A fast-twitch oxidative-glycolytic muscle with a robust inward calcium current

This report describes the histochemical and physiological properties of a rat skeletal muscle with a robust activity-dependent slow inward Ca2+ current. The muscle, the flexor digitorum brevis (FDB), is a small plantar flexor from the hindfoot. It is a homogeneous muscle consisting of approximately 90% fast-twitch oxidative-glycolytic (type IIA) fibers. Stimulation of the FDB with repetitive stimulus trains (30 or 50 Hz for 330 ms, 1 train/s for 2-5 min) produced a slow increase in the base-line or resting tension of the muscle between trains. This progressive increase in resting tension appears to be due to the activation of voltage-dependent slow Ca2+ channels, since it could be eliminated (i) by stimulating the muscle in a medium containing 2 mM EGTA and without Ca2+, and (ii) by the addition of either Co2+ or verapamil. The presence of a slow current may be associated with an increase in K+ efflux as stimulation continues, and with a prolongation of relaxation time. We also propose that the slow Ca2+ current may contribute to the allosteric activation of phosphorylase kinase during muscle activity. The FDB provides an excellent preparation to investigate the regulation of muscle metabolism by intra- and extra-cellular Ca2+ during exercise.