Characterization of two voltage-dependent, inwardly rectifying currents in dibutyryl-cyclic AMP treated cortical astrocytes

In astrocytes, a simultaneous influx of chloride (Cl-) along with potassium (K+) ions has been postulated for explaining the process of the extracellular K+ buffering via the mechanism of local accumulation. With the patch-clamp technique we have investigated in primary culture (2-4 weeks) of rat cortical astrocytes the presence of K+ and Cl- conductances which are active around the resting membrane potential. Whole-cell currents were recorded in GFAP positive cells after treatment (1-2 weeks) of the cultures with 0.25 mM db-cAMP. In intra- and extracellular physiological solutions the cell currents elicited in response to hyperpolarized step potentials from an holding potential of – 60 mV revealed the existance of two different conductances. The first on had characteristics comparable to a voltage-dependent, invar-rectifying potassium current (K-ir): it displayed a quasi-instantaneous activation kinetic, a voltage- and time-dependent inactivation for potential more negative than -120 mV and was fully and reversibly blocked by 0.1 mM barium (Ba2+). In 4 mM extracellular K+ solution this conductance was already activated at resting potential; by increasing the external K+ from 4 mM to 12 mM the current-voltage relationship showed an increase in slope conductance while replacement of external 100 mM sodium (Na+) with N-methyl-D-glucamine (NMDG+) decreased the rate and the extent of the inactivation. The secondo ne was a voltage-dependent but not inactivating current which developed at membrane potential more negative than -40 mV. It had a slow activation and deactivation kinetics requiring 3-6 sec to reach the peak amplitude and it was insensitive to 0.1 mM Ba2+. Changes in the external monovalent cations content did not modify neither the amplitude nor the current kinetics. With the intra- and extracellular K+ and Na+ substituted by NMDG+ and in symmetrical Cl- the reversal potential was at 0 mV. The findings suggest that this second conductance i san inward-rectifying chloride current (Cl-ir) with different gating properties from the Cl- currents previously described in astrocytes. These data all together demonstrate the existance in cortical astrocytes of conductances which may be involved in the process f K+ extracellular clearing, playing also a crucial role in the mechanism of swelling which occur in physiophatological conditions.