To evaluate the roles of FSH and LH in follicular growth, GnRH-immunized anestrous heifers (n = 17) were randomly assigned (Day 0) to one of three groups (n = 5 or 6). Group 1 received i.m. injections of 1.5 mg porcine FSH (pFSH) 4 times/day for 2 days; group 2 received i.v. injections of 150 μg pLH 6 times/day for 6 days; group 3 received both pFSH and pLH as described for groups 1 and 2. After slaughter on Day 6, measurements were made of follicle number and size, and follicular fluid concentrations of progesterone (P4), estradiol (E2), and aromatase activity. Injection of pFSH increased (P < 0.01) the serum concentrations of FSH between 12 and 54 h. Infusion of pLH increased (P < 0.05) mean and basal concentrations of LH and LH pulse frequency. Serum E2 concentrations were higher (P < 0.05) for heifers given pFSH pLH than those given either pFSH or pLH alone. There was no difference (P ≥ 0.24) between treatments in the number of small follicles (<5 mm). Heifers given pFSH or pFSH pLH had more (P ≤ 0.02) medium follicles (5.0–9.5 mm) than those that were given pLH alone (none present). Heifers given pFSH pLH had more (P = 0.04) large follicles (≥10 mm) than those given either pLH or pFSH alone (none present). Overall, only 1 of 35 small follicles and 2 of 96 medium follicles were E2-active (i.e., E2:P4 >1.0), whereas 18 of 21 large follicles (all in the pFSH pLH treatment) were E2-active; of these, 8 of 18 had aromatase activity. Concentrations of E2 and E2 activity in follicular fluid were correlated (r ≥ 0.57; P < 0.0001) with aromatase activity in heifers given pLH pFSH. In conclusion, pLH failed to stimulate follicle growth greater than 5 mm; pFSH stimulated growth of medium follicles that were E2-inactive at slaughter and failed to increase serum E2 concentrations; whereas pFSH pLH stimulated growth of medium follicles and E2-active large follicles, and a 10- to 14-fold increase in serum E2 concentrations.
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