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 studi ed using the whole-cell patch clamp technique at 19-21 degrees C. Memb rane 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 s odium current two components of inward current were observed in respon se to depolarization already during the early stages of myogenesis: th e well-known slow dihydropyridine (DHP)-sensitive calcium current (I-C a,I-s), and a fast-activated current. Both components persisted in the presence of 2 mu mol/l tetrodotoxin. The fast-activated component was enhanced upon addition of 6 mmol/l Ca2+ or Ba2+ to the external recor ding solution and was decreased when the standard external solution wa s replaced by Ca2+ free solution. Thus, the fast component of the inwa rd current was also carried by Ca2+ (I-Ca,I-f). Unlike I-Ca,I-s, it wa s not blocked with 30 - 150 mu mol/l DHP nifedipine. During 7 s depola rization I-Ca,I-f was detected at approximately -50 mV, 20 mV more neg ative than the membrane potentials at which I-Ca,I-s appeared. At vari ous test potentials t(0.5) for activation of I-Ca,I-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-depen dent Ca2+ channels in early stages of development of frog myocytes, bo th known in mature frog skeletal muscle fibres.