Metabolic clearance and production rates of testosterone at different times of the year in male caribou and reindeer

The metabolic clearance rate (MCR) of testosterone at different times of the year in male caribou and reindeer was determined by calculating the volume of plasma cleared of a single injection of tritiated testosterone per unit time. The MCR was then used to estimate the daily production rate, from the arithmetic product of the clearance rate and the plasma testosterone concentration before the injection. During midrut, plasma testosterone levels of 13–22 ng/ml and MCRs of 601–782 ℓ/day gave production rates of 10–13 mg/day. During the nonreproductive period, low plasma testosterone levels (0.4–0.6 ng/ml) and higher clearance rates (1142–2535 ℓ/day) gave production rates of 0.6–1.4 mg/day. The unexpectedly low MCRs that were obtained during the rut were associated with lower volumes of distribution (v1 and v1 + v2), which suggests that the binding capacity of the plasma is higher at this lime. Since only the unbound fraction is available for entry into the cells, information about the binding capacity of the plasma is required before the MCR can be evaluated in terms of the uptake and metabolism of testosterone by the tissues.