Effect of repeated sampling on concentrations of testosterone,LH and prolactin in blood of yearling angus bulls

Blood was collected at intervals of 29 to 31 min for 5 hr from six Angus bulls (15 mo of age) unaccustomed to capture, restraint and jugular venipuncture (stress) to evaluate temporal changes in certain hormones. Concentrations of testosterone and luteinizing hormone (LH) but not prolactin were decreased significantly after the first hour. Testosterone in plasma decreased (P < .01) about 11-fold between 0 hr and 5 hr (9.9 ± 1.7 to .85 ± .16 ng/ml) as described by equation log_e testosterone = log_e 2.4649 ? .5266 hr (r = .83; P < .01). Concentrations of LH in plasma remained low after the first hour and those of prolactin were high at all times and varied significantly only among bulls (27 ± 6 to 84 ± 14 ng/ml). Testosterone but not LH was measured with equal repeatability among duplicate measurements either in whole blood or plasma but its average concentration in whole blood was 66% that of plasma. This study demonstrated that sequential collection of blood from bulls unaccustomed to capture and restraint cannot be used to evaluate normal temporal variations in concentrations of testosterone.