The Influence of Potassium- and Sodium-Free Solutions on Sodium Emux from Squid Giant Axons

The scnsitivity of sodium cfltux to the removal of potassium ions from the cxtcrnal solution and the change in sodium cffiux occurring when sodium ions arc also removed were obscrvcd to be rclated. When Tris was used to replace cxtcrnal sodium ions, increases in sodium effiux were always observed whether the sensitivity of sodium cfflux to external potassium ions was weak or strong. Greater percentage increases in sodium cfflux occurrccl, however, the greater the sensitivity of sodium effiux to external potassium ions. When lithium ions were used to replace external sodium ions, increases in sodium efliux occurred if the sensitivity of cfliux to external potassium ions was strong whcrcas decreases in sodium ctiIux took placc if the sensitivity of cfflux to external potassium ions was weak. Intermediate sensitivities of efflux to external potassium resulted in no change in cfliux upon substitution of lithium ions for external sodium ions. In the presence of 10 -s M ouabain, substitution of Tris for external sodium ions always resuhed in a small decrease in sodium efflux. Thc data can be described in terms of a modcl which assumes the prcscncc of cffiux stimulation sites that arc about 98 % selective to potassium ions and about 2 % selective to sodium or lithium ions. I N T R O D U C T I O N The ratio of the magnitude of sodium efflux in K-free seawater to that in 10 mM K seawater, the "K-free effects" varies in squid giant axons between the extremes of 0.2 and 0.8 (Hodgkin and Keynes, 1955; Caldwell et al., 1960; Sjodin and Beaug$, 1967, 1968 a; Mullins and Brinley, 1967). The response of sodium efflux to the removal of external sodium ions also varies in squid giant axons (Hodgkin and Keynes, 1955; Frumento and Mullins, 1964; Keynes, 1965; Mullins and Brinley, 1967; Sjodin and Beaug~, 1967, 1968; Baker, 1968). The purpose of this investigation was to see whether these variations are in any way correlated.