AEQUORIN-CALCIUM TRANSIENTS IN MAMMALIAN FAST AND SLOW MUSCLE FIBERS

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The aequorin-Ca²⁺ transient evoked by depolarizing clamp pulses was faster in the fast twitch extensor digitorum longus (e.d.l.) muscle than in the slow twitch soleus muscle of the rat at temperatures examined (15-40°C). The Q₁₀ of the decay time constants were 2.3 and 2.1 for the e.d.l. and soleus, respectively. The potential-amplitude relation of the Ca²⁺ transient in both muscles was sigmoid and tended to plateau at around +50 mV. The maximum amplitude evoked by strong and long pulses was greater in the e.d.l. than in the soleus. The Ca²⁺ transients in both muscles could be evoked in Ca²⁺-free (EGTA) Ringer, indicating that the Ca²⁺ transient is caused by Ca²⁺ released from intracellular stores. During a prolonged depolarization, Ca²⁺ transients gradually declined in both muscles due probably to inactivation of the excitation-Ca²⁺ release coupling process. The decline was faster in the e.d.l. than in the soleus. After denervation, the Ca²⁺ transients in diaphragm muscle became slower in parallel with a slowing of contraction time. It is concluded that the difference in the time course of the Ca²⁺ transient is a factor responsible for the difference in contraction time in mammalian muscles.