Relationship between intracellular calcium concentration and relaxation of rat fast and slow muscles.

Following injection of aequorin into small bundles of rat soleus and extensor digitorum longus muscles, simultaneous measurements of light (proportional to the intracellular free calcium ion concentration ([Ca2+]i)) and force were recorded during muscle twitches over a range of temperatures from 10 to 37 degrees C. The Arrhenius plots for rate of decline of [Ca2+]i following a twitch were linear and yielded the same activation enthalpy (Ea) for both muscle types, indicating that the same process controls Ca2+ uptake in both fast and slow muscles. Arrhenius plots of force decay rate constants showed a break around 20 to 25 degrees C with the Ea's above the break being similar to the Ea's for decline of [Ca2+]i and indicating a close relationship between rate of relaxation and decline of [CA2+]i above this temperature.