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

Abstract

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 μM Ca2+). Identical results were obtained after the addition of 5 mM EGTA (≤ 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.