LH/hCG-binding sites were measured in crude membrane fractions of porcine uteri. Specific high affinity and low capacity receptors for LH/hCG were found in all (n = 17) membrane preparations of myometrium but in only 5 of 17 crude membrane fractions of endometrium of porcine uteri. There was very little competition between hCG and porcine GH (pGH), bovine TSH, pFSH, and pPRL (0.5%, 0.3%, 0.2%, and less than 0.005%, respectively). Specificity of [125I]hCG binding to other tissues was determined by incubating crude membrane preparations of heart, skeletal muscle, liver, and kidney. Numbers and affinities of available LH/hCG-binding sites were characterized in all samples of myometrium and 5 endometrium membrane preparations that were positive for LH/hCG receptors. The results indicate that the number of uterine LH-binding sites in myometrium (0.66 +/- 0.17 fmol/mg) is 10 times less than the receptor capacity in porcine corpora lutea (7.46 +/- 0.54 fmol/mg) when expressed per mg protein of crude membrane preparation. However, it is approximately 60 times less when expressed per mg DNA equivalent of initial homogenate (1.31 +/- 0.28 vs. 81.18 +/- 3.64 fmol/mg, respectively). Receptor affinities of uterine LH/hCG-binding sites remained comparable to those of corpora lutea receptors (Ka = 7.8 X 10(10) M-1). Concentrations of LH/hCG-binding sites in myometrium taken from gilts in the late follicular phase of the estrous cycle (0.13 +/- 0.06 fmol/mg protein; n = 5) were significantly less (P less than 0.05 and P less than 0.01) compared to those in myometrium from luteal phase (0.85 +/- 0.22 fmol/mg protein; n = 6) or early pregnancy (1.03 +/- 0.15 fmol/mg protein; n = 6), respectively. This is probably the first evidence demonstrating specific binding of [125I]hCG by LH receptors in female uterine tissue.

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Short-term feed restriction in prepubertal gilts suppresses episodic LH secretion in the absence of changes in body weight or composition. To assess non-gonadotropin-mediated effects of realimentation at the ovarian level, 52 gilts were assigned to six treatments after 7 days (Days 1-7) of maintenance feeding (approximately 30% ad libitum). Groups R12 and R9 were maintenance-fed Days 8-12 or Days 8-9, respectively; A12 and A9 were fed to appetite Days 8-12 or Days 8-9, respectively. Groups R9P and A9P were fed as groups R9 and A9 were but received 750 IU eCG at 1500 h on Day 8. Groups R12 and A12 were ovariectomized at 1500 h on Day 12, and all other groups were ovariectomized at 1500 h on Day 9. All gilts received oral progestogen (15 mg allyl trenbolone) from Day 1 to ovariectomy, to antagonize the usual increases in endogenous gonadotropins that follow realimentation. Blood samples were obtained at 10-min intervals during selected windows during the experiment. Ovarian follicles were analyzed for development and steroidogenesis, and plasma samples were analyzed by RIA to determine concentrations of LH, FSH, insulin, and insulin-like growth factor-1 (IGF-1). Allyl trenbolone abolished pulsatile LH secretion, and realimentation did not stimulate LH or FSH secretion, with the exception of FSH secretion on Day 8 in A9 gilts. Postprandial insulin concentrations on Day 9 were greater after feeding to appetite (A9, A9P, and A12) than after feed restriction (R9, R9P, and R12). Pre- and postprandial IGF-1 concentrations were higher in re-fed gilts on Day 9 (A9 and A12) and Day 12 (A12) than in feed-restricted gilts. Follicular diameter, fluid volume, and basal granulosa cell estradiol synthesis per follicle were greater in A12 gilts than in R12 gilts, although there was no difference between A9 and R9 gilts. There was no effect of realimentation on follicular fluid concentrations of estradiol or testosterone, or on androgen-driven granulosa cell estradiol synthesis. Treatment with eCG increased follicular diameter, fluid volume, basal and androgen-driven estradiol synthesis, and fluid estradiol concentrations without interaction with feeding level. In conclusion, in the absence of LH elevations, realimentation over 5 days exerts effects at the ovary, increasing follicular growth and estradiol synthesis. These effects may be mediated by insulin, IGF-1, or unmeasured growth factors and would be expected to synergize with increases in endogenous gonadotropin that follow realimentation.

The current experiment was conducted with beef cows during the first 2 weeks postpartum (PP) to determine the effects of suckling and low-level increases of systemic progesterone on secretory characteristics of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in peripheral plasma. Variables measured included mean gonadotropin concentrations, FSH/LH pulse frequencies, pulse amplitudes and synchrony of coincident release. Suckled (S) cows had lower (P less than 0.01-P less than 0.05) mean concentrations of FSH and LH in plasma, lower (P less than .05) FSH and LH pulse frequencies and a lower (P less than 0.05) pulse synchrony (21.6 vs 72.3% coincident pulses) than nonsuckled (NS) cows on Day 7 PP. Neither FSH nor LH pulse amplitude was affected by suckling. Similar differences existed for mean gonadotropin concentrations, pulse frequencies and pulse synchrony on Day 14 PP between S and NS cows. Implanting cows with progesterone implants on Day 7 PP, which chronically increased plasma progesterone to 0.5-0.6 ng/ml, increased (P less than 0.05) mean plasma FSH and LH concentrations, FSH and LH pulse frequencies and pulse synchrony (87.5 vs. 66.3%) in NS-implanted (NSI) versus NS-nonimplanted (NSNI) cows. Progesterone implants had no beneficial effect in S cows. Thus, three major findings seem pertinent: 1) associated with the suckling-induced depression of episodic gonadotropin release was a marked decline in FSH/LH pulse synchrony; 2) a high degree of FSH/LH pulse synchrony (72-88%) was restored in the absence of suckling when gonadotropin pulse frequency increased to 4-5/6 h; 3) the absence of suckling, followed by the provision of low-level progesterone stimulation for 7 days, appeared to have additive effects on FSH and LH secretion. These results provide evidence that the neuroendocrine block associated with suckling in early PP beef cows is, in addition to being associated with depressed LH release, comprised of characteristic anomalies in FSH secretion, FSH/LH pulse synchrony and failure to respond to a putative hypothalamo-hypophyseal potentiator, progesterone.