Fluorometric assay of potential change of Bombyx mori midgut brush border membrane induced by delta-endotoxin from Bacillus thuringiensis

Bacillus thuringiensis δ-endotoxin, CryIA(a), increased ion permeability of brush border membrane vesicles isolated from midgut epithelia of Bombyx mori larvae. This ion permeability change was measured with a potential-sensitive fluorescent dye, 3,3′-dipropylthiadicarbocyanine iodide. This effect was observed at concentrations of the toxin that correspond to normal effective doses in vivo. CryIA(b) and heat-treat CryIA(a) did not show this effect. CryIA(a) did not show this effect on rat renal brush border membranes. The effects depended on the toxin dose in saturable manner. These suggest that this assay system reflects the mode of action of δ-endotoxin in vivo. The toxin increased various ion permeabilities, suggesting that δ-endotoxin forms non-selective cation pores on brush border membranes.