Whole-cell currents were measured with the perforated patch clamp technique in cultured rat astrocytes to analyze the underlying ionic mechanism for a P,purinoceptor-mediated depolarization. ATP (100 pM) induced an inward current with a mean amplitude of 130 pA and an EC,, of 17 pM. The response desensitized during a 1 min application. Replacement of extracellular Na+ with NMDG or K + abolished the ATP-evoked inward current. Replacement of Na+ with choline, however, resulted in an ATP-evoked response of one-third the amplitude in normal solution. This is indicative of a cation rather than Na+ channel. However, due to difficulties in voltageclamping these gap junction-coupled cells at voltages different from the membrane resting potential, the current reversal potential could not be determined. Measurements with K+-sensitive microelectrodes showed that 100 p M ATP lowered the intracellular K + concentration. Replacement of extracellular Ca2+ or C1-did not alter the ATP-induced inward currents. Fura-2 imaging experiments revealed a transient rise of the intracellular Ca2+ concentration during ATP application. Removal of extracellular Ca2 + did not influence the peak response; it did, however, shorten the time course. These results and previous observations that the permeability changes are caused by a P, receptor are indicative of an ATP-sensitive cation conductance. In addition, cytoplasmic Ca2+ is increased by mobilization from intracellular stores, and by additional influx across the cell membrane. Extracellular ATP released by neurons could evoke K + release from astrocytes as well as be a mediator for cation changes that signal cell activation processes when released by damaged cells.