Characterization of an inwardly rectifying chloride conductance expressed by cultured rat cortical astrocytes

The biophysical and pharmacological properties of the inwardly rectifying Cl− conductance (IClh), expressed in rat type‐1 neocortical cultured astrocytes upon a long‐term treatment (1–3 weeks) with dibutyryl‐cyclic‐AMP (dBcAMP), were investigated with the whole‐cell patch‐clamp technique. Using intra‐ and extra‐cellular solutions with symmetrical high Cl− content and with the monovalent cations replaced with N‐methyl‐D‐glucamine, time‐ and voltage‐dependent Cl− currents were elicited in response to hyperpolarizing voltage steps from a holding potential of 0 mV. The inward currents activated slowly and did not display any time‐dependent inactivation. The rising phase of the current traces was best fitted with two exponential components whose time constants decreased with larger hyperpolarization. The steady‐state activation of IClh was well described by a single Boltzmann function with a half‐maximal activation potential at −62 mV and a slope of 19 mV that yields to an apparent gating charge of 1.3. The anion selectivity sequence was Cl− = Br− = I− > F− > cyclamate ≥ gluconate. External application of the putative Cl− channel blockers 4,4 diisothiocyanatostilbene‐2,2 disulphonic acid or 4‐acetamido‐4‐isothiocyanatostilbene‐2,2‐disulphonic acid did not affect IClh. By contrast, anthracene‐9‐carboxylic acid, as well as Cd2+ and Zn2+, inhibited, albeit with different potencies, the Cl− current. Taken together, these results indicate that dBcAMP‐treated cultured rat cortical astrocytes express a Cl− inward rectifier, which exhibits similar but not identical features compared with those of the cloned and heterologously expressed hyperpolarization‐activated Cl− channel ClC‐2. GLIA 21:217–227, 1997. © 1997 Wiley‐Liss, Inc.