Kinetics and energetics of calcium efflux from intact squid giant axons

The Ca efflux from intact squid axons consists of three major components: one that is activated by Cao, one that is activated by Nao and a residual flux that persists in the nominal absence of both Cao and Nao. The properties of these components have been investigated in unpoisoned axons and in axons poisoned with cyanide. 2. Under all conditions the shape of the curve relating Cao to Cao-activated Ca efflux approximates to a section of a rectangular hyperbola, consistent with simple Michaelis activation. 3. The external Ca concentration giving half-maximal activation of Cao-activated Ca efflux is about 2 muM in unpoisoned axons immersed in Na-ASW, but on poisoning changes progressively to values in the range 1-10 mM. The residual efflux from unpoisoned axons may reflect activation by traces of Ca present immediately external to the axolemma. 4. The apparent affinity for Cao of Cao-activated Ca efflux is very similar in unpoisoned axons immersed in sea waters containing Na, Li, Tris or K as major cation, whereas in poisoned axons the affinity in Na and Li is about the same but higher than that in choline and Tris. 5. In unpoisoned axons Ca influx increases linearly as Cao is increased from 2 muM to 110 mM. The absolute value of the Ca influx from 10 muM-Cao is less than 1% of the Cao-activated Ca efflux at this external Ca concentration. In poisoned axons the sizes of Cao-activated Ca efflux and Ca influx were similar at all Ca concentrations examined. 6. The shape of the curve relating Nao to Nao-activated Ca efflux approximates to a section of rectangular hyperbola in unpoisoned axons but is clearly sigmoidal in axons that have been fully poisoned with cyanide. The sigmoidal shape develops progressively during poisoning. ...