Calcium-activated force responses in fast- and slow-twitch skinned muscle fibres of the rat at different temperatures.

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1. Force responses from mechanically skinned fibres of rat skeletal muscles (extensor digitorum longus and soleus) were measured at different temperatures in the range 3-35 degrees C following sudden changes in Ca2+ concentration in the preparations. 2. At all temperatures there were characteristic differences between the slow- and fast-twitch muscle fibres with respect to the relative steady-state force-[Ca2+] relation: such as a lower [Ca2+] threshold for activation and a less steep force-pCa curve in slow-twitch muscle fibres. 3. At 3-5 degrees C the force changes in both types of muscle fibres lagged considerably behind the estimated changes in [Ca2+] within the preparations and this enabled us to perform a comparative analysis of the Ca2+ kinetics in the process of force development in both muscle fibre types. This analysis suggest that two and six Ca2+ ions are involved in the regulatory unit for contraction of slow- and fast-twitch muscle fibres respectively. 4. The rate of relaxation following a sudden decrease in [Ca2+] was much lower in the slow-twitch than in the fast-twitch muscle at 5 degrees C, suggesting that properties of the contractile apparatus could play an essential role in determining the rate of relaxation in vivo. 5. There was substantial variation in Ca2+ sensitivity between muscle fibres of the same type from different animals at each temperature. However the steepness of the force-[Ca2+] relation was essentially the same for all fibres of the same type. 6. A change in temperature from 5 to 25 degrees C had a statistically significant effect on the sensitivity of the fast-twitch muscle fibres, rendering them less sensitive to Ca2+ by a factor of 2. However a further increase in temperature from 25 to 35 degrees C did not have any statistically significant effect on the force-[Ca2+] relation in fast-twitch muscle fibres. 7. The effect of temperature on the Ca2+ sensitivity of slow-twitch muscle fibres was not statistically significant, mainly because of the large variation in sensitivity amongst these preparations at room temperature. 8. Two types of oscillatory processes not associated with intracellular membranes were observed in the force response of all slow-twitch muscle fibres when submaximally activated (less than 60% maximum force) at 25 and 35 degrees C, but never at 3-5 degrees C. The frequency of oscillations increased with temperature. 9. Maximum Ca2+-activated force in both muscle fibre types was greatly dependent upon temperature over the range 0-25 degrees C, but increased only slightly above 25 degrees C. 10. Experiments on the rigor state suggest that the number of possible actomyosin interacting sites diminishes considerably as temperature is decreased below 25 degrees C.

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