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 ($) over bar 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-1 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 (mu 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 affect in 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.