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Am J Physiol Regul Integr Comp Physiol 278: R87-R94, 2000;
0363-6119/00 $5.00
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Vol. 278, Issue 1, R87-R94, January 2000

Postcontractile force depression in humans is associated with an impairment in SR Ca2+ pump function

R. Tupling, H. Green, S. Grant, M. Burnett, and D. Ranney

Department of Kinesiology, University of Waterloo, Waterloo Ontario, Canada N2L 3G1

To investigate the hypothesis that intrinsic changes in sarcoplasmic reticulum (SR) Ca2+-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 Ca2+ uptake and maximal Ca2+-ATPase activity were made on homogenates obtained before and after the induction of PCD. Eight untrained females, age 20.6 ± 0.75 yr (mean ± 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 Ca2+-ATPase activity (nmol · mg protein-1 · min-1) and maximal SR Ca2+ uptake (nmol · mg protein-1 · min-1) were depressed (P < 0.05) by 15 min of exercise [192 ± 45 vs. 114 ± 8.7 and 310 ± 59 vs. 205 ± 47, respectively; mean ± 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 Ca2+-ATPase and Ca2+ uptake was preserved throughout exercise and during recovery. These results illustrate that during PCD, Ca2+ uptake is depressed and that the reduction in Ca2+ uptake is due to intrinsic alterations in the Ca2+ pump. The role of altered Ca2+ sequestration in Ca2 release, cytosolic-free calcium, and PCD remains to be determined.

isometric exercise; muscle; excitation-contraction coupling


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