Tension development and calcium sensitivity in skinned muscle fibres of the frog.

Calcium activated isometric tension development was measured in single skinned muscle fibres of the ileofibularis muscle of the frog. The experiments were carried out at 5 degrees C, pH = 6.9, 1 mM free Mg2+ and an ionic strength of 160 mM. A Hill curve was fitted to the isometrically developed tension at different Ca2+ concentrations by means of a non-linear least mean square approximation. At a sarcomere length of 2.15 micron, the Ca2+ concentration for half maximum tension (K) was 1.6 microM. This Ca2+ concentration decreased with increasing sarcomere length; at 2.7 micron, K was 1.1 microM and at 3.1 micron, K was 0.9 microM. Therefore, Ca sensitivity is increased at larger sarcomere lengths. Consequently, the optimal sarcomere length for tension development shifted to larger values when the Ca2+ concentration was lowered. Osmotic compression of the fibre at 2.15 micron by means of 5% Dextran also caused an increase in Ca sensitivity (K was 1.0 microM). At 2.7 micron, addition of 5% Dextran hardly affected the Ca sensitivity. The possible role of the interfilament spacing in the explanation of these results discussed.