Biophysical and pharmacological characterization of inwardly rectifying K+ currents in rat spinal cord astrocytes.
1. Whole cell and cell-attached patch-clamp recordings were obtained from rat spinal cord astrocytes maintained in culture for 6-14 days. It was found that the resting conductance in these astrocytes is primarily due to inwardly rectifying K+ (Kir) channels. 2. Two types of astrocytic Kir channels were identified with single-channel conductances of approximately 28 and approximately 80 pS, respectively. Channels displayed some voltage dependence in their open probability, which was largest (0.8-0.9) near the K+ equilibrium potential (Ek) and decreased at more negative potentials. The resting potential closely followed Ek, so it can be assumed that Kir channels have a high open probability at the resting potential. 3. The conductance of inwardly rectifying K+ currents (Kir) depended strongly on [K+]o and was approximately proportional to the square-root of [K+]o. 4. Kir currents inactivated in a time- and voltage-dependent manner. The Na+ dependence of inactivation was studied with ion substitution experiments. Replacement of [Na+]o with choline or Li+ removed inactivation. This dependence of current inactivation on [Na+]o resembles the previously described block of Kir channels in other systems by [Na+]o. 5. Kir currents were also blocked in a dose-dependent manner by Cs+ (Kd = 189 microM at -140 mV), Ba2+ (Kd = 3.5 microM), and tetraethylammonium (TEA; 90% block at 10 mM) but were insensitive to 4-aminopyridine (4-AP; 5 mM). In the current-clamp mode, Ba2+ and TEA inhibition of Kir currents was associated with a marked depolarization, suggesting that Kir channel activity played a role in the establishment of the negative resting potential typical of astrocytes. 6. These biophysical features of astrocyte inwardly rectifying K+ channels are consistent with those properties required for their proposed involvement in [K+]o clearance: 1) high open probability at the resting potential, 2) increasing conductance with increasing [K+]o, and 3) rectification, e.g., channel closure, at positive potentials. It is proposed, therefore, that the dissipation of [K+]o following neuronal activity is mediated primarily by the activity of astrocytic Kir channels.
Ransom
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CB
Department of Neurology, Yale University School of Medicine, New Haven, Connecticut 06510.
Sontheimer
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D015761:4-Aminopyridine / Q000494:pharmacology; D000818:Animals; D001253:Astrocytes / Q000502:physiology*; D001464:Barium / Q000494:pharmacology; D002586:Cesium / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000494:pharmacology*; D017680:Potassium Compounds / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D013116:Spinal Cord / Q000502:physiology*
D015221:Potassium Channels; D017680:Potassium Compounds; D002586:Cesium; D001464:Barium; D015761:4-Aminopyridine