Secretory dynamics of bioactive and immunoactive LH during the oestrous cycle of the sheep

Blood samples were collected every 15 min for 6 h during the follicular (1 day before oestrus), and early (Days +1 to +3), mid- (Days +4 to +8), and full (Days +9 to +14) luteal phases of the oestrous cycle. Serum concentrations of immunoactive LH were measured by radioimmunoassay. The biological activity of serum LH was determined by an in-vitro bioassay that uses LH-induced testosterone production from mouse interstitial cells as an endpoint. Only ovine and bovine LH and hCG had appreciable activity in this bioassay. The temporal pattern of secretion of bioactive LH paralleled the secretory pattern of immunoactive LH at all stages of the ovine oestrous cycle. However, the secretory pattern itself varied regularly through the oestrous cycle. The frequency of secretory excursions of LH was highest during the follicular phase (6.2 +/- 0.9 pulses/6 h) and was progressively reduced through the luteal phase (1.1 +/- 0.1 pulses/6 h during full luteal phase). Conversely, amplitude of secretory excursions of immunoactive LH was low during the follicular phase (0.79 +/- 0.08 ng/ml) and significantly (P less than 0.05) increased during the mid- and full luteal phases (1.49 +/- 0.10 and 2.37 +/- 0.20 ng/ml, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)     

Sex differences in acute luteinizing hormone responses to gonadectomy remain after progesterone antagonist and dopamine agonist treatment.

In male rats, LH pulse frequency and amplitude increase dramatically by 24 h after gonadectomy; in females they increase only slightly by this time. Mean FSH levels increase significantly in both sexes by 24 h after gonadectomy. The objectives of the present studies were to compare pulsatile LH, FSH, and prolactin (PRL) secretion in intact versus gonadectomized and in male versus female rats, and to determine whether the acute postovariectomy lag in LH rise is due to a lingering effect of the higher PRL and/or progesterone (P) levels seen in intact females. LH pulse amplitude, frequency, and mean levels increased significantly by 24 h after gonadectomy in both sexes, but the increases were greater in the males. FSH mean levels, but not pulse amplitude or frequency, increased similarly in both sexes by 24 h after gonadectomy. PRL did not change with gonadectomy. Treatment with CB-154 (a dopamine agonist), with or without RU486 (a P antagonist), 1 h before gonadectomy significantly suppressed pulsatile PRL secretion 1 day later in both sexes. There was no effect of either treatment on LH secretion. We have demonstrated that there is a sex difference in LH, but not FSH or PRL, pulsatility at 24 h after gonadectomy, and that female rats' higher PRL and P levels do not account for their slow rate of LH rise after ovariectomy.     

The effects of intracerebroventricular infusion of prolactin in luteinizing hormone, testosterone and growth hormone secretion in male sheep

This study tested a hypothesis that the enhancement of the prolactin (PRL) concentration within the central nervous system (CNS) disturbs pulsatile luteinizing hormone (LH) and growth hormone (GH) secretion in rams that are in the natural breeding season. A 3h long intracerebroventricular (icv.) infusion of ovine PRL (50 microg/100 microl/h) was made in six rams during the daily period characterized by low PRL secretion in this species (from 12:00 to 15:00 h); the other six animals received control infusions during the same time. Blood samples were collected from 9:00 to 18:00 h at 10 min intervals. A clear daily pattern of LH secretion was shown in control animals, with the lowest concentration at noon and an increasing basal level around the time of sunset (P < 0.001). No significant changes in LH concentration occurred in PRL-infused animals and the concentration noted after infusion of PRL was significantly (P < 0.05) lower than after the control infusion. The frequency of LH pulses tended to decrease in rams after PRL treatment. The changes in LH secretion clearly carried over to the secretion of testosterone in the rams of both groups. The GH concentrations changed throughout the experiment in both groups of rams, being higher after the infusions (P < 0.001). However, the mean GH concentration and GH pulse amplitude noted after PRL infusion were significantly lower (P < 0.001 and P < 0.05, respectively) from those recorded in the control. The continued fall in PRL secretion observed in rams following PRL infusion (P < 0.05 to P < 0.001) indicates a high degree of effectiveness of exogenous PRL at the level of the CNS. In conclusion, maintenance of an elevated PRL concentration within the CNS leads to disturbances in the neuroendocrine mechanisms responsible for pulsatile LH and GH secretion in sexually active rams.     

Influence of gonadotropin-releasing hormone pulse amplitude, frequency, and treatment duration on the regulation of luteinizing hormone (LH) subunit messenger ribonucleic acids and LH secretion

The effects of GnRH pulse amplitude, frequency, and treatment duration on pituitary alpha and LH beta subunit mRNA concentrations were examined in castrate-testosterone replaced male rats. Experimental groups received iv GnRH pulses (5, 25, or 125 ng) at 7.5-, 30-, or 120-min intervals for 8, 24, or 48 h. Saline pulses were given to control rats. Acute LH secretion was measured in blood drawn before and 20 min after the last GnRH pulse. In saline controls, alpha and LH beta mRNAs (150 +/- 14, 23 +/- 2 pg cDNA bound/100 micrograms pituitary DNA) fell to 129 +/- 14 and 18 +/- 2, respectively, after 48 h. In animals receiving GnRH pulses (7.5-min intervals), the 125-ng dose stimulated a slight increase (P less than 0.01) in alpha mRNA levels after 8 and 24 h and both LH subunit mRNAs were increased by the 25- and 125-ng doses after 48 h. The 30-min pulse interval injections (25- and 125-ng doses) increased LH beta mRNA levels after 8 h, but alpha mRNAs were not elevated until after 24 h. Maximum (3-fold) increases in alpha and LH beta mRNAs were seen in rats receiving 25-ng pulses every 30 min for 48 h. Using 120-min pulses, LH subunit mRNAs were not increased by any GnRH dose through 48 h. Acute LH release was not seen in rats receiving 5 ng GnRH pulses at any pulse interval.(ABSTRACT TRUNCATED AT 250 WORDS)     

Induction of precocious puberty in ewe lambs by pulsatile administration of GnRH

Circhoral administration (250 ng/h, i.v.) of GnRH induced a preovulatory-like surge of LH and subsequent luteal function in 4 of 4 ewe lambs 1 month before expected date of puberty. Within 12h of the start of pulsatile delivery of GnRH, mean concentrations of immunoactive and bioactive LH increased significantly (P less than 0.05) and the LH surge occurred by 1.8 +/- 0.6 days of treatment. Mean concentrations of serum progesterone were elevated significantly (P less than 0.001) 3 days after the surge. The biopotency of LH (bioactive LH/immunoactive LH) before the GnRH-induced surge of LH did not differ from LH biopotency in ewe lambs receiving circhoral delivery of saline (0.41 +/- 0.05 and 0.46 +/- 0.04, respectively). Biopotency of LH declined markedly at the GnRH-induced LH surge (0.25 +/- 0.04), but biopotency of serum LH was significantly augmented (P less than 0.05) during the period of luteal activity (0.70 +/- 0.07). Regular oestrous cycles were observed in 3 of 4 ewe lambs after the 10-day GnRH treatment period. These results indicate that pulsatile delivery of GnRH is effective in inducing precocious puberty in ewe lambs. Increase in LH biopotency does not appear to be required in the pubertal transition to reproductive cyclicity in this species. Augmented LH biopotency may be important in support of luteal function after first ovulation.     

Seasonally Dependent Effect of Ectopic Pituitary Grafts on 24-Hour Rhythms in Serum Prolactin and Gonadotropins in Rats

To assess to what extent the presence of an ectopic pituitary differentially affected circulating prolactin (PRL) and gonadotropin levels at different times of the year, rats kept under 12h light, 12h dark (12:12 LD) photoperi-ods and receiving a pituitary graft or a sham operation in summer or winter were examined 3 months later. In both male and female sham-operated rats, a circadian variation in serum PRL levels was found, with an acrophase varying from 21:53h to 00:54h and the mesor and amplitude higher in spring than autumn in males and higher in autumn than in spring in females. After grafting a pituitary, changes in serum PRL related to time of day were no longer observed. In pituitary-grafted male rats killed during spring, serum PRL levels were higher than controls at only a few time points throughout the 24h cycle, whereas in rats killed during autumn, there were no significant differences in PRL levels between grafted and control rats. Pituitary-grafted female rats killed during spring showed serum PRL levels significantly higher than those of sham-operated rats, while in female rats killed in autumn, PRL levels of pituitary-grafted and sham-operated rats did not differ. Significant variations of luteinizing hormone (LH) related to time of day were found in sham-operated male rats only, with acrophases at 23:52h and 00:24h for spring and autumn, respectively, and the mesor and amplitude of the rhythm significantly higher in autumn. Pituitary transplants suppressed 24h variations in circulating LH and depressed its levels during the two seasons examined. As far as follicle-stimulating hormone (FSH), pituitary grafts decreased circulating levels, with the extent of decrease higher during autumn than in spring. The results indicate that some endocrine consequences of the grafting of an ectopic pituitary are dependent on time of year.     

Seasonally Dependent Effect of Ectopic Pituitary Grafts on 24-Hour Rhythms in Serum Prolactin and Gonadotropins in Rats

To assess to what extent the presence of an ectopic pituitary differentially affected circulating prolactin (PRL) and gonadotropin levels at different times of the year, rats kept under 12h light, 12h dark (12:12 LD) photoperi-ods and receiving a pituitary graft or a sham operation in summer or winter were examined 3 months later. In both male and female sham-operated rats, a circadian variation in serum PRL levels was found, with an acrophase varying from 21:53h to 00:54h and the mesor and amplitude higher in spring than autumn in males and higher in autumn than in spring in females. After grafting a pituitary, changes in serum PRL related to time of day were no longer observed. In pituitary-grafted male rats killed during spring, serum PRL levels were higher than controls at only a few time points throughout the 24h cycle, whereas in rats killed during autumn, there were no significant differences in PRL levels between grafted and control rats. Pituitary-grafted female rats killed during spring showed serum PRL levels significantly higher than those of sham-operated rats, while in female rats killed in autumn, PRL levels of pituitary-grafted and sham-operated rats did not differ. Significant variations of luteinizing hormone (LH) related to time of day were found in sham-operated male rats only, with acrophases at 23:52h and 00:24h for spring and autumn, respectively, and the mesor and amplitude of the rhythm significantly higher in autumn. Pituitary transplants suppressed 24h variations in circulating LH and depressed its levels during the two seasons examined. As far as follicle-stimulating hormone (FSH), pituitary grafts decreased circulating levels, with the extent of decrease higher during autumn than in spring. The results indicate that some endocrine consequences of the grafting of an ectopic pituitary are dependent on time of year.     

Effect of sampling interval on serum concentrations of luteinizing hormone, follicle-stimulating hormone, and prolactin in prepubertal, ovariectomized, and cycling gilts.

Three experiments were conducted to determine the effect of sampling interval on serum concentrations of LH, FSH, and prolactin (PRL) in prepubertal, ovariectomized, and cycling gilts. In all experiments, blood samples were drawn at 2-min intervals for 4 h from indwelling jugular catheters. Mean serum hormone concentrations, mean number of peaks, and mean and maximum peak heights of LH, FSH, and PRL were calculated using values reflecting 2-, 6-, 10-, 20-, 30-, and 60-min sampling intervals. For LH, FSH, and PRL, mean serum concentrations can be obtained through blood samples drawn at hourly intervals. Since LH peaks are very distinct in pigs, the number of secretory peaks and mean peak height can be obtained via samples drawn at 20-min intervals. Since FSH and PRL peaks are less well defined, a more frequent sampling interval (10 min) is needed to determine number of peaks and mean peak height. To obtain the maximum peak height or the number of minutes for LH, FSH, or PRL to rise from its nadir to zenith, blood samples need to be drawn at 2-min intervals. Regardless of reproductive state, these data indicate that the sampling interval needed to characterize serum concentrations of LH, FSH, and PRL in the gilt is dependent upon the parameter in question.     

Pulsatile secretion of luteinizing hormone and follicle stimulating hormone and their relationship to secretion of estradiol and onset of estrus in weaned sows

The objective of this experiment was to characterise temporal changes in estradiol and pulsatile secretion of luteinizing hormone (LH) and follicle stimulating hormone (FSH) during the interval between weaning and estrus in the sow. Five multiparous sows were sampled at 10-min intervals for 3 h beginning 8 h after weaning and continuing every 12 h until estrus. Interval to estrus was 102 ± 2 h (range 96–108) after litters were weaned, and interval to preovulatory LH and FSH surges was 109 ± 5 h (range 92–116). With the exception of the period of the preovulatory surge, neither average nor basal concentrations of LH or FSH changed over time. Number of LH peaks per 3 h reached a maximum of 2.8 at 48 h before the preovulatory surge, then declined to 0.8 at 12 h before the surge. Peak amplitude for LH and peak frequency and amplitude for FSH also declined with time before preovulatory surges. Relative ranks were computed for individual sows based on the mean concentration of LH or FSH for each bleeding period. Rankings were consistent over the periods, but were not correlated with interval to estrus. Estradiol concentrations peaked (88 ± 7 pg/ml) at 36 h before preovulatory surges, coincident with the decline in peak frequency of LH. We conclude that pulsatile secretion of LH and FSH changes during the interval between weaning and estrus but secretion of these two hormones may be controlled by different mechanisms.     

Influence of endogenous prolactin on the luteinizing hormone stimulation of testicular steroidogenesis and the role of prolactin in adult male rats

The role of endogenous prolactin (PRL) in the control of testosterone (T) secretion and T responses to LH treatment was evaluated in adult male rats. Rats were actively immunized three times against ovine PRL in Freund's adjuvant-saline mixture (PRL-IMM rats), and control rats were treated with adjuvant-saline mixture (ADJ-CON rats). On day 110 after initial immunization, rats in each of these two groups were divided into three subgroups. Rats in subgroups 1 and 2 were injected with saline while those in subgroup 3 received 200 micrograms ovine PRL in saline, twice a day for a total of 7 injections. On day 113, the seventh injection was given 3 h before the termination of the experiment. On the same day, 2.5 h before the rats were sacrificed, rats in subgroups 1 and 3 were treated with saline; animals in subgroup 2 received 25 micrograms ovine LH in saline. Blood samples were obtained throughout the study, and sera were used for measurement of PRL antibodies, gonadotropins, progesterone (P), and T. PRL antibodies were detected in the sera of all rats actively immunized with PRL. Administration of PRL increased serum T levels in ADJ-CON rats, and this effect was eliminated in rats actively immunized against PRL. LH treatment significantly increased serum T levels in ADJ-CON rats. In PRL-IMM rats, this increase was attenuated while circulating P concentrations were elevated. These data demonstrate that PRL treatment can increase T secretion and that endogenous PRL is required for the complete expression of the stimulatory action of LH on T secretion in adult male rats.     

Circadian and pulsatile variations in plasma levels of inhibin, FSH, LH and testosterone in adult Murrah buffalo bulls

The present study investigated pulsatile and circadian variations in the circulatory levels of inhibin, gonadotrophins and testosterone. Six adult buffalo bulls (6 to 7 yr of age) were fitted with indwelling jugular vein catheters, and blood samples were collected at 2-h intervals for a period of 24 h and then at 15-min interval for 5 h. Plasma concentrations of inhibin, FSH, LH and testosterone were determined by specific radioimmunoassays. Plasma inhibin levels in Murrah buffalo bulls ranged between 0.201 to 0.429 ng/mL, with a mean of 0.278 +/- 0.023 ng/mL. No inhibin pulses could be detected during the 15-min sampling interval. Plasma FSH levels ranged between 0.95 to 3.61 ng/mL, the mean concentration of FSH over 24 h was 1.66 +/- 0.25 ng/mL. A single FSH pulse was detected in 2 of 6 bulls. The LH levels in peripheral circulation ranged between 0.92 to 9.91 ng/mL, with a mean concentration of 3.33 +/- 1.02 ng/mL. Pulsatility was detected in LH secretion with an average of 0.6 pulses/h. Plasma testosterone levels in 4 buffalo bulls ranged from 0.19 to 2.99 ng/mL, the mean level over 24 h were 1.34 +/- 0.52 ng/mL. Testosterone levels in peripheral circulation followed the LH secretory pattern, with an average of 0.32 pulses/h. The results indicate parallelism in inhibin, FSH and LH, and testosterone secretory pattern. Divergence in LH and FSH secretory patterns in adult buffalo bulls might be due to the presence of appreciable amounts of peripheral inhibin.     

Effects of hyperprolactinemia on prolactin and LH pulsatile pattern in female rats.

Prolactin (PRL) and luteinizing hormone (LH) secretions are very closely-related. To further understand these mechanisms, the pulsatile secretion pattern of both hormones in experimentally-induced hyperprolactinemia has been studied in adult female rats. Hyperprolactinemia was induced by the transplanting of two pituitary glands. Nine days after the transplant operation, rats were bled (75 or 100 microliters/7 min for 3 h). Serum samples were analyzed for prolactin and LH values by RIA. Hyperprolactinemia modifies pulsatile PRL secretion by increasing the absolute amplitude and duration of the peaks together with a decrease in their frequency. Also, the mean values of the hormone during the whole studied period were increased. Hyperprolactinemia was followed by an increase in the mean values of LH and in the absolute amplitude of the peaks. All these results suggest that hyperprolactinemia induced by pituitary grafting in adult female rats, is followed by a significant change in prolactin and LH pulsatility, which may explain, to some extent, the effects of hyperprolactinemia on reproduction.     

Acute effect of suckling on gonadotropin, prolactin and glucocorticoid concentrations in serum of intact and ovariectomized beef cows

Suckling may prolong the anovulatory period postpartum by 1) a neural-mediated inhibition of luteinizing hormone-releasing hormone (LHRH)-induced gonadotropin secretion, or 2) an inhibitory effect of hormones released by suckling on gonadotropin secretion and/or action at the ovary. In the present investigation we considered whether a suckling event caused 1) acute inhibition of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion, and 2) release of glucocorticoids and/or prolactin (PRL). Six Hereford cows remained intact and six were ovariectomized (ovx) on day 7 postpartum. Calves remained with their dams continuously. Cows were bled at 10-min intervals during 6 consecutive hr on days 14, 28 and 42 postpartum. Both LH and FSH were released episodically by day 14 in intact and ovx cows, but suckling did not acutely affect LH and FSH secretion. A PRL release accompanied suckling 67, 96 and 95% of the time. However, among all instances where PRL was released on days 14, 28 and 42 postpartum, 67, 29 and 37% occurred independent of a suckling event. Glucocorticoids were not released by suckling in intact cows but were released in ovx cows. We conclude that suckling does not acutely affect LH or FSH concentrations in serum of cows postpartum, that PRL concentrations usually increase in serum coincident with suckling but can be released at other times, and suckling-induced glucocorticoid release depends upon the presence of the ovary.     

Interaction between dietary intake and ovariectomy on concentrations of insulin-like growth factor-I, GH and LH in plasma of heifers

The objective of this study was to determine if alterations in dietary intake and(or) ovariectomy influence plasma concentrations of IGF-I, GH and LH in heifers. Cyclic heifers (n = 23) were individually fed for 10 wk either 1) 1.8% of body weight in dry matter per day (GAIN; n = 7) 2) 1.1% of body weight in dry matter per day (MAINT; n = 8); or 3) 0.7% of body weight in dry matter per day (LOSE; n = 8). After 10 wk of dietary treatment, heifers were ovariectomized 36 to 40 h following the second injection of prostaglandin F2alpha analog (2 injections 11 d apart). Heifers weighed 444 +/- 13, 387 +/- 8, and 349 +/- 9 kg in the GAIN, MAINT and LOSE groups, respectively, at the time of ovariectomy; the average daily weight gains during the 10-wk period were 0.96, 0.17 and -0.31 kg, respectively (P < 0.001), for the 3 groups. Blood plasma was collected for 6 h at 15-min intervals 1 d before and 2 wk after ovariectomy. The MAINT group of heifers had greater IGF-I concentrations than either the LOSE or GAIN groups; IGF-I decreased (P < 0.05) by 23 and 35% after ovariectomy in the MAINT and GAIN groups, respectively, but did not change (P > 0.10) in the LOSE groups. Dietary restriction tended to increase (P < 0.10) GH pulse frequency and mean GH. Ovariectomy had no effect (P > 0.10) on mean GH or GH pulse frequency but increased (P < 0.05) GH pulse amplitude in the GAIN groups. Dietary treatment had no effect (P > 0.10) on mean LH, or LH pulse amplitude and frequency. However, across dietary treatments, ovariectomy increased mean LH and LH pulse frequency but did not affect (P > 0.10) LH pulse amplitude. In summary, dietary restriction increased GH secretion while ovariectomy increased LH secretion. There appears to be a dichotomy of response between GH and IGF-I in the way heifers respond to dietary treatment and(or) ovariectomy.     

Effects of intraventricular injection of delta receptor antagonist ICI 154, 129 on the secretion of luteinizing hormone and prolactin in the proestrous rat.

In order to ascertain the role of delta receptors in the control of gonadotropin secretion, a preferential delta receptor antagonist ICI 143,129 was microinjected into the third ventricle through chronically implanted cannulae and the effects on the serum concentration of luteinizing hormone (LH) and prolactin (PRL) were determined in female rats in proestrus. When the injection was given at 1030 h, ICI 154,129 (50 micrograms) exerted no significant effects on either LH or PRL. However, in the rat given a microinjection of ICI 154,129 at 1300 h, an afternoon rise in LH occurred in advance and was of greater magnitude, with the peak time more than 1 h earlier and the peak amplitude approximately 100% greater than that in the control rat, respectively. The injection also suppressed the PRL rise during the plateau phase. The results indicate that delta receptors are involved in the mediation of the inhibitory influence of endogenous opioids on the surge of LH in proestrus, and that delta receptors mediate the facilitatory influence of opioids on the PRL surge during the plateau phase.     

Effect of [D-Trp6]LHRH infusion on prolactin secretion by perifused rat pituitary cells

The effect of a superactive agonistic analog of luteinizing hormone-releasing hormone (LHRH), [D-Trp6]LHRH on prolactin (PRL) secretion by perifused rat pituitary cells was investigated. Constant infusion of [D-Trp6]LHRH (0.5 ng/min) for 2-3 h elicited a significant decrease in PRL secretion by these cells. This decrease in PRL release started ca. 30 min after the beginning of the infusion with the LHRH analog and lasted up to 1.5-2 h. [D-Trp6]LHRH significantly stimulated luteinizing hormone (LH) secretion during the first 30 min of peptide infusion; thereafter, LH levels began to return to control values. In animals pretreated in vivo with 50 micrograms of [D-Trp6]LHRH (s.c.) 1 h before sacrifice, PRL secretion by the rat pituitary cell perifusion system was significantly lower than vehicle-injected controls throughout the entire [D-Trp6]LHRH infusion period. On the other hand, thyrotropin-releasing hormone (TRH)-stimulated PRL secretion was slightly, but significantly imparied by [D-Trp6]LHRH infusion, while dopamine (DA) inhibition of PRL release was unaffected by this same treatment. These results reinforce previous observations of a modulatory effect of [D-Trp6]LHRH, probably mediated by pituitary gonadotrophs, on PRL secretion by the anterior pituitary. In addition, our findings suggest that basal PRL secretion by the lactotroph may be dependent on a normal function of the gonadotroph. The collected data from this and previous reports support the existence of a functional link between gonadotrophs and lactotrophs in the rat pituitary gland.     

The influences of GnRH, oxytocin and vasoactive intestinal peptide on LH and PRL secretion by porcine pituitary cells in vitro.

The aim of the present study was to evaluate the possible direct effects of GnRH, oxytocin (OT) and vasoactive intestinal peptide (VIP) on the release of LH and PRL by dispersed porcine anterior pituitary cells. Pituitary glands were obtained from mature gilts, which were ovariectomized (OVX) one month before slaughter. Gilts randomly assigned to one of the four groups were treated: in Group 1 (n = 8) with 1 ml/100 kg b.w. corn oil (placebo); in Group 2 (n = 8) and Group 3 (n = 8) with estradiol benzoate (EB) at the dose 2.5 mg/100 kg b.w., respectively, 30-36 h and 60-66 h before slaughter; and in Group 4 (n = 9) with progesterone (P4) at the dose 120 mg/ 100 kg b.w. for five consecutive days before slaughter. In gilts of Group 2 and Group 3 treatments with EB have induced the negative and positive feedback in LH secretion, respectively. Isolated anterior pituitary cells (10(6)/well) were cultured in McCoy's 5a medium with horse serum and fetal calf serum for 3 days at 37 degrees C under the atmosphere of 95% air and 5% CO2. Subsequently, the culture plates were rinsed with fresh McCoy's 5A medium and the cells were incubated for 3.5 h at 37 degrees C in the same medium containing one of the following agents: GnRH (100 ng/ml), OT (10-1000 nM) or VIP (1-100 nM). The addition of GnRH to cultured pituitary cells resulted in marked increases in LH release (p < 0.001) in all experimental groups. In the presence of OT and VIP we noted significant increases (p < 0.001) in LH secretion by pituitary cells derived from gilts representing the positive feedback phase (Group 3). In contrast, OT and VIP were without any effect on LH release in Group 1 (placebo) and Group 2 (the negative feedback). Pituitary cells obtained from OVX gilts primed with P4 produced significantly higher amounts (p < 0.001) of LH only after an addition of 100 nM OT. Neuropeptide GnRH did not affect PRL secretion by pituitary cells obtained from gilts of all experimental groups. Oxytocin also failed to alter PRL secretion in Group 1 and Group 2. However, pituitary cells from animals primed with EB 60-66 h before slaughter and P4 produced markedly increased amounts of PRL in the presence of OT. Neuropeptide VIP stimulated PRL release from pituitary cells of OVX gilts primed with EB (Groups 2 and 3) or P4. In contrast, in OVX gilts primed with placebo, VIP was without any effect on PRL secretion. In conclusion, the results of our in vitro studies confirmed the stimulatory effect of GnRH on LH secretion by porcine pituitary cells and also suggest a participation of OT and VIP in modulation of LH and PRL secretion at the pituitary level in a way dependent on hormonal status of animals.     

Perioestrous patterns of circulating LH,FSH, prolactin and oestradiol-17β in the gilt

Concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL) were measured in jugular blood and those of oestradiol-17β (E₂17β) in utero-ovarian blood. Samples were taken from five intact gilts every 15 min for 108 h starting between day 15 and day 18 of the oestrous cycle. In the late luteal/early follicular phase, high pulsatile LH secretion, close to one pulse per hour, was observed. This could be the stimulus necessary for the final maturation of the ovarian follicles.Thereafter, frequency and amplitude of pulses, and the baseline value, decreased and were low at least between 36 and 12 h before the preovulatory LH surge. PRL and FSH concentrations also declined. This was probably due to the increase of oestrogen secretion. As E₂17β concentrations were still high, the surge of LH which was accompanied by increase in FSH and PRL, occurred for approximately 13 to 20 h. While LH and PRL mean levels decreased, FSH concentrations continued to increase. Peaks of PRL were observed during the late luteal/early follicular phase and during the LH discharge. During the period of estrus, each exposure to the boar was immediately followed by one of these peaks, which could play a role in the sexual behavior of the gilt.     

The relationship between 13,14-dihydro-15-keto PGF2 alpha and LH secretion in bulls

Half-life (t1/2), volume of distribution (Vd) and total body clearance (TBC) of 13,14-dihydro-15-keto PGF2 alpha (PGFM) were measured in order to determine optimal sampling frequency for accurate measurement of PGFM. Three yearling Holstein bulls (349.2 +/- 6.7 kg) and 3 yearling Holstein steers (346.7 +/- 7.0 kg) were utilized in a 3 X 3 Latin square design. Animals were given 0, 25 or 50 micrograms PGF2 alpha I.V.; blood samples collected every 2 min and plasma PGFM determined. The t1/2, Vd and TBC of PGFM were 2.3 +/- .2 min, 43.3 +/- 3.3 liters and 13.7 +/- 1.9 liters/min, respectively and were similar for 25 and 50 micrograms doses. To determine the relationship between endogenous PGFM and LH secretion in bulls, blood samples were collected every 2 min for 12 h in 4 yearling Angus bulls (489.1 +/- 11.6 kg). All animals elicited at least one LH surge and PGFM concentrations were measured in samples coincident with the LH surge. Mean plasma PGFM concentrations were greater prior to the LH surge than during the LH surge. In addition, mean plasma PGFM concentration and frequency of PGFM peaks appeared to increase prior to the LH surge suggesting an association between PGFM and pulsatile LH secretion in the bull.     

Ontogeny of growth hormone, prolactin, luteinizing hormone, and testosterone secretory patterns in the ram

The ontogenetic changes that occur in secretory patterns of growth hormone (GH), prolactin (Prl), luteinizing hormone (LH), and testosterone (T) in rams maintained in constant photoperiod were examined. Nine ram lambs were moved to individual pens in a controlled environment (12L: 12D cycle; 18-24 degrees C temperature) at 66 days of age. Blood samples were collected via indwelling cannulae at 15-min intervals for an 8-h period at 80, 136, 192, 248, and 304 days of age. Plasma concentrations of GH, Prl, LH, and T were quantitated and parameters of the secretory patterns determined. Mean concentration of GH tended to decline with age, probably because the amplitude of secretory peaks was significantly reduced with age. There were no age-associated changes in basal concentration of GH or incidence of GH peaks. There was an increase in Prl secretion (as estimated by mean concentration) at 136 and after 248 days of age. Significant age-associated changes occurred in all parameters of LH and T secretion. At the younger ages, testosterone concentrations were low and LH concentrations were elevated. At the older ages the relationship was reversed, with LH low and testosterone high. There were no significant correlations between frequency and magnitude of LH and T peaks. The significant correlations present among parameters of LH and T secretion were between basal concentration of LH and overall mean concentration and basal concentration of T. These results suggest that LH may not be the sole tropic stimulator of acute T secretion.