Key result
In fast-twitch muscles of rats, mice, and shrews, muscle relaxation preceded Ca2+ decay, whereas in slow-twitch rat soleus, force half-relaxation time was three times longer than Ca2+ decay.
The study reveals a surprising property of fast-twitch muscles where relaxation precedes calcium decay, proposed to be due to a right-shifted pCa-tension curve.
Should not alter clinical management of muscle disorders; leaves open whether pCa-tension shifts govern fast-twitch relaxation in humans.
Isometric single-twitch force and intracellular Ca2+ transients were recorded simultaneously, using fura-2, from slow- and fast-twitch muscle fibres of the rat, mouse and Etruscan shrew Suncus etruscus. In the slow-twitch rat soleus, force half-relaxation time was three times longer than the 50% decay time of the fura-2 signal. In contrast, in the fast-twitch extensor digitorum longus muscles of all three animals, muscle relaxation preceded Ca2+ decay. It is proposed that this surprising property of fast-twitch muscles is due to their pCa-tension curve, which is shifted to the right compared with that of slow-twitch muscle.
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Wetzel et al. (1998) studied this question. Fast-twitch muscle fibers vs. Slow-twitch muscle fibers was evaluated on Force half-relaxation time relative to 50% decay time of the fura-2 signal. In fast-twitch muscles of rats, mice, and shrews, muscle relaxation preceded Ca2+ decay, whereas in slow-twitch rat soleus, force half-relaxation time was three times longer than Ca2+ decay.
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