A possible function of Ca store site in cultured chick myotubes was examined by recording contraction of the myotube with special reference to the effect of caffeine. Caffeine at low concentrations (below 1 rnM), applied focally on the myotube through a micropipette with a pressure pulse, elicited focal contraction without membrane potential changes. Procaine inhibited t h e caffeine contracture. Deuterium oxide also inhibited the caffeine contracture at low concentrations, but enhanced the maximal contracture. These observations are similar to those in t h e mature frog muscle fiber in which the sarcoplasmic reticulum (SR) is a main site of caffeine action. On the basis of these similarities, it was considered that caffeine acts on SR to elicit contracture in the myotube. The ability of SR to accumulate and release Ca ion seemed to be low, because caffeine contracture decreased or disappeared in a Ca-free solution in many myotubes.
Contraction of the mature skeletal muscle fiber is elicited by Ca ion released from the sarcoplasmic reticulum (SR) following excitation of the cell membrane. In the cultured chick myotube, the excitation is represented by a spike potential and a prolonged depolarization, which are evoked spontaneously or following electrical stimulation (