Temperature-dependent Calcium Sensitivity Changes in Skinned Muscle Fibres of Rat and Toad

Single mechanically skinned muscle fibres of different types (fast- and slow-twitch mammalian; slow and twitch amphibian) were successively activated in solutions of various Ca2+ concentrations at different temperatures. An increase in temperature from 5 to 22 degrees C reversibly shifted the isometric steady-state force-pCa curves towards higher Ca2+ concentration for individual fibres of each of the muscle types. A further increase in temperature to 35 degrees C in mammalian fibres resulted in an additional decrease in Ca2+ sensitivity. The temperature dependence of Ca2+ sensitivity was greater in the 'faster' fibre types: fast-twitch greater than slow-twitch; twitch greater than slow. The maximum isometric force response, P0, of both rat and toad skinned fibres was found to be strongly dependent on temperature below 22 degrees C. No detectable force could be induced by Ca2+ in mammalian muscle fibres at 0-1 degree C while in toad fibres P0 decreased by about 90% when temperature dropped from 20 to 0 degree C. Since in mechanically skinned fibres of other amphibians (Bufo bufo, Rana species) P0 is only marginally affected it is likely that the P0-temperature relations are indicative of the range of temperature over which the muscles are normally functional. The P0-temperature relations of skinned muscle fibres closely resembled the P0-temperature relations of tetanically stimulated intact muscle preparations from the same species of animals suggesting that the contractile apparatus is mainly responsible for the variation of force response with temperature in intact muscle.