Extracellular ATP-induced currents in astrocytes: involvement of a cation channel.
Whole-cell currents were measured with the perforated patch clamp technique in cultured rat astrocytes to analyze the underlying ionic mechanism for a P2-purinoceptor-mediated depolarization. ATP (100 microM) induced an inward current with a mean amplitude of 130 pA and an EC50 of 17 microM. 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 voltage-clamping 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 microM ATP lowered the intracellular K+ concentration. Replacement of extracellular Ca2+ or Cl- 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 P2x 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.
Walz
W
W
Institut für Neurobiologie, Universität Heidelberg, Germany.
Gimpl
G
G
Ohlemeyer
C
C
Kettenmann
H
H
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D001253:Astrocytes / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D004594:Electrophysiology; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects; D008839:Microelectrodes; D009431:Neural Conduction / Q000187:drug effects; D011188:Potassium / Q000378:metabolism / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D018048:Receptors, Purinergic P2 / Q000187:drug effects
D007473:Ion Channels; D018048:Receptors, Purinergic P2; D000255:Adenosine Triphosphate; D011188:Potassium; D002118:Calcium