Effects of external calcium reduction on the kinetics of potassium contractures in frog twitch muscle fibres.

1. The amplitude and time course of K contractures (Cl‐ constant) of single twitch muscle fibres of the frog have been analysed in three external Ca2+ concentrations. 2. The resting potential, effective resistance, threshold for the Na current and K‐induced depolarizations were not modified by replacing 1.8 mM‐Ca2+ by 3 mM‐Mg2+ in absence (low‐Ca saline: 3‐6 micro M‐Ca2+) or in the presence of 5 mM‐EGTA (Ca‐free saline: less than or equal to 10(‐9) M‐Ca2+). 3. The 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. 4. When the concentration of Mg2+ was increased to 5 mM in Ca‐free saline K contractures were slower and smaller than those in 3 mM‐Mg2+. 5. In Ca‐free saline the activation curve (peak tension vs. logarithm of external K+ concentration) shifted by 3‐5 mV towards more positive potentials while the inactivation curve (peak tension of the test contracture vs. logarithm of external K+ concentration during the conditioning period) shifted by 16‐18 mV towards more negative potentials. Both curves became steeper in Ca‐free saline. 6. The effects of external Ca2+ reduction were not modified by replacing all chloride for methanesulphonate. 7. Direct effects of external Ca2+ on excitation‐contraction coupling during K contractures could involve the inward Ca current and/or specific interactions between external Ca2+ ions and the coupling mechanism.