Cotransport of 2-methyl-aminoisobutyric acid and chloride in rabbit small intestine.

This article examines the nature of the Cl- dependence of the rabbit intestinal imino acid carrier using 2-methyl-aminoisobutyric acid (MeAIB) as substrate for measuring influx across the luminal membrane of rabbit distal ileum. Reduction of mucosal Cl- concentration from 140 to 6 mM successively increased the concentration at which MeAIB influx is half-maximal from 3.6 +/- 0.2 (SD) to 21.0 +/- 3.7 mM. Maximal influx of MeAIB was unchanged. Measured at 70 mM Cl-, MeAIB induced a Cl- influx across the brush-border membrane (delta JClmc) of 5.01 +/- 0.65 mumol.cm-2 x h-1 (SE, n = 16; 45%), and an identical Cl(-)-independent influx of leucine induced a delta JClmc of 1.82 +/- 0.52 mumol.cm-2 x h-1 (SE, n = 12; 16%) (P < 0.001). At 24 mM Cl-, delta JClmc induced by leucine did not differ significantly from zero, whereas the effect of MeAIB was maintained. The ratio between Na+ influx across brush-border membrane (delta JNamc) and the Na(+)-dependent flux measured at 140 mM Cl- was 2.4 +/- 0.7 (n = 8) for MeAIB, 1.4 +/- 0.4 (n = 10) for leucine, and 1.1 +/- 0.2 (n = 13) for D-glucose. The Hill coefficient of Na+ activation (+/- SD) in distal ileum was 0.82 +/- 0.19 for leucine and 1.14 +/- 0.03 for D-glucose. A Hill coefficient of 1.60 +/- 0.05 was obtained for D-glucose in the jejunum. Concentrations of MeAIB and D-glucose, chosen to given equal delta JNamc, induced a 40 and 21% increment of JClmc (P < 0.05), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)