External calcium and contractile activation during potassium contractures in twitch muscle fibres of the frog.

Effects of external Ca2+ concentration reduction on the amplitude and time course of K+ contractures were studied in single muscle fibres. The resting potential, effective resistance, threshold for the Na current, action potential and K+-induced depolarizations did not change when 1.8 mM Ca2+ was replaced by 3 mM Mg2+ (3--6 microM Ca2+). Identical results were obtained after the addition of 5 mM EGTA (less than or equal to 10(-9) M Ca2+; Ca-free saline). The rate of tension development during the initial phase of K+ contractures was independent of external Ca2+ while the amplitude, the duration, and the time constant of spontaneous relaxation decreased progressively as Ca2+ concentration was diminished. The activation curve shifted by 3--5 mV towards more positive potentials while the inactivation curve shifted by 16--18 mV in the opposite direction and both curves became steeper in Ca-free saline. External Ca2+ may play a role in excitation--contraction coupling during K contractures either via the inward Ca current or via specific interactions between external Ca2+ ions and the coupling mechanism or both.