Action of extracellular pH on Na+ and K+ membrane currents in the giant axon ofLoligo Vulgaris

Abstract

Voltage-clamp currents and resting membrane potential of squid giant axons have been studied at extracellular pH varying between 4 and 10. The membrane currents, analyzed according to the Hodgkin-Huxley equations, showed that sodium permeability,P _Na (E), and potassium conductance,g _K (E), curves were shifted toward positive voltages by different amounts and slightly depressed as the external pH was lowered. Under the same conditions, τ_m(E) and τ_n(E) were found to be enhanced and shifted to a larger extent in the same direction. The rate constants α_m and α_n were shifted substantially toward positive voltages, but β_m and β_n changed hardly at all. The shift of the β_m(E) curve was analyzed in terms of a fixed surface charge model; it indicates that unspecific negative groups with an approximate pK_a of 4.5 are located in the vicinity of sodium active sites with an average charge separation of 8 Å. A similar figure is obtained for the potassium system from the shift of the β_n(E) curve.