Rheogenic transport of basic and acidic amino acids across the brush border of Necturus small intestine

Abstract

In studies with isolated Necturus intestine, glutamate (Glu-) and Na+ each enhanced the mucosal influx of the other. Measurement of apical membrane potential, Va, with microelectrodes revealed a rapid depolarization with addition of 10 mM mucosal Glu-. This depolarization was Na+ dependent. Upon complete removal of Cl- from the bathing medium Va hyperpolarized and the Glu- -induced depolarization increased significantly. However, removal of Cl- did not alter the total Glu- influx. These data suggest that external Cl- attenuates the rheogenicity of Na+/Glu- cotransport in the apical membrane of the absorptive cells. We have presented a model consistent with these observations in which Cl- competes with one -COO- group of Glu- for its binding site on the carrier. The two complexes which may form, carrier/Glu-/2Na+ or carrier/Glu-/2Na+/Cl-, allow for either electrogenic or electroneutral transport of Glu-, depending on the ratio [Glu-]/[Cl-] in the extracellular fluid. In other experiments, addition of mucosal L-lysine (Lys+) induced a rapid depolarization of Va. In the presence of Na+, the depolarization appeared to be saturable with respect to Lys+ concentration. In Na+-free media, however, the depolarization increased with Lys+ concentration up to a maximum at 10 mM and then decreased to near zero at 30 mM. These data are consistent with a model for Lys+ entry in which an anionic site of the carrier can bind either Na+ or the epsilon-NH3+ group of Lys+. In this model transport of either complex, carrier-/Lys+ or carrier-/Lys+/Na+ (and return of the carrier to the extracellular surface) is rheogenic. However, at higher Lys+ concentrations, the epsilon-NH3+ group of a second Lys+ molecule may bind to the carrier forming a complex, carrier-/2Lys+, which is not transported.