Modulation of sodium-cotransport systems by other ions

After a brief review of Na+-cotransport systems which also accept other ions as co-ions or modifiers, modulation of the Na+-L-glutamate transport system in rabbit renal brush border membranes by K+ and H+ is discussed in more detail. Intravesicular K+ increases the initial uptake rate and electrogenicity of the cotransport. This effect of K+ is attributed to the formation of a K+-carrier complex that moves much more rapidly than do the other complexes. The resulting shift in rate limitancy (relative increase in overall rate over the relative increase in rate of step under consideration) from an electroneutral towards a charge-translocating pathway unmasks the electrogenicity of the initial L-glutamate uptake. A positive correlation between relative rate limitancy of the electrogenic pathway and electrogenicity is demonstrated supporting this model. Protons, in addition to acting as co-ions, modify Na+-glutamate cotransport by increasing both the initial rate and the electrogenicity of uptake. This phenomenon is assumed to represent a transition of the transport system from a carrier-like to an open channel-like translocation mode. Thus, the intrinsic properties of Na+-cotransport systems may vary under the influence of other ions. This holds true in particular for the electrogenicity of the initial transport rate which may change independently of alterations in charge stoichiometry.