Voltage dependent ionic currents in frog cultured skeletal myocytes

The voltage dependent ionic currents in cultured embryonic skeletal myocytes at stages of development ranging from 1 to 6 day were studied using the whole-cell patch clamp technique. Sodium (INa) and calcium (ICa) inward and potassium (IK) outward currents were observed at all stages. INa did not differ from that described in adult frog striated muscle fibres. Slow ICa was mediated by current through dihydropyridine sensitive Ca channels and it did not differ in its kinetics from corresponding slow ICa in frog adult twitch muscle fibres. In about 10% of cells examined for ICa, this current was significantly slower and similar to ICa described in frog tonic muscle fibres. In some cases two slow calcium currents with distinguishable kinetics were recorded in the same myocytes. Fast dihydropyridine-insensitive noninactivating ICa could also be observed. At least 6 types of IK were registered, with approximate time-to-peak (at test pulse of -10 mV) 5, 12, 20, 30, 50 ms (fast IK) and more than 7 s (slow IK). Three of them (5, 20 and 30 ms) predominated in 3-day cultures and disappeared in 6-day-old cultures. IK in myocytes did not correspond fully in the kinetics to IK reported in adult frog skeletal muscles. Channels associated with transient fast and noninactivating slow IK were shown to be highly sensitive to low temperature (+5 degrees C).