Failure of short term stimulation to reduce sarcoplasmic reticulum Ca2+-ATPase function in homogenates of rat gastrocnemius

To examine the effect of short term intense activity on sarcoplasmic reticulum (SR) Ca²⁺ sequestering function, the gastrocnemius (G) muscles of 11 anaesthetized male rats (weight, 411±8 g,X±SE) were activated using supramaximal, intermittent stimulation (one train of 0.2 msec impulses per sec of 100 msec at 100 Hz). Homogenates were obtained from stimulated white (WG-S) and red (RG-S) tissues, assayed for Ca²⁺ uptake and maximal Ca²⁺ ATPase activity and compared to contralateral controls (WG-C, RG-C). Calcium uptake (nmoles/mg protein/min) determined using Indo-l and at [Ca²⁺]_f concentrations between 300–400 nM was unaffected (p>0.05) by activity in both WG (6.14+0.43 vs 5.37+0.43) and RG (3.21+0.18 vs 3.07+0.20). Similarly, no effect (p>0.05) of contractile activity was found for maximal Ca²⁺ ATPase activity (mole/mg protein/min) determined spectrophotometrically in RG (0.276+0.03 vs 0.278+0.02). In WG, Ca²⁺ ATPase activity was 15% higher in WG-S compared to WG-C (0.412+0.03 vs 0.385+0.04). Repetitive stimulation resulted in a reduction in tetanic tension of 74% (p<0.05) by 2 min in the G muscle. By the end of the stimulation period, ATP concentration was reduced (p<0.05) by 57% in the WG and by 47% in the RG. These results indicate that the repeated generation of maximal tetanic force, at least for short term periods, need not adversely affectin vitro homogenate determination of Ca²⁺ sequestering function in spite of severe alterations in energy potential and that some other mechanism must be involved to explain the depression in Ca²⁺ uptake and Ca²⁺ ATPase activity previously noted with short term intense exercise.