POTASSIUM-DEPENDENT CALCIUM INFLUX IN ACUTELY ISOLATED HIPPOCAMPAL ASTROCYTES

Potassium depolarization can increase the intracellular ionized calcium concentration ([Ca2+](i)) of cultured astrocytes, but it is not known if astrocytes that have matured in the intact CNS also exhibit voltage-dependent [Ca2+](i) signalling. To address this issue, fluorometric measurements of [Ca2+](i) were obtained from astrocytes acutely isolated from young adult rat hippocampus. In control artificial cerebrospinal fluid containing 5 mM [K+](0), average resting [Ca2+](i) was 195 nM. Elevation of [K+](0) to 50 mM caused [Ca2+](i) to increase 150 nM to 1 mu M above resting levels. The threshold [K+](0) necessary to evoke an elevation in [Ca2+](i) was 20-25 mM, and the magnitude of the [Ca2+](i) signal grew progressively with increasing [K+](0) (up to 50 mM). These [Ca2+](i) increases were blocked completely by removal of external Ca2+, and markedly suppressed by the calcium channel blockers verapamil (30 mu M and greater) and Co2+ (1 mM). Neither reversal of Na+-Ca2+ exchange, nor Ca2+-activated Ca2+ release, nor Ca2+ influx through stretch-activated channels contributed to the [Ca2+](i) increase. These results suggest that [K+](0)-evoked [Ca2+](i) signals are mediated by influx through voltage-gated calcium channels. In contrast to results from cultured astrocytes and acutely isolated neurons, these [Ca2+](i) increases were insensitive to dihydropyridine compounds. We conclude that increases in interstitial [K+], observed in situ during several pathological conditions, trigger voltage-dependent [Ca-2+](i) signals in astroglial cells. This may constitute an important form of neuron-to-glial communication.