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 Bombyx mori were performed immediately after the fourth ecdysis. A clear dose-response relationship in the frass-failure assay was recorded after 48 h for topically treated silkworms, whereas the oral treatment caused the maximal decrease of frass production even at the lowest dose (2.5 fg/larva). Preincubation of midgut brush border membrane vesicles from untreated larvae with variable amounts of FC reduced leucine uptake in vesicles from the anterior-middle midgut region, suggesting a toxic effect on the plasma membrane, 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 μg reduced leucine transport. Conversely, the forced ingestion of the same doses always caused 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 presence of an unidentified component (X), which decreases following FC treatment. Interestingly, after treatment, the increases of saturated or unsaturated fatty acids is balanced by the loss of the X component. The results presented here suggest a multiple mode of action of fenoxycarb, involving a hormonal-like stimulation of leucine absorption with low-dose topical applications or as a long-term result of oral applications and a modification of brush border membrane lipid composition, which both affect amino acid absorption in the larval midgut.