Characteristics of Na+ current in Schwann cells cultured from frog sciatic nerve

Characteristics of voltage-dependent currents in cultured frog Schwann cells were investigated by the whole-cell clamp technique. An inward current was detectable at a membrane potential level more positive than -50 mV and reached a maximum value at about -10 mV, while no rectifying channel was present. The inward current was carried by Na+ ions, because the extrapolated reversal potential of the current agreed with the calculated ENa, and the current was sensitive to tetrodotoxin. The membrane potential for half-maximal inactivation was -82 mV. The inactivation curve indicated that more than 90% of the Na+ channels were inactivated at the resting membrane potential, suggesting that the cultured frog Schwann cells could not generate an action potential under physiological conditions. The time constant for the inactivation at a maximum current was 5.3 ms (-10 mV, 13 degrees C). The electrophysiological characteristics of the Na+ current in the cultured frog Schwann cells were compared with those in other tissues. This Na+ current was quantitatively different from that observed in the amphibian node of Ranvier but was similar to that in the mammalian Schwann or glial cells, especially in the more hyperpolarized half-maximal inactivation potential and in the slower inactivation time course.