Ion regulation in the larval lepidopteran midgut and the response to Bacillus thuringiensis delta -endotoxin

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 super(+) 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 super(+) flux from lumen to haemolymph is electrical. In contrast to K super(+), the H super(+) chemical gradient is exceedingly large, with luminal pH values of 11-12 and haemolymph/lumen H super(+) ratios as high as 10 super(5). At equilibrium, the steep proton gradient is consistent with a passive distribution of H super(+) across the midgut epithelium as predicted from the Nernst equation. In B mori larvae, ingestion of a lethal dose of Bt delta -endotoxin produces an increase in K super(+) 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 super(+) and pH and a decline in luminal pH. Midgut K super(+) activity, however, remains unchanged. An important factor in the pathogenesis of Bt is irreversible alkalization of the epithelial cells as H super(+) is redistributed across the midgut to reach a new Nernst equilibrium.