DECREASED PULSATILE LH-SECRETION IN HEIFERS SUPEROVULATED WITH ECG ORFSH

This study was designed to test the hypothesis that treatment with sup erovulatory drugs suppresses endogenous pulsatile LH secretion. Heifer s (n=5/group) were superovulated with eCG (2500 IU) or FSH (equivalent to 400 mg NIH-FSH-P1), starting on Day 10 of the estrous cycle, and w ere injected with prostaglandin F-2 alpha on Day 12 to induce luteolys is. Control cows were injected only with prostaglandin. Frequent blood samples were taken during luteolysis (6 to 14 h after PG administrati on) for assay of plasma LH, estradiol, progesterone, testosterone and androstenedione. The LH pulse frequency in eCG-treated cows was signif icantly lower than that in control cows (2.4 +/- 0.4 & 6.4 +/- 0.4 pul ses/8 h, respectively; P<0.05), and plasma progesterone (3.4 +/- 0.4 v s 1.8 +/- 0.1 ng/ml, for treated and control heifers, respectively; P< 0.05) and estradiol concentrations (25.9 +/- 4.3 & 4.3 +/- 0.4 pg/ml, for treated and control heifers, respectively; P<0.05) were higher com pared with those of the controls. No LH pulses were detected in FSH-tr eated cows, and mean LH concentrations were significantly lower than t hose in the controls (0.3 +/- 0.1 and 0.8 +/- 0.1, respectively; P<0.0 5). This suppression of LH was associated with an increase in estradio l (9.5 +/- 1.4 pg/ml; P<0.05 compared with controls) but not in proges terone concentrations (2.1 +/- 0.2 ng/ml; P>0.05 compared to controls) . Both superovulatory protocols increased the ovulation rate (21.6 +/- 3.9 and 23.0 +/- 4.2, for eCG and FSH groups, respectively; P>0.05). These data demonstrate that super-ovulatory treatments decrease LH pul se frequency during the follicular phase of the treatment cycle. This could be explained by increased steroid secretion in the eCG-treated h eifers but not in FSH-treated animals.