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

Ca2+ transients were investigated in single fibers isolated from rat extens or digitorum longus muscles exposed to chronic low-frequency stimulation fa r different time periods up to 10 days. Approximately 2.5-fold increases in resting Ca2+ concentration ([Ca2+]) were observed 2 h after stimulation on set and persisted throughout the stimulation period. The elevated [Ca2+] le vels were in the range characteristic of slow-twitch fibers from soleus mus cle. 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 impairmen t of Ca2+ uptake and, with longer stimulation periods, by an additional los s 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 contrib ution to Ca2+ sequestration.