Temperature effects on isometric contractions of slow and fast twitch muscles of various rodents--dependence on fibre...

The influence of temperature (range 15-37 degrees C) on the isometric contractions of the slow twitch soleus (SOL) and the fast twitch extensor digitorum longus (EDL) muscles of mice, rats and guinea pigs were investigated in vitro. Cooling of the bathing solution prolonged the time parameters of single twitches and tetanic contractions in a non-linear manner in both muscle types of all animals. In muscles containing predominately fast twitch fibres like the EDL of all animals cooling was followed by an increase of the single twitch tension (cold potentiation) with a maximum of 160-180% at about 20 degrees C. The influence of a decrease of the temperature on the twitch tension was different in the SOL of the three animals. In SOL of mice (containing about 50% slow twitch fibres) the twitch tension was virtually unchanged, in SOL of rats (about 70% slow twitch fibres) a moderate cold depression and in SOL of guinea pigs (composed by slow twitch fibres only) a strong cold depression was observed. The maximum tetanic tension decreased progressively on cooling in all muscles and independently of their fibre type composition. Cooling increased the twitch-tetanus-ratio in fast twitch and decreased it in pure slow twitch muscles. It is concluded that the temperature dependence of the single twitch and the twitch-tetanus-ratio can be used as a physiological measure of the fibre type composition of a given muscle.