Calcium entry into cultured mouse astrocytes

Abstract

The barium-sensitive component of transmembrane influx of 45Ca into cultured mouse astrocytes was investigated in order to find clues for an involvement of a glial Ca 2+ uptake system in Ca 2+ movements of the CNS. The Ca 2+ turnover was very high amounting to a rate constant of 1.4 min −1. The magnitude of the Ca 2+ flux was, however, only about 2% of the K + flux. It had the same magnitude as the Cl − flux. The Ca 2+ influx was not sensitive to the organic Ca 2+ channel blockers verapamil and nifedipine; however, cadmium, cobalt and barium blocked the flux in millimolar concentrations. The Ca 2+ influx was a linear function of the external Ca 2+ concentration up to 1.8 mM. A further increase in external Ca 2+ did not increase the magnitude of the influx. Increases in external K + to the physiological ceiling level of 12 mM (which corresponds to a 15 mV depolarization) and to 54 mM (48 mV depolarization) had no effect on the system. We conclude that the Ca 2+ entry system of glial cells is not involved in the decrease of the external Ca 2+ concentration during neuronal activity. The barium-evoked spontaneous depolarizations of astrocytes reported in the literature are probably due to the blockade of K + channels by barium, since 50 μM barium did not affect the Ca 2+ flux and increased the total Ca 2+:K + flux ratio from 1:50 to 1:2.5.