Calcium transients in single fibers of low-frequency stimulated fast-twitch muscle of rat

Ca2+ transients were investigated in single fibers isolated from rat extensor digitorum longus muscles exposed to chronic low-frequency stimulation far different time periods up to 10 days. Approximately 2.5-fold increases in resting Ca2+ concentration ([Ca2+]) were observed 2 h after stimulation onset and persisted throughout the stimulation period. The elevated [Ca2+] levels were in the range characteristic of slow-twitch fibers from soleus muscle. In addition, we noticed a transitory elevation of the integral [Ca2+] per pulse with a maximum (similar to 5-fold) after 1 day Steep decreases in rate constant of [Ca2+] decay could be explained by an immediate impairment of Ca2+ uptake and, with longer stimulation periods, by an additional loss of cytosolic Ca2+ binding capacity resulting from a decay in parvalbumin content. A partial recovery of the rate constant of [Ca2+] decay in 10-day stimulated muscle could be explained by an increasing mitochondrial contribution to Ca2+ sequestration.