The basic asymmetry of Na +-dependent glycine transport in Ehrlich cells

Abstract

Influx and efflux of glycine have been examined as a function of external and internal Na + concentrations, respectively, when Δ μ Na = 0 . With Δ μ Na = 0 it was found that at comparable external and cellular Na + levels, the K m for efflux was larger by an order of magnitude than the value for influx and the V for efflux was several times greater than the V for influx. For both fluxes the major effect of Na + was to decrease the K m value. The observations are consistent with the conclusion that the Na +-dependent transport system is asymmetric per se. Influx and efflux of glycine were increased in a near linear manner by increasing the Na + concentration from 13 to 100 mM, the half-time for glycine equilibration being a function of the Na + concentration in absence of an electrochemical potential difference for Na +. In Na +-free media ( [ Na +] < 5 mM ) equilibration of glycine between cells and medium was not achieved after 60 min at 25°C. With Δ μ Na = 0 , efflux (or uptake) of glycine was not affected by internal (or external) K + between 20 and 120 mM suggesting that K + plays no direct role in Na +-dependent transport of glycine in Ehrlich cells.