Effect of fenoxycarb on leucine uptake and lipid composition of midgut brush border membrane in the silkworm, Bombyx mori (Lepidoptera, Bombycidae)

Topical or oral applications of fenoxycarb (FC) to fifth instar larvae of B ombyx mori were performed immediately after the fourth ecdysis. A clear don e-response relationship in the frass-failure assay was recorded after 48 h for topically treated silkworms, whereas the oral treatment caused the maxi mal decrease of frass production even at the lowest dose (2.5 fg/larva). Pr eincubation of midgut brush border membrane vesicles from untreated larvae with variable amounts of FC reduced leucine uptake in vesicles from the ant erior-middle midgut region, suggesting a toxic effect on the plasma membran e, absent in vesicles from the posterior region. The topical application of 2.5 fg of FC in vivo caused a significant increase of K+-dependent leucine uptake and accumulation into membrane vesicles of both the anterior-middle and posterior midgut regions, whereas application of 2.5 mug reduced leuci ne transport. Conversely. the forced ingestion of the same doses always cau sed an increase in leucine uptake and accumulation in both midgut regions. The brush border membrane composition is affected by both oral and topical treatment, but membranes from the posterior midgut region are less affected , especially by the oral treatment. Regardless of the considered gut region , the analysis of the fatty acid composition of the membrane revealed the p resence of an unidentified component (X), which decreases following FC trea tment. interestingly, after treatment, the increases of saturated or unsatu rated fatty acids is balanced by the loss of the X component. The results p resented here suggest a multiple mode of action of fenoxycarb. involving a hormonal-like stimulation of leucine absorption with low-dose topical appli cations or as a long-term result of oral applications and a modification of brush border membrane lipid composition. which both affect amino acid abso rption in the larval midgut. (C) 2001 Academic Press.