Force-calcium relations in skinned twitch and slow-tonic frog muscle fibres have similar sarcomere length dependencies.

The sarcomere length (SL) dependence of the calcium sensitivity of force was measured in skinned single twitch and slow-tonic muscle fibres from frog and toad. Twitch and slow-tonic fibres were characterized by location, appearance, physiological response to calcium and by protein band patterns from sodium-dodecyl-sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Force-calcium relations were determined for each fibre type at two sarcomere lengths, 2.4 and 3.1 microns. Bathing solution ionic strength (IS) was 200 mM and solution pH was 7.0, 6.0 or 5.5; experiments were also done at IS = 120 mM and pH 7.0. At all pHs and ionic strengths tested, slow-tonic fibres exhibited a slower time course of force development and were more sensitive to calcium than were twitch fibres. Lowering IS increased calcium sensitivity and lowering pH decreased calcium sensitivity in both fibre types. Increasing SL increased the calcium sensitivity of force in both twitch and slow-tonic fibres at pH 7.0 and at both 200 and 120 mM IS. Lowering pH caused a decrease in the length dependence of calcium sensitivity of both fibre types; at pH 5.5 the calcium sensitivity of force in slow-tonic fibres exhibited a slight decrease with increasing SL.