Independent changes of intracellular calcium and pH in identified leech glial cells

The intracellular Ca2+ (Ca2+i) and the intracellular pH (pHi) were measured in identified neuropile glial cells in the central nervous system of the leech Hirudo medicinalis, using the fluorescent dye fura-2, and double-barrelled, neutral carrier, pH-sensitive microelectrodes. Different stimuli were used to elicit Ca2+i and/or pHi changes, such as application of ammonium, high external K(+)-concentration, and low external pH. Ammonium (20 mM) and high external K+ (20 mM), which depolarized the glial membrane by 20-30 mV, evoked rapid and large rises of Ca2+i. In contrast to the Ca2+i changes, amplitude and direction of the pHi changes were dependent on the presence of CO2/HCO3- in the saline. The addition of CO2/HCO3-, and the subsequent reduction of external pH from 7.4 to 7.0, had no effect on Ca2+i, but caused significant changes of pHi. The results suggest that the ammonium- and K(+)-induced Ca2+i rises were due to the membrane depolarization leading to a Ca2+ influx through voltage-gated Ca2+ channels in the glial membrane, while the pHi changes resulted from movements of ammonia and from the activation or inhibition of the Na(+)-HCO3- cotransporter. This indicates that changes of intracellular Ca2+ and pH can occur independently of each other, suggesting that the homeostasis of these ions is not necessarily interrelated in these glial cells.