Free intracellular calcium ([CA2+]i) in opioid sensitive cells of the porcine anterior pituitary

A new experimental model was utilized to study calcium involvement in the mechanism of opioid influence on cultured porcine pituitary cells. The in vitro model involved interactive argon laser cytometry of pituitary cells pre-loaded by three dyes (fluo-3AM, fura-red and naloxone-conjugated to fluorescein). We compared: 1) the kinetics of free intracellular calcium ([Ca2+]i) in anterior pituitary cells of pregnant pigs (day 25-30) treated in vitro with naloxone (NAL) or gonadotrophin-releasing hormone (GnRH) and 2) the distribution of the opioid-sensitive cells by image analysis of doubly loaded cells. In experiment 1, the changes in [Ca2+]i of pituitary cells pre-loaded with fluo-3 AM (488(ex)/520(em) nm) in response to NAL (10(-6) M) or to GnRH (10(-8) M) were compared to a control cell group. Repetitive line scans across cells were performed and the fluorescence emission from individually selected cells was measured in a time-dependent manner (in 0.5 seconds intervals during periods of 50 seconds). Analysis of data indicated significant increases of [Ca2+]i in NAL- (P<0.001) and GnRH-treated cells (P<0.05) in comparison to the control group. In experiment 2, the fluorescence intensity of doubly-loaded pituitary cells (fura-red, 488(ex)/605(em) nm, as principal calcium indicator and NAL-conjugated with fluorescein, 488(ex)/520(em) nm, to distinguish opioid-sensitive cells) were measured using dual detector image analysis. We found that only approximately 8% of the entire population of anterior pituitary cells exhibited sensitivity to the opioid antagonist treatment. This paper demonstrates calcium involvement in the opioid action on anterior pituitary cells from pregnant pigs and provides a useful model for studies at the individual pituitary cell level and in time-dependent manner.

Prediction of fat-free mass of pigs from 50 to 130 kilograms live weight

Seventy-two Duroc x Hampshire x Yorkshire pigs were used to evaluate bioelectrical impedance procedures to predict fat-free mass of live pigs. Pigs were allotted by sex, ancestry, and weight. Pigs (12 gilts and 12 barrows) averaging 50+/-2.4 kg were slaughtered to establish a baseline for body composition. A pen of six gilts and a pen of six barrows were randomly selected for slaughter when the pen averaged either 70, 90, 110, or 130 kg. Pigs were weighed, then a four-terminal plethysmograph was used to measure resistance (omega) and reactance (omega), and length (cm) was measured between detector terminals. Pigs were slaughtered 12 h later, and carcasses were chilled for 24 h. The right side was ground twice and mixed and samples were frozen for later analyses of fat content. Actual fat-free mass (ActFFM) was determined from the weights and percentage of fat. Predicted fat-free mass (PredFFM) was calculated using the following equation: Pred FFM = .486 (live weight) - .881 (resistance) + .48 (length) + .86 (reactance) + 7.959. The correlation coefficients between ActFFM and PredFFM ranged from .66 to .91 overall. Correlation coefficients approximating slaughter weight (90 kg) were .94 (P < .02). Fat-free mass was underestimated by the prediction equation at all slaughter weights, but the predicted fat-free mass was highly correlated to the actual fat-free mass, except for the 110-kg gilts (r = .68, P = .15) and the 130-kg barrows (r = .65, P = .16). The data support the use of bioelectrical impedance to measure fat-free mass over a wide range of weights for finishing pigs.

The effects of post-weaning progestagen treatment (Regumate) of early-weaned primiparous sows on subsequent reproductive performance

This study investigated the effects of feeding the orally active progestagen, altrenogest (Regumate) post-weaning on the subsequent reproductive performance of early weaned sows. Ninety (90) Large White/Landrace first parity sows were randomly assigned to three treatments. Treatment 1 (EW) and treatment 3 (CW) sows were weaned on day 12 and day 24 post-partum, respectively while treatment 2 sows (EW-R) were weaned on day 12 post-partum and received an individual daily dose of 20 mg of Regumate on days 13 to 24 post-partum inclusive. Each sow was mated naturally at least twice at the first post-weaning or post-treatment oestrus and slaughtered on days 25-28 of pregnancy to determine the number of corpora lutea and embryos. Regumate-to-oestrus and weaning-to-oestrus intervals were similar for EW-R and CW sows (6.2 vs. 5.6 days). However, both intervals were significantly shorter (P < 0.01) than the weaning-to-oestrus interval of EW sows (7.3 days). An excellent synchronization of oestrus was achieved with Regumate treatment with 97% of treated sows in oestrus within 7 days of Regumate withdrawal compared with 64% for EW sows (P < 0.01) and 87% for CW sows (P > 0.05). Treatment with Regumate resulted in a significant increase in ovulation rate (16.9 vs. 15.4 and 14.9 for treatments EW-R, EW and CW, respectively; P < 0.05) and a non-significant increase in early embryonic survival (77% vs. 68% vs. 68% for treatments EW-R, EW and CW, respectively; P > 0.05). These results indicate that Regumate feeding is a potential management tool to alleviate the diminished reproductive performance associated with early weaning regimes since it leads to successful control of oestrus, higher ovulation and embryo survival rates and thus a greater potential litter size.

Myometrial LH/hCG receptors during the estrous cycle and pregnancy in pigs

Receptors for luteinizing hormone/human chorionic gonadotropin (LH/hCG) have been identified in porcine, rabbit, rat, and human myometrium. To determine the estrous cycle and pregnancy related changes in the receptor capacity and affinity, radioreceptor assays were performed with membrane homogenates of porcine uterine tissues. Cycling gilts were divided into four experimental groups: I (n = 6), day 1-2; II (n = 5), day 6-7; III (n = 5), day 11-12; and IV (n = 6), day 18-20 of the estrous cycle. Pregnant pigs were divided into three experimental groups: I (n = 5), day 35-40; II (n = 5), day 65-70; and III (n = 4), day 95-105 of pregnancy. The concentrations [femtomoles/mg protein (fmol/mg protein)] and affinities of unoccupied LH/hCG binding sites were characterized in all samples of myometrium. Receptor concentrations were highest (P < 0.01) in groups II and III (19.3 +/- 2.5 and 35.8 +/- 2.1 fmol/mg protein, respectively), and was lowest in groups I and IV (5.3 +/- 1.4 and 7.5 +/- 0.7 fmol/mg protein, respectively). Receptor affinity constants (Ka) were consistent (P > 0.05) throughout the estrous cycle [I, (5.1 +/- 1.5) x 10(9); II, (3.0 +/- 0.8) x 10(9); III, (3.2 +/- 0.9) x 10(9); IV, 5.5 +/- 0.7 x 10(9) 1m-1]. Plasma hormone concentrations of progesterone, estrogen and LH were typical of values noted at these times. During pregnancy, receptor concentrations were greatest (P < 0.05) in group II (85.4 +/- 18.5 fmol/mg protein). In groups I and III receptor numbers were 10.8 +/- 2.3 and 26.7 +/- 6.6 fmol/mg protein, respectively. The Ka in group I was 10 times greater (P < 0.05) than Ka in groups II and III, (I, 3.1 +/- 0.9 x 10(10) lm-1; II, 3.4 +/- 0.3 x 10(9) lm-1; III, 3.3 +/- 1.1 x 10(9) lm-1). Plasma hormone concentrations typically found during pregnancy were noted. The function of these LH/hCG binding sites remains unknown; however, changes in receptor capacity during the estrous cycle and pregnancy support a role for modulation of the receptor by hormonal factors.

Gonadotropin secretion in ovariectomized Chinese Meishan and hybrid large white gilts; responses to challenges with estradiol benzoate, gonadotropin-releasing hormone, or porcine follicular fluid

Castrated Meishan (MS) and hybrid Large White (hLW) gilts were used in a series of challenge experiments to establish differences in the control of gonadotropin secretion. Acutely ovariectomized gilts (72 h) were injected i.m. with 10 micrograms/kg estradiol benzoate (EB) and serially bled for evaluation of estradiol clearance from the blood and patterns of LH and FSH release. Estradiol was more rapidly metabolized from the circulatory system of the acutely ovariectomized MS gilts than from the hLW gilts; as a consequence, hLW gilts had higher concentrations of estradiol than MS gilts at 24-32 h (p < 0.05) and 48-60 h (p < 0.01) post-EB treatment. The hLW but not the MS gilts exhibited a classic negative/positive LH response to EB; MS gilts responded with a random release of large episodes of LH. Plasma FSH concentrations were consistently greater (p < 0.05) in acutely ovariectomized MS gilts than in hLW gilts. Both breeds demonstrated a slight negative FSH response to EB and then exhibited increased variability in plasma FSH with time after treatment. Mean plasma LH was immediately elevated in chronically ovariectomized (> 14 days) hLW and MS gilts given 1 microgram/kg of GnRH i.m., but the magnitude of the LH response was greater (p < 0.01) in the hLW gilts. Comparable differences (p < 0.01) in FSH secretion were also observed, indicating the presence of greater pools of releasable FSH and LH in the hLW gilts, or a greater sensitivity to GnRH.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolactin as a luteotrophin during late pregnancy in pigs

The effect of prolonged hyperprolactinaemia on the secretion of LH, progesterone and oestradiol, and its relationship to the maintenance of pregnancy was examined in pigs. Twelve crossbred, pregnant gilts were injected i.m. with 1.5 mg haloperidol kg-1 body weight (n = 6) or vehicle (n = 6) once a day from day 60 to day 66 of pregnancy. Blood samples were collected at 08:00, 12:00, 16:00, 20:00, 24:00 h from day 60 to day 67 and every 15 min for 4 h (08:00-12:00 h) on days 60, 63 and 66. Plasma concentrations of prolactin were higher (P < 0.001) in haloperidol-treated gilts than in control gilts (121.3 +/- 4.3 ng ml-1 and 13.6 +/- 0.4 ng ml-1, respectively). Hyperprolactinaemia completely inhibited the pulsatile secretion of LH and diminished (P < 0.001) basal peripheral concentrations of LH (hyperprolactinaemia, 0.3 +/- 0.04 ng ml-1 and control, 0.6 +/- 0.005 ng ml-1). Despite the inhibition of LH release in hyperprolactinaemic gilts, plasma concentrations of progesterone were higher (P < 0.001) than in the control group (20.8 +/- 0.6 and 12.6 +/- 0.2 ng ml-1, respectively). Oestradiol concentrations were not different between groups, although oestradiol tended to be higher in hyperprolactinaemic gilts than in the control group throughout the sampling period (29.1 +/- 1.9 versus 23.7 +/- 1.6 pg ml-1, respectively). Abortion did not occur in any of the gilts. These results are the first to demonstrate that induced hyperprolactinaemia during the second half of pregnancy (days 60-66) will drastically suppress the major porcine luteotrophin but not affect pregnancy maintenance in pigs.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparisons of endocrinology and behavioural events during the periovulatory period in Meishan and large-white hybrid gilts

Attainment of puberty, cycle lengths, ovulation rate and endocrinology during the periovulatory period were studied in Meishan (MS) and European Large-White hybrid (LW) gilts. The mean age at onset of puberty of 115 days in MS (n = 20) gilts was younger (P < 0.001) than the 235 days in LW (n = 23). In the MS population studied, ovulation rate was not different (P > 0.1) during the third and fourth oestrous cycles, nor were there differences (P > 0.1) in the mean cycle length over the first three cycles. Overall changes in plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH) and oestradiol did not differ significantly (P > 0.1) between the breeds (MS, n = 6; LW, n = 5) during the periovulatory period, but plasma inhibin concentrations were significantly (P < 0.05) higher in the MS. The time intervals from the oestradiol peak concentration and the onset of the LH surge until the onset of behavioural oestrus were significantly different (P < 0.005) between the breeds, with oestrus occurring earlier in the MS. However, no difference (P > 0.1) was found between the groups when the intervals from the peak oestradiol concentration to the onset of the LH surge were compared. These results indicate differences between the breeds, particularly in terms of the age of attainment of puberty and the timing of the onset of behavioural oestrus relative to the oestradiol and LH surges.

Comparison of follicular heterogeneity and ovarian characteristics in Meishan and large-white hybrid pigs

Comparisons were made between characteristics of pre-ovulatory follicles recovered from prolific Chinese Meishan gilts (n = 12) and from European Large-White hybrid gilts (n = 13) in the late follicular phase preceding their fifth oestrous cycle, to determine whether there is an ovarian basis for the enhanced prolificacy in the Meishan. A total of 177 follicles per breed was classified as pre-ovulatory, based on follicular fluid oestradiol concentrations. Results obtained demonstrated high variability in all follicular characteristics in both breeds and no decrease in heterogeneity was evident in the Meishan. The Meishan follicles tended to be smaller (P < 0.06) and had less follicular fluid (P < 0.005), but total oestradiol content per follicle was similar (P < 0.1) with the result that the concentration of oestradiol in follicular fluid tended to be higher (P < 0.06) in Meishan than Large-White hybrid pigs. There were no differences between breeds in terms of testosterone concentrations in follicular fluid, hCG binding to granulosa cells or total DNA content of granulosa cells. Concentrations of inhibin in follicular fluid were similar in both breeds (P > 0.1) which resulted in a trend towards less total inhibin content in Meishan than Large-White hybrid follicles (P = 0.065). Corpora lutea were recovered from both breeds (n = 12 per breed) on days 27-31 of pregnancy after mating at first, second and third oestrus:corpora lutea were smaller (P < 0.001) and contained less progesterone per corpus luteum in the Meishan (P < 0.05) than in Large White hybrid pigs.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of opioids on LH secretion in gilts during the estrous cycle

The effect of endogenous opioid peptides (EOP) on LH secretion is variable during different physiological states. A series of experiments concerning the role of EOP on LH secretion in cyclic gilts was performed. They were comprised of (1) an administration of an opioid antagonist or agonist in gilts during the estrous cycle and in ovariectomized (OVX) gilts in which the LH surge was induced with estradiol benzoate (EB) and (2) in vitro studies on GnRH release from the stalk median eminence (SME) of cyclic gilts and OVX estrogen and progesterone primed gilts in response to naloxone (NAL). Naloxone and met-enkephalin analogue (FK 33-824) administration as a single independent injections did not affect LH secretion during the early (Day 16) or late (Day 19 or 20) follicular phase. However, continuous infusion of FK 33-824 for 4 h decreased LH secretion during the infusion period on Day 19 of the estrous cycle. Morphine also exerts an inhibitory effect on the EB-induced LH surge during the positive feedback phase (60-64 h after EB administration) in OVX gilts. On the contrary, NAL infusion in OVX gilts during the negative feedback phase (30-34 h after EB administration) did not alter LH secretion. A single injection of FK 33-824 in luteal phase gilts decreased the number of LH pulses for a 3 h period. This allows to hypothesize that EOP participates in the regulation of pulsatile LH secretion in pigs during the luteal phase. In vitro studies indicate that influence of EOP on LH secretion also takes place at the SME level. GnRH efflux from the SME of gilts during the luteal and late follicular phases was augmented in the presence of NAL. Unexpectedly, the priming of OVX gilts with estrogens caused the highest increase in GnRH release from the SME in vitro in response to NAL. These results confirm the variety of functional links between the opioid system and LH secretion in gilts during different stages of the estrous cycle.

Nutrition-endocrine interrelationships in the lactating and weaned sow

Previous studies had identified the effects of nutrient restriction on delayed occurrence of estrus when body condition is low at farrowing or levels of intake fall below 2.5 kg (8.35 Mcal ME/h/d) during lactation. It appears that the failure via the GnRH pulse generator to stimulate sufficient LH pulsatility to trigger steroidogenesis may play a major role in the lack of reinitiation of cyclicity. Base-line concentrations, because of their variability between individual animals, may not be a major factor in predicting the time to return to estrus. Prolactin appears to be definitely antigonadotropic during lactation but may be involved in folliculogenesis effects during the postweaning period. Previous results would indicate that metabolic factors in concert with the major reproductive hormones play a key role in reestablishing the cycle postweaning. To achieve a greater understanding of the complexities of these two systems--nutrition and reproduction-greater emphasis must be placed on more precisely regulating the integral factors modulating the return to cyclic behavior.

Gonadotropin-independent mechanisms participate in ovarian responses to realimentation in feed-restricted prepubertal gilts

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.

Ovarian steroid secretion changes after hCG stimulation in early pregnant pigs

Administration of human chorionic gonadotropin (hCG) to promote ovarian steroid secretion near the time of recognition of pregnancy was evaluated. Neither 500 or 1000 IU of hCG caused a significant increase in luteal function as determined by progesterone (P(4)) concentrations in peripheral blood following treatment on Day 12. Estradiol concentrations were elevated (P<0.01) for the 500 IU hCG group on Days 13, 14, 15 and 16 versus the control group. The 1000 IU of hCG group had three-to five-fold greater (P<0.01) estradiol concentrations than controls on Days 14, 15 and 16 post mating. Treatment with hCG also reduced (P<0.05) the number of resorbed embryos. The results suggest that hCG treatment on Day 12 of pregnancy reduced embryo loss and influenced peripheral estradiol secretion patterns.

Effect of human chorionic gonadotropin on preovulatory luteinizing hormone surge and ovarian hormone secretion in gilts

The influence of varying doses of human chorionic gonadotropin (hCG) on the preovulatory luteinizing hormone (LH) surge, estradiol-17 beta (E2) and progesterone (P4) was studied in synchronized gilts. Altrenogest (AT) was fed (15 mg X head-1 X d-1) to 24 cyclic gilts for 14 d. Pregnant mares serum gonadotropin (PMSG; 750 IU) was given im on the last day of AT feeding. The gilts were then assigned to one of four groups (n = 6): saline (I), 500 IU hCG (II), 1,000 IU hCG (III) and 1,500 IU hCG (IV). Human chorionic gonadotropin or saline was injected im 72 h after PMSG. No differences in ovulation rate or time from last feeding of AT to occurrence of estrus were observed. All gilts in Groups I and II expressed a preovulatory LH surge compared with only four of six and three of six in Groups III and IV, respectively. All groups treated with hCG showed a rapid drop (P less than .01) in plasma levels of E2 11, 17, 23 h after hCG injection when compared with the control group (35 h). The hCG-treated gilts exhibited elevated P4 concentrations 12 h earlier than the control group (3.1 +/- .5, 3.4 +/- .72, 3.1 +/- .10 ng/ml in groups II, III and IV at 60 h post-hCG vs .9 +/- .08 ng/ml in group I; P less than .05). These studies demonstrate that injections of ovulatory doses of hCG (500 to 1,500 IU) had three distinct effects on events concomitant with occurrence of estrus in gilts: decreased secretion of E2 immediately after hCG administration, failure to observe a preovulatory LH surge in some treated animals and earlier production of P4 by newly developed corpora lutea.

Evidence for the presence of luteinizing hormone/human chorionic gonadotropin-binding sites in the porcine uterus

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.

Prolactin administration during the luteal and follicular phase of the oestrous cycle of gilts

In Exp. I infusions of prolactin (0.5 mg in 2 ml sterile saline) were repeated every 2 h for 36 h on Days 12-13 of the cycle. In Exp. II infusions of prolactin were administered from Days 17 to 19 (60 h) at 2-h intervals. Control gilts were given 2 ml sterile saline at similar intervals during the same period. Basal prolactin concentrations before initiation of infusions ranged from 1.3 +/- 0.1 to 5.6 +/- 2.2 ng/ml in both experiments. By 5 min after a prolactin infusion, mean plasma prolactin concentration ranged from 74.9 +/- 5.8 to 113.0 +/- 9.5 ng/ml, but then declined to approximately equal to 10 ng/ml just before the next infusion of prolactin. Administration of prolactin during the luteal phase of the oestrous cycle of the gilts had no effect on basal levels of progesterone, oestradiol or LH. During the follicular phase there were no differences (P greater than 0.05) between control and prolactin-treated gilt progesterone and LH concentrations, but oestradiol plasma values were decreased (P less than 0.05) on the 2nd and 3rd day of prolactin treatment. Our results would indicate that prolactin does not play a major role in the regulation of the oestrous cycle of the pig.

Utero-ovarian venous concentrations of prostaglandin E2 (PGE2) and prostaglandin F2 alpha (PGF2 alpha) following PGE2 intrauterine infusions

The uterine horns and utero-ovarian veins of nine crossbred mature gilts were bilaterally cannulated on day 9 of the estrous cycle (day O - first day of estrus). Each uterine horn in treated gilts (N = 5) was infused with 150 micrograms PGE2 in 3 ml of saline at 0900 h on day 12, 15 and 18 of the estrous cycle. Control gilts (N = 4) received 3 ml saline intrauterine infusions on the corresponding day. Blood samples were collected from the utero-ovarian veins 15 min before each infusion and for the following 6 h with 15, 30 and 60 min intervals through the first, second and third two-hour periods, respectively. Venous concentrations of PGE2 and PGF2 alpha were determined by radioimmunoassay procedures. Infusion of PGE2 resulted in an immediate elevation in PGE2 concentration in utero-ovarian venous drainage. Coincident elevations of PGF2 alpha utero-ovarian venous concentrations were observed after PGE2 infusion. Plasma PGF2 alpha concentrations in the utero-ovarian veins were elevated (P less than .01) in PGE2 treated gilts for one hour post-treatment. The duration of PGE2 and PGF2 alpha elevations as well as the peak values were influenced by day of the cycle.

The effect of repeated mating at short intervals on reproductive performance of gilts

Three trials involving 87 gilts were completed to investigate the role of additional sexual stimulation on various reproductive parameters. Forty-five gilts were mated twice at 12-h intervals at second pubertal estrus with intact boars beginning 12 h after detection of sexual receptivity (control group). Forty-two gilts were exposed to a vasectomized boar 15 min following each fertile mating to provide additional copulatory stimulus (treatment group). This additional exposure improved (P less than .01) conception rate (100 vs 84%). No statistical differences were observed in number of corpora lutea (CL), number of viable embryos (VE) or number of total embryos (TE) at d 30. These results suggest that additional mating stimuli may have a beneficial effect on conception rate. While the use of additional males resulted in more pregnant females, it likely did not have an effect on litter size.

Evidence against chronic teat stimulation as an autonomous effector of diminished gonadotropin release in beef cows

The current experiment was performed to examine the acute and cumulative effects of chronic manual teat stimulation on the tonic pattern of luteinizing hormone (LH) secretion in beef cows. Additionally, we characterized the plasma profile of prolactin and cortisol release to evaluate whether changes in the concentrations of these hormones occurred in response to teat stimulation and whether such changes were related to gonadotropin secretion. Six weeks after ovariectomy, nonlactating beef females were paired by age and assigned randomly to a control group (n = 3) or a stimulation group (n = 3). Stimulated cows were subjected to 20 min of continuous manual teat stripping every 4 h for 24 h. Blood samples were collected from all cows at 10-min intervals beginning 1 h before and continuing for 2 h after the onset of each stimulation period in treated animals. Numerous episodes of prolactin and cortisol release were observed in control and treated animals throughout the 24-h experiment. The percentage of stimulations accompanied by prolactin and cortisol releases for each of the three treated animals was 100 and 100, 16.7 and 50, and 50 and 100, respectively. The number of prolactin peaks observed the hour after onset of teat stimulation was greater (P less than .06) than the number observed the hour before. However, the number of cortisol peaks was not statistically related to teat stimulation. Overall, mean concentrations of prolactin and cortisol were not increased by teat stimulation. Luteinizing hormone pulse frequency (1.6 +/- .1 pulses/h) and mean LH concentrations (12.1 +/- .6 ng/ml) were not acutely or chronically affected by teat stimulation and were not related to prolactin or cortisol release. We conclude that mechanical stimulation of the teat is not singly effective in altering the pattern or quantity of tonic LH release in ovariectomized cows.

The effect of intrauterine infusions of prostaglandin E2 on luteal function in nonpregnant gilts

Two trials were conducted to study the effects of intrauterine infusions of prostaglandin E2 (PGE2) on luteal function in nonpregnant gilts. Cannulae were surgically implanted on day 9 postestrus into the lumen of each horn with a cephalic vein cannula inserted for collection of peripheral blood. Intrauterine infusions of 0, 25, 75 or 200 microg of PGE2 were initiated at 0900 h on day 12 and administered thereafter every 12 hr until estrus or day 22 in the first trial. The second trial protocol included an increase in the dose of PGE2 administered as well as the frequency of infusion. Infusion of 0, 200, 300 or 400 microg PGE2 was begun at 0300 h on day 12 and continued every 6 hr until estrus or day 22. Cephalic plasma samples for progesterone analysis were collected every six hours from 0300 h on day 11 to 2100 h on day 26 in both trials. In Trial 1 mean plasma progesterone concentrations for all treatments were not different (P>0.05) from the controls on any given day of the estrous cycle. Interestrous interval was unaffected by intrauterine infusion of PGE2. The mean plasma progesterone concentrations for all treatments were not different (P>0.05) from the controls on days 11-18 of the estrous cycle in Trial 2. However, plasma progesterone concentrations for the 200-microg and 300-microg PGE2 groups appeared to be greater than the controls on days 14 and 15, indicating a possible delay in the decline of progesterone for these groups. The mean plasma progesterone concentrations for the treatment groups were lower (P<0.05) than the controls on days 20-26 of the cycle. treatment cycle length did not differ (P>0.05) from previous cycle length; thus treatment with PGE2 had no effect on interestrous interval. PGE2 may have retarded the decline of progesterone secretion by the corpus luteum in some cases, but at these dosages and frequencies of administration PGE2 was ineffective in prolonging luteal maintenance.

Coincident secretion of follicle-stimulating hormone and luteinizing hormone in early postpartum beef cows: effects of suckling and low-level increases of systemic progesterone

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.

Pituitary and ovarian responses of postpartum dairy cows to progesterone priming and single or double injections of gonadotropin-releasing hormone

Utilizing single or double pulses of gonadotropin-releasing hormone (GnRH), with or without progesterone pretreatment, we induced ovulation in dairy cows on day 14 postpartum. In experiment 1, neither progesterone priming nor repetitive injection of GnRH enhanced pituitary LH or FSH secretion compared to a single GnRH injection. However, pretreatment with 100 mg progesterone tended (P<0.1) to enhance luteal progesterone secretion during the induced cycle. We confirmed this observation in a second experiment by utilizing a larger number of cows. Cows given 100 mg progesterone prior to a single 200 microg injection of GnRH exhibited higher (P<0.05) concentrations of serum progesterone on days 12 and 16 of the induced cycle (days 26 and 30 postpartum). These results suggest that progesterone pretreatment may influence luteal progesterone secretion following ovulation. This appears to occur via an ovarian mechanism which is independent of pituitary gonadotropin secretion.

Time of the preovulatory LH surge in the gilt and sow relative to the onset of behavioral estrus

Two experiments were conducted to study the time of occurrence of the preovulatory LH surge in pigs. Sampling every ten minutes in six cycling gilts before and after onset of standing estrus revealed the preovulatory surge began from 8 hr before to 12 hr after the lordosis reflex was elicited. Three of six gilts initiated the preovulatory LH release coincident with the onset of estrus. Data from 28 postpartum sows, with samples drawn every six hours commencing with the onset of estrus, indicated maximum LH levels were present at the first observance of estrus. Six of the 28 sows had an LH peak 18-24 hr after the onset of estrus.

Porcine LH levels during the estrous cycle, gestation, parturition and early lactation

Thirty-five sows were used in a study of plasma luteinizing hormone (LH) concentrations during the estrous cycle, including first and second estrus postweaning, during gestation, at parturition and during the first 5 d of lactation. After cannulation of the anterior vena cava via the cephalic vein, blood samples were collected four times daily for 2 d before estrus, during estrus and for 3 d after estrus and once daily at 1100 h throughout the rest of the estrous cycle. One sample was collected from each sow on alternate days (1100 h) during the gestation period. Four daily samples were obtained on d 60 through 68 of gestation and for 5 d after parturition. Preovulatory surges of LH occurred 8 to 32 h before standing estrus in four of seven sows during first estrus and in three of seven sows during second estrus. The mean preovulatory LH surge during the first estrus postweaning (3.00 +/- .46 ng/ml) was lower than that during second estrus (4.24 +/- .60 ng/ml). Throughout the first 12 d of gestation, LH concentrations were relatively high and variable, ranging from 1.2 to 2 ng/ml. They then decreased to a range of .42 to .62 ng/ml after d 24. Similar LH concentrations were noted throughout the remainder of gestation except for some slight fluctuations. Higher concentrations (P less than .05) were noted with evening collections (1900 h, 1:17 +/- .09 ng/ml) than in daytime collections (0700 h, .64 +/- .08: 1100 h, .61 +/- .06; 1500 h, .72 +/- .06 ng/ml) during d 60 to 68 of gestation. Low plasma concentrations of LH (.3 to .5 ng/ml) were observed for the 5 d before and during parturition. After parturition, considerable variation of LH occurred, with rising concentrations observed on d 5 postpartum. Results of this study illustrate the occurrence of LH surges before behavioral estrus with increased plasma LH during implantation.

Effect of suckling on pituitary responsiveness to gonadotropin-releasing hormone throughout the early postpartum period of beef cows

The objective of this study was to compare pituitary responsiveness to gonadotropin-releasing hormone (GnRH) throughout the early postpartum period (PP) in beef cows suckling in single calf vs cows which had calves removed at birth. The experiment utilized 48 cows in a 2 X 4 factorial. Main effects studied were suckling status (suckled or nonsuckled) and days PP (d 3, 10, 20 or 30). Covariates studied were pre-treatment concentrations of progesterone (P), estradiol-17 beta (E), luteinizing hormone (LH) and follicle stimulating hormone (FSH). Suckled (S) cows nursed one calf, while nonsuckled (NS) cows had calves removed at birth. Treatment was a single im injection of 100 micrograms GnRH dissolved in saline. Pre-GnRH plasma concentrations of E, P, LH and FSH and post-treatment plasma concentrations of LH and FSH were determined at 15 min intervals (-30 to 360 min) in blood samples collected via jugular cannulae. Post-treatment plasma concentrations of P were determined at 6 (n = 23) or 8 (n = 23) days after treatment to monitor ovarian response. Concentrations of E, LH and FSH before GnRH injection did not differ (P greater than .1) between S and NS cows at any PP stage and data were pooled for statistical analyses. Pre-treatment LH, but not FSH, increased (r = .43; P less than .002 (with days PP and concentrations of LH were greater (P less than .05) at 30 d than at 3 d PP. Pre-treatment P was greater (P less than .05) in NS than in S cows on d 30 PP and P was correlated (r = .51; P less than .01) with days PP in NS cows. However, the frequency of cows exhibiting plasma P greater than or equal to 1 ng/ml prior to GnRH was low on all days studied and was similar for both groups. LH release after GnRH was correlated with increasing days PP in both S (r = .73; P less than .001) and NS (r = .61; P less than .002) cows, Releasable FSH was not related to days PP in either group. Mean peak LH and total LH released were greater (P less than .025 and P less than .05), respectively) in S than in NS cows at 30 d PP. Mean peak FSH and total FSH released were greater (P less than .05 and P less than .07, respectively) at 10 and 30 d PP in S cows than in NS cows. Pre-treatment E, but no P, LH or FSH was positively related (P less than .01) to GnRH-mediated LH release in both groups. When pre-treatment E was partitioned into separated regressions for S (ES) and NS (ENS) cows, increased pituitary LH response in S cows on d 30 appeared to be due in part to a greater sensitivity of S cows to E modulation. We found no evidence to suggest that pituitary responsiveness per se is decreased in beef cows suckling a single calf.

Effect of mating on the luteinizing hormone surge in the pig

Twenty-eight cannulated crossbred gilts were assigned to one of four treatments: natural mating, artificial insemination (AI, whole semen), AI (seminal plasma) or control (nonmated). Blood samples were collected four times daily (0800, 1200, 1600 and 2000 hr) on the day before standing estrus, the day of the onset of standing estrus and the first day after the onset of standing estrus. Thereafter, blood samples were collected once daily (1200 hr) until 15 days postestrus. Analysis of luteinizing hormone (LH) in the plasma revealed similar concentrations for all groups on the day of estrus, but higher (P less than .05) values for naturally mated gilts the following morning. Mean concentrations of LH on the first day after the onset of standing estrus were higher (P less than .01) for the mated gilts (2.75 +/- .54 ng/ml) than for gilts in the AI whole semen (.89 +/- .10 ng/ml), AI seminal plasma (1.02 +/- .13 ng/ml) and control groups (.92 +/- .11 ng/ml). Concentration of LH in the plasma of pregnant gilts tended to be higher after day 9. These results suggest that coitus may prolong the LH surge.