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+ (K-ir) channels. 2. Two types of astrocytic K-ir channels were identified with single-channel conductances of similar to 28 and similar to 80 pS, respectively. Channels displayed some voltage dependence in their open probability, which was largest (0.8-0.9) near the K+ equilibrium potential (E(k)) and decreased at more negative potentials. The resting potential closely followed E(k), so it can be assumed that K-ir channels have a high open probability at the resting potential. 3. The conductance of inwardly rectifying K+ currents (K-ir) depended strongly on [K+](o) and was approximately proportional to the square-root of [K+](o). 4. K-ir 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 K-ir channels in other systems by [Na+](o). 5. K-ir currents were also blocked in a dose-dependent manner by Cs+ (K-d = 189 mu M at -140 mV), Ba2+ (K-d 3.5 = mu M), 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 K-ir currents was associated with a marked depolarization, suggesting that K-ir 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 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 K-ir channels.