The effects of benomyl and its breakdown products carbendazim and butyl isocyanate on the ultrastructure and function of tracheal ciliated cells

The effects of benomyl (methyl-1-(butylcarbamoyl) benzimidazole 2yl-carbamate) and its breakdown products carbendazim (methyl-2-benzimidazole carbamate) and butyl isocyanate on the ciliary function and ultrastructure of canine ciliated tracheal epithelial cells were examined. Ciliary beat frequencies from cultured ciliated cells in the tissue outgrowth were measured with an optical spectrum analysis system. A decrease in ciliary frequency described the extent to which these cells were affected by the test compounds over time. Serial dilutions of the test compounds were prepared in 100 percent corn oil and applied to the ciliated cell cultures over 90 minute or 6 hour exposure times with frequencies obtained at 15 minute or 1 hour intervals. Subsequent to exposures, the cell cultures were prepared for examination with a transmission electron microscope.

Benomyl and butyl isocyanate caused dose-dependent decreases in ciliary beat frequencies of tracheal epithelial cells within ninety minutes of exposures. Benomyl at a concentration of 300 μg/ml (3 mM) caused ciliostasis within seventy-five minutes exposure. Butyl isocyanate at a molar concentration three times lower than benomyl (1 mM, 300 μg/ml) caused a similar response although, within thirty minutes. The frequency data obtained from across the ranges of concentrations tested were used to calculate the ED₅₀ concentration (effective dose which caused a fifty percent drop in ciliary frequency) for both compounds. The ED₅₀ for benomyl was 0.75 mM, while that for butyl isocyanate was 0.52 mM. Carbendazim did not cause statistically significant decreases in frequency over ninety minute exposures. However, with exposures up to 6 hours, moderate decreases were measured with statistical significance seen at 3 hours exposure.

Benomyl exposure caused damage to mitochondria of ciliated epithelial cells which was seen as moderate to severe swelling of mitochondria, disruption of inner mitochondrial membranes, and loss of mitochondrial matrix. Other cellular organelles appeared normal, although a slight proliferation of smooth vesicles was suggested. Butyl isocyanate exposure did not cause the same mitochondrial damage in ciliated epithelial cells as seen with benomyl exposure. Mitochondria, as well as other cellular organelles, appeared normal.

Although ciliostatic responses were seen with both benomyl and butyl isocyanate, the sites and/or modes of action of these two compounds may be different. The toxic effects of benomyl to ciliated tracheal epithelial cells may possibly be attributed primarily to butyl isocyanate and secondarily to carbendazim after benomyl enters the cell and breaks down to its toxic components.