Modulation of leucine absorption in the larval midgut of Bombyx mori (Lepidoptera, Bombycidae)

In the larval midgut of Bombyx mori a K+-dependent transporter for leucine and amino acids with a hydrophobic side chain is responsible for the absorp tion of most essential amino acids. We investigated if a modulation of its activity occurred as a result of starvation or after hormonal treatments. W e measured amino acid uptake in brush border membrane vesicles (BBMV) purif ied from the anterior-middle (AM) and posterior (P) regions of the midgut i n fifth instar larvae. Silkworms were either starved or topically treated w ith low dosages of fenoxycarb, a molecule often used as a juvenile hormone mimic. The maximal uptake value of K+-driven leucine transport was increase d in BBMV of AM- and P-midgut regions of starved larvae. The initial uptake rates of serine and glutamine, two amino acids transported by the same cot ransporter as leucine, were also increased. Leucine kinetics proved that V- max was the kinetic parameter modified by starvation in both midgut regions . Topical applications of fenoxycarb at a dose of 2.5 fg/larva immediately after the fourth ecdysis, induced an increase of leucine initial uptake rat es and of intravesicular accumulation of leucine in both AM- and P-BBMV. Ki netic analysis of leucine uptake indicated again that V-max was increased i n BBMV from both midgut regions in treated larvae. (C) 2001 Elsevier Scienc e Inc. All rights reserved.