Ionic permeabilities induced by Bacillus thuringiensis in Sf9 cells.

The effect of Bacillus thuringiensis insecticidal toxins on the monovalent cation content and intracellular pH (pHi) of individual Sf9 cells of the lepidopteran species Spodoptera frugiperda (fall armyworm) was monitored with the fluorescent indicators potassium-binding benzofuran isophthalate (PBFI) and 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF). The sequential removal of K+ and Na+ from the medium, in the presence of CryIC, a toxin which is highly active against Sf9 cells, caused sharp shifts in the fluorescence ratio of PBFI, demonstrating a rapid efflux of these ions. In Sf9 cells, pHi depends strongly on the activity of a K+/H+ exchanger. In the absence of toxin, removal of K+ from the external medium resulted in a reversible acidification of the cells. In the presence of CryIC, pHi equilibrated rapidly with that of the bathing solution. This effect was both time- and concentration-dependent. In contrast with CryIC, CryIIIA, a coleopteran-specific toxin, and CryIA(a), CryIA(b) and CryIA(c), toxins which are either inactive or poorly active against Sf9 cells, had no detectable effect on pHi. B. thuringiensis endotoxins thus appear to act specifically by increasing the permeability of the cytoplasmic membrane of susceptible cells to at least H+, K+ and Na+.