur with ischemia, it appears that two components are involved in the reductions in SR Ca2+ ATPase activity noted during reperfusion: one that reduces the SR Ca2+ ATPase activity to below normal and one that simply reverses the ischemic-induced increase in SR Ca2+ ATPase activity. The former component appears to be more pronounced in the EDL muscle.
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