VOLTAGE-DEPENDENT SODIUM AND POTASSIUM CURRENTS IN CULTURED TROUT ASTROCYTES

Voltage-gated ionic currents were recorded from cultured trout astrocy tes with the whole-cell variation of the patch-clamp technique. In a s ubpopulation of astrocytes depolarizations above -40 mV activated a fa st transient inward current that was identified as a sodium current by ion substitution experiments, its current reversal potential, and its TTX-sensitivity. Regarding threshold of activation, peak current volt age, and amplitude this current closely resembled those previously des cribed for mammalian astrocytes. Voltage-dependence of inactivation an d kinetics, however, markedly differed from the ''glial-like'' sodium current occurring in mammalian hippocampal or optic nerve astrocytes, since the sodium current of trout astrocytes exhibited a faster time c ourse of activation and decay and a more depolarized steady-state inac tivation curve with midpoints close to -60 mV. During a period of 2 we eks in culture the biophysical properties of the sodium current did no t change significantly, albeit a continuous decrease in current densit y was observed. At depolarizing voltage steps positive to -40 mV, addi tionally voltage-gated potassium outward currents were evoked, which c ould be separated into a steady-state current with delayed rectifier p roperties and an inactivating component resembling the A-type current. Moreover, in a subpopulation of astrocytes an inward potassium curren t was elicited at hyperpolarizing potentials, which exhibited biophysi cal features consistent with the potassium inward rectifier of mammali an astrocytes. (C) 1994 Wiley-Liss, Inc.