Voltage dependent fast calcium current in cultured skeletal myocytes of the frog Rana temporaria.

Voltage dependent calcium currents in cultured frog embryonic skeletal myocytes at stages of development ranging from 2 to 9 days were studied using the whole-cell patch clamp technique at 19-21 degrees C. Membrane currents were recorded in the presence of 2 mmol/l Ca2+ (outside), and 60 mmol/l CsCl and 50 mmol/l TEACl (inside). In the absence of sodium current two components of inward current were observed in response to depolarization already during the early stages of myogenesis: the well-known slow dihydropyridine (DHP)-sensitive calcium current (ICa,s), and a fast-activated current. Both components persisted in the presence of 2 mumol/l tetrodotoxin. The fast-activated component was enhanced upon addition of 6 mmol/l Ca2+ or Ba2+ to the external recording solution and was decreased when the standard external solution was replaced by Ca2+ free solution. Thus, the fast component of the inward current was also carried by Ca2+ (ICa,f). Unlike ICa,s, it was not blocked with 30-150 mumol/l DHP nifedipine. During 7 s depolarization ICa,f was detected at approximately -50 mV, 20 mV more negative than the membrane potentials at which ICa,s appeared. At various test potentials t0.5 for activation of ICa,f was 8-20 ms, and the current declined during depolarization with tau in of 500-800 ms. These results indicate the existence of two types of voltage-dependent Ca2+ channels in early stages of development of frog myocytes, both known in mature frog skeletal muscle fibres.