In neurones, intracellular calcium ([Ca²⁺]_i) plays a crucial role in the regulation of the neuronal activity and therefore needs to be precisely controlled. To do this, neurones have developed several homeostatic systems: Ca²⁺ binding proteins (CaBP), plasma membrane (PMCA) and endoplasmic (SERCA) Ca²⁺-ATPases, sodium- calcium exchanger (NCX) and mitochondrial uptake mechanisms. Using perforated-patch electrophysiological recording techniques combined with indo-1 measurement of [Ca²⁺]_i in neurones dissociated from superior cervical ganglion (SCG: isolated from humanely killed rats; see Trouslard et al. 1993), homeostatic systems involved in the maintenance of resting [Ca²⁺]_i and the clearance of small Ca²⁺ transients (< 500 nM) following activation of voltage-gated Ca²⁺ entry were investigated.

In these neurones resting [Ca²⁺]_i was ~100 nM. This decreased when extracellular Ca²⁺ was removed or 100 µM La³⁺ was added to the extracellular solution. This effect was not mimicked by Cd²⁺ (200 µM), an inhibitor of voltage-gated Ca²⁺ currents. PMCA inhibition by intra-cellular carboxyeosin (50 µM) or extracellular alkalisation (pH 9.0) prolonged the recovery of small Ca²⁺ transients (Table 1) and induced an increase in resting [Ca²⁺]_i. This rise in resting [Ca²⁺]_i was abolished in a Ca²⁺-free extracellular solution and also by La³⁺ but not by Cd²⁺.

Depletion of the intracellular Ca²⁺ stores, following SERCA inhibition with 100 nM thapsigargin, had no effect on the recovery phase (Table 1) of small Ca²⁺ transients but induced a persistent rise in resting [Ca²⁺]_i, which was also blocked by La³⁺. In a Ca²⁺-free solution, the thapsigargin-induced rise in [Ca²⁺]_i was abolished and the decrease in resting [Ca²⁺]_i was more pronounced.

These results suggest that the PMCA is the principal Ca²⁺ extrusion system at low [Ca²⁺]_i and plays an important role in the maintenance of basal [Ca²⁺]_i. Resting [Ca²⁺]_i appears to be set by a passive, La³⁺-sensitive Ca²⁺ influx counterbalanced by Ca²⁺ extrusion via the PMCA. The intracellular stores may also participate in the control of resting [Ca²⁺]_i by releasing or sequestering Ca²⁺ and stimulating a Ca²⁺ influx through the plasma membrane.This work was supported by the UK Medical Research Council and Wellcome Trust.

Trouslard, J., Marsh, S.J. & Brown, D.A. (1993). J. Physiol. 468, 53-71. abstract