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

Abstract

Influx ofd-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 andl-Glu share a transport system with a maximum transport rate of 1 μmol ⋅ cm−2 ⋅ h−1and an apparent affinity constant ( K ½) of ∼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 handlesd-Asp andl-Glu. The characteristics of this transport system match those of system[Formula: see text].l-Glu and Ser share a transporter for which the inhibitor constant ( K i) ofl-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 ∼10 μmol ⋅ 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 μmol ⋅ cm−2 ⋅ h−1at 1 mM Ser) are the same at both pH values. Thel-Glu-sensitive transport of Ser together with the contribution of system bo,+ account for ∼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 ofl-Glu and thel-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.