Sodium-alanine cotransport in oocytes ofXenopus laevis: Correlation of alanine and sodium fluxes with potential and current changes

Abstract

The sodium-dependentl-alanine transport across the plasma membrane of oocytes ofXenopus laevis was studied by means of [¹⁴C]-l-alanine,²²Na⁺ and electrophysiological measurements. At fixed sodium concentrations, the dependence of alanine transport on alanine concentration follows Michaelis-Menten kinetics; at fixed alanine concentrations, the transport varies with sodium concentration with a Hill coefficient of 2. In the presence of sodium the uptake of alanine is accompanied by a depolarization of the membrane. Under voltage-clamp conditions this depolarization can be compensated by an inward-directed current. Assuming that this current is carried by sodium we arrive at a 2∶1 stoichiometry for the sodium-alanine cotransport. The assumption was confirmed by direct measurements of both sodium and alanine fluxes at saturating concentrations of the two substrates, which also yielded a stoichiometry close to 2∶1. The sodium-l-alanine cotransport is neither inhibited by furosemide (0.5 mmol/liter) nor by N-methyl amino isobutyric acid (5 mmol/liter). A 20-fold excess ofd-alanine overl-alanine caused about 60% inhibition.