Electrogenicity of sodium/L-glutamate cotransport in rabbit renal brush-border membranes: a reevaluation

Abstract

In order to clarify contradictory reports on the electrogenicity of sodium/L-glutamate cotransport, this cotransport was studied using brush-border membrane vesicles isolated from rabbit renal cortex. Beforehand, the claim that the symport of L-glutamate with Na+ is linked to simultaneous antiport with K+ has been confirmed by the demonstration that equilibrium exchange of L-glutamate is inhibited by potassium. Concerning the electrogenicity of the system, the following results are reported: net uptake of sodium-dependent L-glutamate uptake was stimulated when the transmembranal electrical potential difference was increased by replacing a sodium sulfate gradient by a sodium nitrate gradient. At 100 mM Na+ the 'relative electrogenicity' of the initial uptake in the presence of intravesicular potassium was 2-times higher than in its absence. At a sodium concentration of 20 mM, when overall uptake was reduced, the relative electrogenicity in the presence of K+ was even 3-fold higher than in K+-free media. The relative electrogenicity of sodium/D-glucose cotransport measured under the same experimental conditions was not affected by K+. These results are discussed in terms of a model where the apparent electrogenicity of a cotransport system is dependent on the extent to which the charge translocating step is rate limiting ('rate limitancy'). It is proposed that potassium antiport, while decreasing charge stoichiometry of Na+/glutamate transport, increases the relative rate limitancy of the transport step translocating three cations (probably two Na+, one H+) together with one glutamate. Thereby the positive electrogenicity of glutamate uptake increases, in complete contrast to what would be expected from simple considerations of charge stoichiometry.