Ionic permeabilities of anAplysia giant neuron

Summary

In a giant neuron ofAplysia californica, permeabilities and conductances obtained by measuring net fluxes of Na⁺, K⁺ and Cl⁻ with ion-specific microelectrodes were compared with those obtained by measuring transmembrane current and potential changes when the three ions were varied in the external solution. Net fluxes were measured with ion-specific microelectrodes, after blocking metabolic processes, thus allowing movement of ions down their electrochemical gradients. Permeabilities and conductances obtained from the “chemical” measurements (i.e., ion-specific electrodes) were generally comparable to the values obtained from “electrical” measurements. Where discrepancies occurred, they could be explained by showing that some of the assumptions necessary to use the “electrical” method were not quantitatively true in this system. The absolute magnitudes of the permeabilities are significantly less than those found in many axonal preparations. There is also a relatively highP _Na/P _K ratio. The selectivity of the membraneagainst ions such as Tris⁺ and MeSO ⁻₃ is not good, Tris⁺ being nearly as permeable as Na⁺ and MeSO ⁻₃ about one-half as permeable as Cl⁻. These properties may be characteristic of somal membranes.