Intracellular calcium transients and potassium current oscillations evoked by glutamate in cultured rat astrocytes

Abstract

Glutamate responses in cultured rat astrocytes from cerebella of neonatal rats were investigated using the perforated-patch configuration to record membrane currents without rundown of intracellular messenger cascades, and microfluorometric measurements to measure the intracellular Ca2+ concentration ([Ca2+]1) and intracellular pH (pH(i)) with fura-2 AM and 2',7'bis-(2-carboxyethyl)-5,6-carboxyfluorescein acefoxy methyl-ester respectively. In the perforated-patch mode, glutamate evoked single or multiple outward current transients in 82% of the cells, which disappeared when the recording technique was converted into a conventional whole-cell mode. The outward current transients were accompanied by [Ca2+]1 transients, whereas pH(i) fell monophasically, without any sign of oscillation. Pharmacological analysis of the glutamate-induced responses indicated that ionotropic receptor activation evoked an inward current but no outward current transients, and metabotropic receptor activation (of the mGluR1/5 type) elicited outward current transients but no inward current. The outward current transients were reduced in frequency, or even abolished, after depletion of the intracellular Ca2+-stores by the Ca2+-ATPase inhibitor cyclopiaconic acid (10 μM). They reversed near -85 mV and were reduced by tetraethylammonium (10 mM), suggesting that they were caused by K+ channel activation. It is concluded that glutamate evoked these K+ outward current transients by oscillatory Ca2+ release mediated by mGluR activation. The corresponding membrane potential waves across the astroglial syncytium could provide spatial and temporal dynamics to the glial K+ uptake capacity and other voltage-dependent processes.