BIOACTIVE AND IMMUNOREACTIVE CONCENTRATIONS OF CIRCULATING LUTEINIZING-HORMONE DURING SEXUAL-MATURATION IN THE BOVINE

The characteristics of circulating LH during sexual maturation in cattle were assessed by examining bioactive and immunoreactive LH concentrations, as well as their ratio (B/I ratio). Male and female intact control (CONT), gonadectomized (GNX; at 241 +/- 3 days of age, Day 0 of the study), and gonadectomized animals administered 17-beta-estradiol (GNXE) were evaluated. Serum samples were collected at 15-min intervals for 24 h at 1, 7, 13, 17, 21, 25, 29, 33, 37, and 43 wk subsequent to Day 0. Bioactive LH was assessed with an in vitro bioassay using mouse testicular interstitial cells. In initial experiments, immunoreactive LH was quantified in RIAs using three different antibodies. The two RIAs employing polyclonal antibodies overestimated low LH concentrations, but the absolute values obtained in each of the three assays were highly correlated. Hence, immunoreactive LH was measured in an RIA using monoclonal anti-bovine LH (bLH) (JR-518B7). No significant changes in the B/I ratios were observed during individual pulses of LH secretion. Accordingly, pools consisting of equal volumes of the serial blood samples collected during the 24-h period for each animal at each stage of maturation (pools) were compared. LH B/I ratios for GNX females increased significantly with time (p < 0.01) and the B/I ratios for GNX males were significantly higher than for GNX females (p < 0.05). Concentrations of LH in most of the pools for GNXE and CONT animals were extremely low or nondetectable until the later bleeding periods. Therefore, a second set of pools, consisting of equal volumes of blood samples known to contain pulses of LH secretion for each animal at each stage of maturation (pools of pulses), was also analyzed. However, no major differences were observed in LH B/I ratios in the pools of pulses during maturation or between the six experimental groups. Thus, the changes in circulatory LH concentrations during sexual maturation and in response to GNX and GNXE were similar in bovine males and females. Furthermore, the secretory patterns of bioactive and immunoactive LH were concordant during bovine sexual development.