title: Postcontractile force depression in humans is associated with an impairment in SR Ca(2+) pump function.
abstract: To investigate the hypothesis that intrinsic changes in sarcoplasmic reticulum (SR) Ca(2+)-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 Ca(2+) uptake and maximal Ca(2+)-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 Ca(2+)-ATPase activity (nmol x mg protein(-1) x min(-1)) and maximal SR Ca(2+) uptake (nmol. mg protein(-1) x 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 Ca(2+)-ATPase and Ca(2+) uptake was preserved throughout exercise and during recovery. These results illustrate that during PCD, Ca(2+) uptake is depressed and that the reduction in Ca(2+) uptake is due to intrinsic alterations in the Ca(2+) pump. The role of altered Ca(2+) sequestration in Ca(2) release, cytosolic-free calcium, and PCD remains to be determined.
authors.affiliation: Department of Kinesiology, University of Waterloo, Waterloo Ontario, Canada N2L 3G1.
authors.forename: R
authors.initials: R
authors.lastname: Tupling
authors.forename: H
authors.initials: H
authors.lastname: Green
authors.forename: S
authors.initials: S
authors.lastname: Grant
authors.forename: M
authors.initials: M
authors.lastname: Burnett
authors.forename: D
authors.initials: D
authors.lastname: Ranney
mesh_terms: D000328:Adult; D000252:Calcium-Transporting ATPases / Q000502:physiology*; D004558:Electric Stimulation; D015444:Exercise / Q000502:physiology; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology*; D018482:Muscle, Skeletal / Q000201:enzymology / Q000378:metabolism / Q000502:physiology*; D012519:Sarcoplasmic Reticulum / Q000201:enzymology* / Q000502:physiology
chemical_list: D000252:Calcium-Transporting ATPases