Estradiol and testosterone metabolic clearance and production rates during puberal development in boars

Changes in metabolic clearance rate (MCR) and production rate (PR) of estradiol-17 beta (E2) and testosterone (T) were evaluated in crossbred boars averaging 35 +/- 1, 91 +/- 3, 118 +/- 4 and 177 +/- 6 kg at 80, 160, 260 and 560 days of age, respectively. A comparison of E2 and T MCR determined in blood or plasma was made in castrate and intact boars at 180 days of age. In the two experiments, estimates of MCR of E2 and T were consistently greater (22.0 and 23.8%) in blood than in plasma. These differences were not influenced by age of boar or castration. The MCR (l X day X kg BW) for E2 and T in plasma was greater (P less than 0.05) for 80-day-old prepuberal boars than the three groups of older boars. Production rates of E2 and T were lower in boars at 80 days of age than in older boars. Thus, a reduction in the MCR and an increase in PR of E2 and T in the boar are involved in the increased concentrations of circulating steroids associated with puberal development. Difference in MCR, determined in blood and plasma for both E2 and T, suggests that the contribution of the cellular component of blood to MCR studies in pigs should not be ignored.