Sodium-calcium exchange and calcium-calcium exchange in internally dialyzed squid giant axons

Summary

The influx and efflux of calcium (as⁴⁵Ca) and influx of sodium (as²⁴Na) were studied in internally dialyzed squid giant axons. The axons were poisoned with cyanide and ATP was omitted from the dialysis fluid. The internal ionized Ca²⁺ concentration ([Ca²⁺] _i ) was controlled with Ca-EGTA buffers. With [Ca²⁺] _i >0.5 μm,⁴⁵Ca efflux was largely dependent upon external Na and Ca. The Na ₀ -dependent Ca efflux into Ca-free media appeared to saturate as [Ca²⁺] _i was increased to 160 μm; the half-saturation concentration was about 8 μm Ca²⁺. In two experiments²⁴Na influx was measured; when [Ca²⁺] _i was decreased from 160 μm to less than 0.5 μm, Na influx declined by about 5 pmoles/cm² sec. The Na ₀ -dependent Ca efflux averaged 1.6 pmoles/cm² sec in axons with a [Ca²⁺] _i of 160 μm, and was negligible in axons with a [Ca²⁺] _i of less than 0.5 μm. Taken together, the Na influx and Ca efflux data may indicate that the fluxes are coupled with a stoichiometry of about 3 Na⁺-to-1 Ca²⁺. Ca efflux into Na-free media required the presence of both Ca and an alkali metal ion (but not Cs) in the external medium. Ca influx from Li-containing media was greatly reduced when [Ca²⁺] _i was decreased from 160 to 0.23 μm, or when external Li was replaced by choline. These data provide evidence for a Ca−Ca exchange mechanism which is activated by certain alkali metal ions. The observations are consistent with a mobile carrier mechanism which can exchange Ca²⁺ ions from the axoplasm for either 3 Na⁺ ions, or one Ca²⁺ and an alkali metal ion (but not Cs) from the external medium. This mechanism may utilize energy from the Na electrochemical gradient to help extrude Ca against an electrochemical gradient.