VOLTAGE-DEPENDENT IONIC CURRENTS IN FROG CULTURED SKELETAL MYOCYTES

The voltage dependent ionic currents in cultured embryonic skeletal my ocytes at stages of development ranging from 1 to 6 day were studied u sing the whole-cell patch clamp technique. Sodium (I(Na)) and calcium (I(Ca)) inward and potassium (I(K)) outward currents were observed at all stages. I(Na) did not differ from that described in adult frog str iated muscle fibres. Slow I(Ca) was mediated by current through dihydr opyridine sensitive Ca channels and it did not differ in its kinetics from corresponding slow I(Ca) in frog adult twich muscle fibres. In ab out 10% of cells examined for I(Ca), this current was significantly sl ower and similar to I(Ca) described in frog tonic muscle fibres. In so me cases two slow calcium currents with distinguishable kinetics were recorded in the same myocytes. Fast dihydropyridin-insensitive noninac tivating I(Ca) could also be observed. At least 6 types of I(K) were r egistered, with approximate time-to-peak (at test pulse of -10 mV) 5, 12, 20, 30, 50 ms (fast I(K)) and more than 7 s (slow I(K)). Three of them (5, 20 and 30 ms) predominated in 3-day cultures and disappeared in 6-day-old cultures. I(K) in myocytes did not correspond fully in th e kinetics to I(K) reported in adult frog skeletal muscles. Channels a ssociated with transient fast and noninactivating slow I(K) were shown to be highly sensitive to low temperature (+5-degrees-C).