Physiologically based modeling of vinyl acetate uptake, metabolism, and intracellular pH changes in the rat nasal cavity

Chronic inhalation exposure to vinyl acetate (VA) causes lesions in the nasal cavity of the rat. This effect appears to be related to tissue exposure to either acetaldehyde (AAld) or acetic acid (AA) metabolites of VA or both. A physiologically based pharmacokinetic model was constructed to describe the deposition of VA in the nasal cavity of the rat and provide estimates of regional tissue exposure to VA, AAld, and AA. Since formation of AA in the nasal tissue should cause intracellular acidification, a submodel which describes free intracellular hydrogen ion concentration and intracellular pH (pH(i)) changes was linked to the VA model. The dosimetry model was applied to data from a series of experiments designed to measure the uptake and metabolism of VA in the isolated upper respiratory tract of the rat at exposure concentrations ranging from 73 to 2190 ppm. Extraction of VA from the nasal cavity was nonlinear with respect to exposure concentration and ranged from 36 to 94%, with the greatest deposition occurring at the lowest VA concentrations. Pretreatment with bis(p-nitrophenyl)phosphate, an inhibitor of carboxylesterases, significantly reduced fractional deposition of VA compared to naive rats exposed to similar VA concentrations. The best model fits for VA extraction and AAld appearance were achieved when a second carboxylesterase isozyme, with high-affinity characteristics, was included. Simulations of 6-h inhalation exposures to VA predicted that the order of nasal tissue exposures will be to AA > AAld > VA. In addition, based on measured tissue hydrolysis rates, sufficient acid should be formed by the metabolism of VA to cause significant changes in pH(i). VA exposures of 200 and 600 ppm were predicted to result in a pH(i) of less than 7.2 and 6.7, respectively. This model provides nasal dosimetry estimates needed to develop mechanistically based risk assessment approaches for human exposures to VA vapor. (C) 1997 Academic Press.