The squid axon as a model for studying plasma membrane mechanisms for calcium regulation.

Calcium movement across plasma membranes occurs mainly by three routes: voltage-dependent calcium channels, adenosine 5'-triphosphate-driven calcium pump, and Na+-Ca2 exchange. The regulation of the intracellular ionized calcium is the consequence of two parallel calcium transport mechanisms: a high affinity, low capacity system responsible for extruding calcium during resting conditions (calcium pump) and a low affinity and high capacity system (Na+-Ca2 antiporter). This last system is designed to extrude calcium ions when intracellular calcium rises above certain levels and also to lead calcium ions into the cell under conditions that favor the reverse mode of operation of the exchanger. This short review provides an analysis of the most conspicuous features of the two membrane transport mechanisms determined in dialyzed squid axons with special emphasis on both the complexity of the Na+-Ca2+ exchange system and its marked asymmetry.