Spontaneous changes in intracellular calcium concentration in type I astrocytes from rat cerebral cortex in primary culture

Measurement of fura‐2 fluorescence in type I astrocytes from rat cerebral cortex showed that the intracellular calcium ion concentration undergoes very large spontaneous changes. These spike‐like changes ranged from resting levels of calcium of 50–250 nM to as high as 1–2 μM. The spikes were found to be irregular in frequency and amplitude and were frequently synchronous in confluent cultures. The synchronous events appeared as propagating waves that spread over many cells. The spontaneous spikes persisted when the extracellular calcium concentration was reduced to below micromolar levels suggesting that the source for the increases in [Ca2+]i was intracellular. Treatment of the astrocytes with tetrodotoxin did not abolish the spontaneous changes, nor did blockade of voltage‐dependent calcium channels with nimodipine and D‐600. Ryanodine, a blocker of the sarcoplasmic reticulum calcium‐induced calcium release channel, was also without effect. These changes in [Ca2+]i were different in character from both agonist‐induced oscillations and depolarization‐induced increases in intracellular calcium concentration. Depolarization using 25–100 mM [K+]o resulted in a prompt rise in intracellular calcium concentration, which returned to near resting levels, and this response was sensitive to removal of extracellular calcium and voltage‐gated calcium channel antagonists. L‐glutamate (0.5–100 μM) caused large increases in [Ca2+]i that were associated with discrete periodic oscillations in some cells. The cellular trigger for the spontaneous spikes is currently not understood. We conclude that spontaneous changes in [Ca2+]i in astrocytes are distinct from agonist‐induced and membrane potential depolarization‐induced changes.