EFFECT OF BACILLUS-THURINGIENSIS TOXINS ON THE MEMBRANE-POTENTIAL OF LEPIDOPTERAN INSECT MIDGUT CELLS

To test whether the ability of Bacillus thuringiensis toxins to form p ores in the midgut epithelial cell membrane of susceptible insects cor relates with their in vivo toxicity, we measured the effects of differ ent toxins on the electrical potential of the apical membrane df fresh ly isolated midguts from gypsy moth (Lymantria dispar) and silkworm (B ombyx mori) larvae. In the absence of toxin, the membrane potential, m easured with a conventional glass microelectrode, was stable for up to 30 min. It was sensitive to the K+ concentration and the oxygenation of the external medium, Addition of toxins to which L. dispar is highl y [CryIA(a) and CryIA(b)] or only slightly [CryIA(c) and CryIC] sensit ive caused a rapid, irreversible, and dose-dependent depolarization of the membrane, CryIF, whose toxicity towards L. dispar is unknown, and CryIE, which is at best poorly active in vivo, were also active in vi tro. In contrast, CryIB and CryIIIA, a coleopteran-specific toxin, had no significant effect. The basolateral-membrane potential was unaffec ted by CryIA(a) or CryIC when the toxin was applied to the basal side of the epithelium. In B. mori midguts, the apical-membrane potential w as abolished by CryIA(a), to which silkworm larvae are susceptible, bu t CryIA(b) and CryIA(c), to which they are resistant, had no detectabl e effect. Although the technique discriminated between active and inac tive toxins, the concentration required to produce a given effect vari ed much less extensively than the sensitivity of gypsy moth larvae, su ggesting that additional factors influence the toxins' level of toxici ty in vivo.