Toxicokinetics of toluene in the rat

Toxicokinetic experiments with toluene that rely on data pooled from several animals have typically provided estimates of rate constants based on single exponential functions; in the present study a within-subject design was used to describe the time course of toluene in the blood to see if additional exponential terms could be resolved. Each rat was exposed for 2 h to either 1 000, 1 780, or 3 000 ppm of toluene. Blood samples were drawn at different times from indwelling jugular cannulae, and blood toluene concentrations were determined by vapor phase equilibrium chromatography. There was substantial variability between the peak blood values of the animals at each exposure concentration. Nonlinear regression was used to determine regression parameters for data drawn from individual animals. A bi-exponential function was found to minimize significantly the residual sum of squares observed during elimination. However, only a single exponential function could be resolved reliably during uptake. The mean half-time during uptake was approximately 34 min; the two half-times for elimination averaged approximately 6 and 90 min. Because of between-subject variability, a more accurate description of the time course of toluene uptake and elimination can be provided by kinetic studies that use a within-subject design.