SPATIAL AND TEMPORAL DISTRIBUTION OF [CA2- A FURA-2 IMAGING STUDY(] IN NORMAL HUMAN MYOTUBES )

The spatio-temporal distribution of intracellular, free calcium ions, [Ca2+](i), induced in human myotubes by electrical stimulation typical ly showed a relatively large increase of [Ca2+](i) in the vicinity of the plasmalemma, The similarity of this distribution, with that observ ed after the application of caffeine, and the lack of any effect of la nthanum, strongly suggest that the main source of Ca2+ participating i n the electrically induced transient is the sarcoplasmic reticulum, An eurally cultured human myotubes therefore display a 'skeletal muscle t ype' coupling between membrane depolarization and calcium release, How ever, the relatively slow time course of the electrically induced tran sients compared to rat and mouse myotubes, together with the inability of Ca2+ released from the sarcoplasmic reticulum to activate the cont ractile machinery, implies that aneurally cultured human myotubes achi eve only a limited degree of differentiation, The relevance this may h ave to an apparent delay between the electrically induced rise in intr anuclear Ca2+ relative to cytosolic Ca2+ remains to be determined but, at this stage of differentiation, there appears to be some form of ba rrier to free diffusion between the two cellular compartments.