Ion regulation in the larval lepidopteran midgut and the response toBacillus thuringiensisδ-endotoxin

Abstract

Fourth‐instar Bombyx mori (silkworm) was used as a model insect to study the effects of Bacillus thuringiensis (Bt) on ion homeostasis in the larval lepidopteran midgut. The K+ chemical gradient across the midgut of B. mori larvae is quite small, sustained by a lumen/haemolymph activity ratio of only 1.6. More than 95% of the driving force causing K+ flux from lumen to haemolymph is electrical. In contrast to K+, the H+ chemical gradient is exceedingly large, with luminal pH values of 11–12 and haemolymph/lumen H+ ratios as high as 105. At equilibrium, the steep proton gradient is consistent with a passive distribution of H+ across the midgut epithelium as predicted from the Nernst equation. In B mori larvae, ingestion of a lethal dose of Bt δ‐endotoxin produces an increase in K+ conductance in the midgut apical epithelium, causing a decrease in the electrical gradient and dissipation of the pH gradient. Larval morbidity can be correlated with a rise in haemolymph K+ and pH and a decline in luminal pH. Midgut K+ activity, however, remains unchanged. An important factor in the pathogenesis of Bt is irreversible alkalization of the epithelial cells as H+ is redistributed across the midgut to reach a new Nernst equilibrium.