Depression of calcium transients after exposure to high K+ solution in Li(+)-loaded frog twitch muscle fibres and its reversal by exogenous myo-inositol.

Using Arsenazo III as a myoplasmic calcium indicator, we have studied the calcium transients evoked by voltage-clamp depolarizing pulses in frog twitch muscle fibres which had been temporarily depolarized by 80 mmol/L K+ in the absence or presence of myoplasmic Li+. After the high K+ exposure, for either a short (15 min) or long (1 h) time, the post-K+ calcium transients could gradually be restored to the level of the pre-K+ ones, if the fibres were not loaded with Li+. In contrast, the post-K+ calcium transients of Li(+)-loaded fibres could not fully recover, and were depressed in a Li+ concentration-dependent manner. The mean amplitude of the post-K+ responses recorded more than 3.5 h after 15 min high K+ exposure was reduced to 56% of pre-K+ control in the fibres which had been loaded with Li+ in 20 mmol/L Li+ Ringer's solution. This depression could be prevented or partially reversed by exogenous myo-inositol. More depression could be induced by 1 h high K+ exposure, but the presence of exogenous myo-inositol could not clearly prevent the post-K+ calcium transients from reduction. Assuming that high K+ exposure caused a depletion of myo-inositol and probably other changes in the metabolism of inositol phospholipids in Li(+)-loaded fibres, we conclude that some metabolites of phosphoinositides may play modulation roles in excitation-contraction coupling in frog twitch muscle fibres.