DEPRESSION OF CALCIUM TRANSIENTS AFTER EXPOSURE TO HIGH K-LOADED FROGTWITCH MUSCLE-FIBERS AND ITS REVERSAL BY EXOGENOUS MYOINOSITOL( SOLUTION IN LI+)

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 hig h K+ exposure, for either a short (15 min) or long (I h) time, the pos t-K+ calcium transients could gradually be restored to the level of th e pre-K+ ones, if the fibres were not loaded with Li+. In contrast, th e post-K+ calcium transients of Li+-loaded fibres could not fully reco ver, and were depressed in a Li+ concentration-dependent manner. The m ean 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 fi bres which had been loaded with Li+ in 20 mmol/L Li+ Ringer's solution . This depression could be prevented or partially reversed by exogenou s myo-inositol. More depression could be induced by 1 h high K+ exposu re, but the presence of exogenous myo-inositol could not clearly preve nt the post-K+ calcium transients from reduction. Assuming that high K + exposure caused a depletion of myo-inositol and probably other chang es in the metabolism of inositol phospholipids in Li+-loaded fibres, w e conclude that some metabolites of phosphoinositides may play modulat ion roles in excitation-contraction coupling in frog twitch muscle fib res.