Effects of pH changes on systems ASC and B in rabbit ileum

Influx of D-aspartate (D-Asp), L-glutamate (L-Glu), and serine (Ser) across the brush-border membrane of the intact mucosa from rabbit ileum has been examined. L-Glu influx is chloride independent and completely sodium depend ent. D-Asp and L-Glu share a transport system with a maximum transport rate of 1 mu mol.cm(-2).h(-1) and an apparent affinity constant (K-1/2) of simi lar to 0.3 mM. The function of this transport system is pH insensitive betw een pH 5.65 and 8.2, and bipolar amino acids do not affect the way in which the transport system handles D-Asp and L-Glu. The characteristics of this transport system match those of system X-AG(-). L-Glu and Ser share a trans porter for which the inhibitor constant (K-i) of L-Glu against Ser decrease s from 54 to 10 mM when pH is reduced from 7.2 to 5.65, while the maximum r ate of transport remains unaffected at similar to 10 mu mol.cm(-2).h(-1). T he K-i values (5 mM) of Ser against L-Glu influx and the L-Glu-sensitive co ntribution to Ser influx (0.8 mu mol.cm(-2).h(-1) at 1 mM Ser) are the same at both pH values. The L-Glu-sensitive transport of Ser together with the contribution of system b(o,+) account for similar to 50% of Ser influx at p H 7.2. The remaining 50% can be ascribed to system B. Transport of Ser by s ystem B is reduced by >95% at pH 5.65. At pH 7.2 K-i of Ser against transpo rt of leucine (Leu) by system B is 18 mM and K-i of Leu against transport o f Ser is 1.7 mM. The low-affinity transport of L-Glu and the L-Glu-sensitiv e transport of Ser are performed by an equivalent of system ASC. Supplement ary experiments using the jejunum confirm the validity of these results for a major portion of the rabbit small intestine.