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1. The effects of Ca2+ deprivation upon mechanical and electrophysiological parameters of single muscle fibres from the m. semitendinosus and the m. iliofibularis of the frog were investigated. 2. When the external free Ca concentration was reduced in steps of one order of magnitude from 10(-3) to 10(-9) M, using up to 10 mM-EGTA and in the presence of 3 mM-Mg2+, the maximum force of K contractures declined by 5-15%, the plateau of maximum force shortened, and in most cases the phase of spontaneous relaxation lengthened. 3. In Ringer solution containing 10(-9) M-Ca2+ and 1 mM-Mg2+ 85% of maximum tetanic force could be maintained for at least 15 sec (5 Hz; 3 degrees C). 4. The reduction of external Ca2+ from 10(-3) to 10(-9) M and its replacement by Mg2+ induced a 20-30 mV shift towards more negative potentials of the 'steady state' inactivation curve (which relates maximum force upon full depolarization to the logarithm of the K concentration or the corresponding membrane potential during the conditioning period). 5. The same alteration in concentrations of divalent cations caused little or no change in the shape and potential dependence of the activation curve (which relates maximum force to the logarithm of the external K concentration of the corresponding membrane potential). 6. The threshold potential for the onset of delayed rectification (point voltage clamp) and that for the initiation of an action potential did not change when external Ca2+ was reduced to 10(-9) M and replaced by Mg2+. 7. When the concentration of EGTA2- was increased to 80 mM (in the presence of 5 mM-Mg2+) twitch height dropped to very small values within a few minutes. However, tetanic force (50 Hz) reaching 20-85% of the original value could still be induced after 1 hr in high EGTA2-. 8. The experiments show that external Ca2+ acts upon excitation-contraction coupling mainly by impeding 'inactivation'. A hypothesis is proposed in which the plateau of maximum force during a contracture is the consequence of a regenerative Cai2+-dependent shift of the inactivation curve towards more positive potentials.

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