Currents related to the sodium-calcium exchange in squid giant axon

We report the measurement of a Cai-activated membrane current in dialyzed squid axon under membrane potential control with a low-noise voltage clamp. Two additional voltage clamp systems were used to clamp the external guard plates to a value that prevented the establishment of potential differences between the central and lateral compartments of the experimental chamber. This reduced to a minimum the contribution of membrane currents generated at the axon ends to the current measured in the central pool. This latter current was reduced by using internal and external solutions designed to diminish at a maximum membrane currents, while maintaining the conditions for optimal operation of the Na+-Ca2+ exchange. Thus TTX was used to block Na+ channels and prolonged exposure to K+-free media was used to eliminate K+ conductance. The maximum concentration of external sodium was 200 mM. The addition of fixed amounts of free ionic calcium to the internal solution, activated a current whose direction and magnitude depended on the thermodynamic driving forces for calcium and sodium. When the experimental conditions determined an inwardly directed current, this depended on the presence of external sodium, and lithium could not substitute for it. The Cai-activated current, was blocked by external lanthanum and showed a high temperature dependence. In experiments in which the reversal potential was measured for the Cai-activated current, it was found to be strikingly similar to the value calculated according to Er = 3ENa - 2ECa, suggesting that the current is the electrical manifestation of the Na+-Ca2+ exchange operating with an stoichiometry of 3Na+:1Ca2+.