THE CONTRIBUTION OF THE SARCOPLASMIC-RETICULUM CA2-INDUCED CA2+ TRANSIENTS OF MURINE SKINNED SKELETAL-MUSCLE FIBERS( TRANSPORT ATPASE TO CAFFEINE)

The present study was carried out to investigate the contribution of t he Ca2+-transport ATPase of the sarcoplasmic reticulum (SR) to caffein e-induced Ca2+ release in skinned skeletal muscle fibres. Chemically s kinned fibres of balb-C-mouse EDL (extensor digitorum longus) were exp osed for 1 min to a free Ca2+ concentration of 0.36 mu M to load the S R 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 = -log(10)[Ca2+]. I n a new experimental approach, we used the pCa/force relationship to t ransform each force transient directly into a Ca2+ transient. The calc ulated Ca2+ transients were fitted by a double exponential function: Y -0 + A(1) . exp (- t/t(1)) + A(2) . exp(t/t(2)), with A(1) < 0 < A(2), t(1) < t(2) and Y-0, A(1), A(2) in micromolar. Ca2+ transients in the presence of the SR Ca2+-ATPase inhibitor cyclopiazonic acid (CPA) wer e 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 th e control transients. Increasing CPA concentrations prolonged the time -to-peak in a dose-dependent manner, following a Hill curve with a hal f-maximal value of 6.5 +/- 3 mu M CPA and a Hill slope of 1.1 +/- 0.2, saturating at 100 mu M. The effects of CPA could be simulated by an e xtended three-compartment model representing the SR, the myofilament s pace and the external bathing solution. III terms of this model, the S R Ca2+-ATPase influences the Ca2+ gradient across the SR membrane in p articular during the early stages of the Ca2+ transient, whereas the s ubsequent relaxation is governed by diffusional loss of Ca2+ into the bathing solution.