In this work, we tested whether L-type Ca2+ channels are involved in the increase of caffeine-evoked tension in frog slow muscle fibers. Simultaneous net Ca2+ fluxes and changes in muscle tension were measured in the presence of caffeine. Isometric tension was recorded by a mechanoelectrical transducer, and net fluxes of Ca2+ were measured noninvasively using ion-selective vibrating microelectrodes. We show that the timing of changes in net fluxes and muscle tension coincided, suggesting interdependence of the two processes. The effects of Ca2+ channel blockers (verapamil and gadolinium) were explored using 6 mm caffeine; both significantly reduced the action of caffeine on tension and on calcium fluxes. Both caffeine-evoked Ca2+ leak and muscle tension were reduced by 75% in the presence of 100 μm GdCl3, which also caused a 92% inhibition of net Ca2+ fluxes in the steady-state condition. Application of 10 μm verapamil to the bath led to 30% and 52% reductions in the Ca2+ leak caused by the presence of caffeine for the peak and steady-state values of net Ca2+ fluxes, respectively. Verapamil (10 μm) caused a 30% reduction in the maximum values of caffeine-evoked muscle tension. Gd3+ was a more potent inhibitor than verapamil. In conclusion, L-type Ca2+ channels appear to play the initial role of trigger in the rather complex mechanism of slow fiber contraction, the latter process being mediated by both positive Ca2+-induced Ca2+ release and negative (Ca2+ removal from cytosol) feedback loops.
The Journal of Membrane Biology – Springer Journals
Published: Nov 25, 2007
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