The amino acid/K + symporters for neutral amino acids along the midgut of lepidopteran larvae: Functional differentiations

Abstract

The functional properties of the K +-dependent symporter for neutral amino acids have been investigated in brush border membrane vesicles prepared from the anterior, middle and posterior portions corresponding to the three morphologically distinguishable regions of the midgut of Bombyx mori larvae. An intravesicular accumulation of leucine was driven by a K +-gradient in the three preparations, but vesicles from the posterior tract displayed much higher uptake and accumulation values. Kinetic analysis of leucine uptake, performed in experimental conditions which mimic as closely as feasible experimentally those occurring in vivo (Δ ψ = −90 mV, pH in7.2 pH out8.7 , [K +] out100 mM), evidenced that the affinity for the amino acid was similar along the midgut (150 μM), but V max in the posterior region was more than 11-fold higher than that of the anterior-middle tract (11.3 ± 0.7 and 0.98 ± 0.07 nmol/7s/mg protein, respectively). Leucine uptake was remarkably influenced by extravesicular pH and by Δ ψ only in vesicles from the posterior midgut: a lowering of pH to 7.2 caused a sevenfold increase of K m , whereas in the absence of Δ ψ, V max decreased threefold. The selectivity sequence for the alkali cations was somewhat different in the two midgut regions, but K + remained the most effective. In the posterior midgut, the selectivity for K + was greatly enhanced when a transmembrane electrical potential was present. Leucine kinetics as a function of external potassium concentration was hyperbolic in the posterior and sigmoidal in the anterior-middle part. Inhibition of leucine uptake induced by a 20-fold excess of different amino acids suggested the presence in both midgut tracts of a broad specificity system for neutral amino acids, with many-but not all-features in common with the B o system of mammal intestinal and renal epithelial brush borders. However, there are differences between the two midgut regions as regard to the ability of the symporters to recognize the different amino acids, which concern the side chain and the presence of the aromatic ring. Altogether these data suggest that two kinds of symporters for neutral amino acids, with different functional properties, are expressed in the anterior-middle and posterior regions of the lepidopteran midgut.