title: Upregulation of Ca2+ removal in human skeletal muscle: a possible role for Ca2+-dependent priming of mitochondrial ATP synthesis.
abstract: In muscle, ATP is required for the powerstroke of the myosin head, the detachment of actin and myosin filaments, and the reuptake of Ca2+ into the sarcoplasmic reticulum. During contraction-relaxation, large amounts of ATP are consumed at the sites of action of the myosin-ATPase and sarcoplasmic reticulum Ca2+-ATPase. The present study addresses the consequences of a reduction in mitochondrial ATP production capacity on sarcoplasmic Ca2+ handling. To this end, myotubes were cultured from patient quadriceps with a biochemically defined decrease in the maximal rate of mitochondrial ATP production and were loaded with indo 1 for imaging of sarcoplasmic Ca2+ changes in real time by confocal microscopy. Myotubes were field-stimulated with 10-ms pulses of 16 V to evoke transient rises in sarcoplasmic Ca2+ concentration ([Ca2+]S). Three single pulses, two pulse trains (1 Hz), and one single pulse were applied in succession to mimic changing workloads. Control myotubes displayed [Ca2+]S transients with an amplitude that was independent of the strength of the stimulus. Intriguingly, the rate of sarcoplasmic Ca2+ removal (CRR) was significantly upregulated during the second and subsequent transients. In myotubes with a reduced mitochondrial ATP production capacity, the amplitude of the [Ca2+]S transients was markedly increased at higher stimulus intensities. Moreover, upregulation of the CRR was significantly decreased compared with control. Taken together, these results are in good agreement with a tight coupling between mitochondrial ATP production and sarcoplasmic Ca2+ handling. Moreover, they support the existence of a relatively long-lasting mitochondrial memory for sarcoplasmic [Ca2+] rises. This memory, which manifested itself as an increase in CRR upon recurrent stimulation, was impaired in patient myotubes with a reduced mitochondrial ATP production capacity.
authors.affiliation: 160 Biochemistry NCMLS, University Medical Center Nijmegen, PO Box 9101, NL-6500 HB Nijmegen, The Netherlands.
authors.forename: Werner J H
authors.initials: WJ
authors.lastname: Koopman
authors.forename: Michel
authors.initials: M
authors.lastname: Renders
authors.forename: Arie
authors.initials: A
authors.lastname: Oosterhof
authors.forename: Toin H
authors.initials: TH
authors.lastname: van Kuppevelt
authors.forename: Baziel G M
authors.initials: BG
authors.lastname: van Engelen
authors.forename: Peter H G M
authors.initials: PH
authors.lastname: Willems
mesh_terms: D000255:Adenosine Triphosphate / Q000096:biosynthesis*; D002118:Calcium / Q000502:physiology*; D004558:Electric Stimulation / Q000379:methods; D006801:Humans; D007700:Kinetics; D008931:Mitochondria, Muscle / Q000378:metabolism* / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000378:metabolism / Q000502:physiology; D018482:Muscle, Skeletal / Q000378:metabolism* / Q000502:physiology; D012519:Sarcoplasmic Reticulum / Q000378:metabolism / Q000502:physiology; D015854:Up-Regulation / Q000502:physiology*
chemical_list: D000255:Adenosine Triphosphate; D002118:Calcium