Calcium efflux from squid axons under constant sodium electrochemical gradient.

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Abstract
The effect of varying Nao and Nai on Ca efflux while maintaining the ratio Nao/Nai constant was explored in squid [Doryteuthis plei] giant axons dialyzed with and without ATP. In the absence of ATP, the Ca efflux increased 3.4 .+-. 0.2-fold when the Nao/Nai concentrations were reduced from 440/80 to 110/20 mM. In the presence of ATP a similar change did not have an appreciable effect. The inhibition of Ca efflux produced by Nai was studied in the presence and in the absence of ATP. In the absence of ATP, inhibition was very marked and was reminiscent of a unimolecular noncompetitive reaction (inactivation constant [KI] of 34 .+-. 5 mM of Nai) whereas in the presence of ATP, the slight inhibition observed indicates that ATP probably increased the KI to 200 mM. From the inhibition of the Ca efflux produced by Nai in the presence or absence of ATP a curve describing the dependence of Nai of the ATP-promoted fraction of Ca efflux was constructed. The effect of Nao on Ca efflux was studied as a function of [Na]i [internal]: at low Nai, an activation constant (KA) of 41 mM for Nao [external] was obtained either in the presence or in the absence of ATP. As intracellular Na was increased in the presence of ATP, Nai seemed to have no effect on the apparent half-activation constant. In the absence of ATP, the KA for activation increased along a sigmoid curve reaching a value of 112 mM at 100 mM Nai. The Ca efflux system uses the energy of the Na electrochemical gradient. The interaction of a single Na+ seemed sufficient to block Ca extrusion whereas several Na+ externally are necessary to activate Ca extrusion.