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 > or = 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.