AREGU January 47/1

Tupling, R., H. Green, S. Grant, M. Burnett, and D. Ranney. Postcontractile force depression in humans is associated with an impairment in SR Ca21 pump function. Am. J. Physiol. Regulatory Integrative Comp. Physiol. 278: R87– R94, 2000.—To investigate the hypothesis that intrinsic changes in sarcoplasmic reticulum (SR) Ca21-sequestration function can be implicated in postcontractile depression (PCD) of force in humans, muscle tissue was obtained from the vastus lateralis and determinations of maximal Ca21 uptake and maximal Ca21-ATPase activity were made on homogenates obtained before and after the induction of PCD. Eight untrained females, age 20.6 6 0.75 yr (mean 6 SE), performed a protocol consisting of 30 min of isometric exercise at 60% maximal voluntary contraction and at 50% duty cycle (5-s contraction and 5-s relaxation) to induce PCD. Muscle mechanical performance determined by evoked activation was measured before (0 min), during (15 and 30 min), and after (60 min) exercise. The fatiguing protocol resulted in a progressive reduction (P , 0.05) in evoked force, which by 30 min amounted to 52% for low frequency (10 Hz) and 20% for high frequency (100 Hz). No force restoration occurred at either 10 or 100 Hz during a 60-min recovery period. Maximal SR Ca21-ATPase activity (nmol ·mg protein21 ·min21) and maximal SR Ca21 uptake (nmol ·mg protein21 ·min21) were depressed (P , 0.05) by 15 min of exercise [192 6 45 vs. 114 6 8.7 and 310 6 59 vs. 205 6 47, respectively; mean 6 SE] and remained depressed at 30 min of exercise. No recovery in either measure was observed during the 60-min recovery period. The coupling ratio between Ca21-ATPase and Ca21 uptake was preserved throughout exercise and during recovery. These results illustrate that during PCD, Ca21 uptake is depressed and that the reduction in Ca21 uptake is due to intrinsic alterations in the Ca21 pump. The role of altered Ca21 sequestration in Ca2 release, cytosolic-free calcium, and PCD remains to be determined.