title: The contribution of the sarcoplasmic reticulum Ca2+-transport ATPase to caffeine-induced Ca2+ transients of murine skinned skeletal muscle fibres.
abstract: The present study was carried out to investigate the contribution of the Ca2+-transport ATPase of the sarcoplasmic reticulum (SR) to caffeine-induced Ca2+ release in skinned skeletal muscle fibres. Chemically skinned fibres of balb-C-mouse EDL (extensor digitorum longus) were exposed for 1 min to a free Ca2+ concentration of 0.36 microM to load the SR with Ca2+. Release of Ca2+ from the SR was induced by 30 mM caffeine and recorded as an isometric force transient. For every preparation a pCa/force relationship was constructed, where pCa = -log10 [Ca2+]. In a new experimental approach, we used the pCa/force relationship to transform each force transient directly into a Ca2+ transient. The calculated Ca2+ transients were fitted by a double exponential function: Y0 + A1 . (-t/t1) + A2 . exp(t/t2), with A1 < 0 < A2, t1 < t2 and Y0, A1, A2 in micromolar. Ca2+ transients in the presence of the SR Ca2+-ATPase inhibitor cyclopiazonic acid (CPA) were compared to those obtained in the absence of the drug. We found that inhibition of the SR Ca2+-ATPase during caffeine-induced Ca2+ release causes an increase in the peak Ca2+ concentration in comparison to the control transients. Increasing CPA concentrations prolonged the time-to-peak in a dose-dependent manner, following a Hill curve with a half-maximal value of 6.5 +/- 3 microM CPA and a Hill slope of 1.1 +/- 0.2, saturating at 100 microM. The effects of CPA could be simulated by an extended three-compartment model representing the SR, the myofilament space and the external bathing solution. In terms of this model, the SR Ca2+-ATPase influences the Ca2+ gradient across the SR membrane in particular during the early stages of the Ca2+ transient, whereas the subsequent relaxation is governed by diffusional loss of Ca2+ into the bathing solution.
authors.affiliation: II. Institute of Physiology, University of Heidelberg, INF 326, D-69120 Heidelberg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Makabe
authors.forename: O
authors.initials: O
authors.lastname: Werner
authors.forename: R H
authors.initials: RH
authors.lastname: Fink
mesh_terms: D000818:Animals; D001692:Biological Transport; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D003198:Computer Simulation; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007211:Indoles / Q000008:administration & dosage / Q000494:pharmacology; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D018482:Muscle, Skeletal / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000201:enzymology*
chemical_list: D004791:Enzyme Inhibitors; D007211:Indoles; D002110:Caffeine; D000252:Calcium-Transporting ATPases; D002118:Calcium; C000543:cyclopiazonic acid