EFFECT OF THE INCREASE OF STEROID BINDING PLASMA-LEVELS AFTER PASSIVE-IMMUNIZATION AGAINST TESTOSTERONE ON THE CONTROL OF LUTEINIZING-HORMONE (LH) SECRETION IN OVARIECTOMIZED UNDERFED DAIRY HEIFERS

The ability of passive immunization against testosterone to increase sex steroid binding levels in plasma and thus to overcome the negative feedback of oestradiol-17-beta (E2) on LH secretion in underfed heifers was investigated. Dairy heifers were ovariectomized and divided in 3 groups: high energy diet (H group, n = 4), low energy diet (L group, n = 3) and low energy diet + E2 implants (LE2 group, n = 4). Twenty-four h before injection of bovine immunoglobulins, the mean concentrations of LH were not different between H and L groups. LH baseline was lower (0.8 vs 1.1 ng/ml, P < 0.03) and the median number of LH pulses was higher (10 vs 5, P < 0.03) in H than in L group. E2 markedly decreased (P < 0.01) the mean and basal concentrations of LH (0.27 ng/ml), and number of LH pulses (0) in the LE2 group (P < 0.05). After injection of anti-testosterone immunoglobulins in the L group, mean and basal LH concentrations tended to decrease. The median number of LH pulses in the L group rose 8 days after immunization (5 vs 7 on day -1 and day +8, P less-than-or-equal-to 0.05). Amplitude of pulses tended to decrease after injection (P = 0.08). In the LE2 group, the mean concentration and baseline of LH were not affected by passive immunization against testosterone, while pulses of LH appeared at day +1 and rose (P = 0.07) at day +8 after immunization with 3.5 pulses. Thus passive immunization against testosterone increased sex steroid binding levels in plasma of underfed heifers and reduced the amount of E2 and/or non-ovarian steroids available for negative feedback effects on LH secretion.