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 dependent. 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 similar to 0.3 mM. The function of this transport system is pH insensitive between 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 transporter for which the inhibitor constant (K-i) of L-Glu against Ser decreases from 54 to 10 mM when pH is reduced from 7.2 to 5.65, while the maximum rate of transport remains unaffected at similar to 10 mu mol.cm(-2).h(-1). The K-i values (5 mM) of Ser against L-Glu influx and the L-Glu-sensitive contribution 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 pH 7.2. The remaining 50% can be ascribed to system B. Transport of Ser by system B is reduced by >95% at pH 5.65. At pH 7.2 K-i of Ser against transport of leucine (Leu) by system B is 18 mM and K-i of Leu against transport of Ser is 1.7 mM. The low-affinity transport of L-Glu and the L-Glu-sensitive transport of Ser are performed by an equivalent of system ASC. Supplementary experiments using the jejunum confirm the validity of these results for a major portion of the rabbit small intestine.