ABSTRACT

The effects of 2-hydroxyoestradiol-17β and oestradiol-17β on the concentration of serum LH was studied in fetal pigs. 2-Hydroxyoestradiol-17β (5 μg/fetus; n = 6), oestradiol-17β (5 μg/fetus; n = 7) or vehicle (n = 6) were injected i.v. into chronically catheterized fetal pigs at gestational ages of 105–108 days. Seven additional fetuses at the same age served as untreated controls. 2-Hydroxyoestradiol-17β but not oestradiol-17β resulted in a rapid decline in plasma LH levels from 0·82 ± 0·21 (s.e.m.) to 0·21 ± 0·05 μg/l within 20 min of injection. Baseline concentrations of plasma LH in the mothers were low (0·72 ±0·2 μg/l) and were not affected by catecholoestrogen or oestrogen treatment of fetuses. The results suggest that catecholoestrogens do not modulate LH secretion in fetal pigs through an oestrogenic action. It is possible that they act by modifying catecholamine metabolism or neurotransmission.

J. Endocr. (1986) 111, 297–300

Abstract

The ontogeny of GH and IGF-I secretion was investigated in the fetal pig. Pulse studies were performed to describe the pattern of GH release. Twenty-four-hour profiles were recorded to examine possible diurnal variations in these hormones.

(I) Pulse studies. Blood samples were obtained at 15-min intervals for 2-h periods from 24 male and 20 female fetuses at various gestational ages (fetal day 89–113; term 113 ± 1 s.d.). Fetuses revealed a pulsatile GH release. The GH pulse frequency did not vary with gestational age in either sex (0·95 ± 0·19 pulses/h). In males the GH pulse amplitude decreased with increasing fetal age (r= −0·41; P<0·02). In female fetuses no significant correlation could be calculated. Mean GH concentrations fell significantly in male fetuses 3 to 4 days before delivery (P<0·05) and the same tendency was observed in females (P<0·06). Between fetal days 94 and 98 GH pulse amplitude and GH and IGF-I concentrations were higher in males than in females (P<0·01, P<0·001 and P<0·02 respectively). Fetal IGF-I secretion showed no ontogenetic changes in both sexes. However, maternal IGF-I concentrations increased with progressing gestation (r=0·46; P<0·001).

(II) 24-h profiles. Eight male and four female late-gestational fetuses (fetal days 104–108) were studied. Blood samples were taken at 30-min intervals over 24 h. Dams and fetuses showed an episodic GH secretion over the 24-h period but no diurnal rhythm was observed. Whereas maternal IGF-I secretion was constant, fetal IGF-I release was characterized by marked fluctuations over the 24 h. In half of the fetuses (n=6) the fluctuations appeared at regular intervals. Again no diurnal rhythm existed.

These data demonstrated that: (1) porcine fetal GH secretion is pulsatile and decreases shortly before birth; (2) a sex difference in GH and IGF-I concentrations exists between fetal days 94 and 98, suggesting that IGF-I is at least partially under the control of GH before birth; (3) fetal GH and IGF-I secretion is episodic over 24 h, but does not vary diurnally; and (4) fetal and maternal GH and IGF-I secretion are regulated independently.

Journal of Endocrinology (1996) 149, 125–133

The effects of gonadal secretions on the release of LH and the stimulation of LH secretion by oestradiol have been investigated in newborn male and female miniature pigs; the differences in the feedback action of testosterone in newborn and pubertal male pigs were also studied.

Hemi-orchidectomy or orchidectomy of 1-week-old pigs had no effect on the level of LH in the plasma; total orchidectomy significantly reduced the levels of testosterone (P<0·01) and progesterone (P<0·05). In female pigs ovariectomized at 1 week of age, the concentration of LH in the plasma decreased, with a strong negative correlation between the level of LH and age (r = −0·41; P < 0·05). The plasma concentration of progesterone was generally low and unaffected by ovariectomy. Orchidectomy and treatment of male pigs, at 1 week of age, with testosterone (6 mg/kg body weight) had no effect on the plasma concentration of testosterone 24 h after treatment. If testosterone propionate was given rather than testosterone, the level of LH was significantly reduced (P< 0·001) 24 h after the injection and the concentration of testosterone in the plasma corresponded to that found in the intact adult male pig. Treatment with oestradiol or oestradiol benzoate did not affect the concentration of LH. Orchidectomy and treatment of pubertal male pigs with testosterone propionate resulted in a significantly (P < 0·001) higher concentration of testosterone in the plasma, compared with newborn pigs treated similarly, but the level of LH was unchanged. This suggests that there is a more rapid rate of clearance of testosterone in the newborn than in the pubertal male miniature pig and that the negative feedback of testosterone is not mediated by aromatization in the newborn animal and it declines before or during puberty.

Treatment of newborn intact male and female and gonadectomized male pigs with oestradiol benzoate produced similar variations in the plasma level of oestradiol in all groups of animals. In the female pigs, however, a surge-like release of LH was observed 60–72 h after the injection of oestradiol benzoate, suggesting that the stimulatory feedback mechanism can operate soon after birth and that the response is sexually dimorphic.

In equine species, luteinising hormone (LH) and prolactin (PRL) release are reduced throughout pregnancy but increase at foaling. The present experiments were designed to study a possible opioidergic regulation of LH and PRL release in pregnant Shetland mares (n=6). At various stages of pregnancy (days 26.4+/-0.6, 75.4+/-5.4, 171.8+/-2.4, 226.2+/-4.8, 282.7+/-3.4 and 319.8+/-2.1), mares were injected with the opioid antagonist naloxone (0.5 mg/kg body weight) and saline. The two treatments were always separated by 2 days, and mares served as their own controls. Two hours after being given naloxone and saline, mares were given the gonadotrophin-releasing hormone (GnRH) analogue buserelin (5 microg per animal). The naloxone experiment was repeated at 2 days after foaling. Blood for the determination of LH and PRL was withdrawn at 15 min intervals for 240 min, and naloxone or saline was injected after 60 min. Naloxone induced significant (P<0.05) LH release on days 172, 226 and 283 of pregnancy but not on days 26, 76 and 320 and 2 days after foaling. Buserelin caused a significant (P<0.05) increase in plasma LH concentrations on days 172, 226, 282 and 320 of pregnancy. The experiments indicate that endogenous opioids are involved in the inhibition of LH release during the second half of pregnancy in equine species. The deactivation of opioid effects on LH release might be a prerequisite for the onset of ovarian activity postpartum. Plasma PRL concentrations increased significantly (P<0.05) after naloxone administration on days 226, 282 and 320 of pregnancy. The naloxone-induced PRL release was most pronounced towards term, indicating an increase in the naloxone-releasable pool and/or the absence of other PRL-release inhibitory mechanisms.

SUMMARY

In a series of experiments on female miniature pigs, the pattern of plasma LH and progesterone levels during the oestrous cycle, late pregnancy and lactation and after ovariectomy were characterized, and the effect of pentobarbitone treatment was tested.

The preovulatory surge of LH occurred in seven out of eight animals between 00.00 and 12.00 h on day 0 of the oestrous cycle (day 1 of standing heat). Plasma progesterone started to decline 8 days before oestrus and reached its lowest value 5 days before the preovulatory LH peak. Increases in progesterone concentration were already noticeable 48 h after the LH surge. During late pregnancy, parturition and lactation, plasma LH was low and showed only minor fluctuations, while plasma progesterone declined 4 to 5 days before parturition. Both hormones remained at low levels throughout lactation. Three weeks before parturition increases in LH were always followed by an increase in progesterone. This dependency was greatly diminished immediately before delivery. Four to 12 days after weaning the animals came into oestrus which was followed by an increase in LH and later an increase in progesterone concentrations.

Ovariectomy during dioestrus resulted in a steady increase in plasma LH levels for 35–39 days. Ovariectomy caused abortion if performed on day 100 of pregnancy. It was followed by a rapid increase of plasma LH concentration. Normal parturition (around day 115) and lactation took place when animals were spayed on day 112 of pregnancy. In this case, plasma LH levels remained even lower than before ovariectomy as long as lactation was maintained. Immediately after weaning a rapid increase in the normal postovariectomy pattern of LH secretion was observed.

Pentobarbitone anaesthesia (30–35 mg/kg body wt, initial dose), during pro-oestrus– oestrus, for less than 5 h had no effect on the preovulatory LH increase. However, pentobarbitone anaesthesia for more than 6 h inhibited the LH peak and ovulation if the animal was under deep anaesthesia before 24.00 h on the day before oestrus. Pentobarbitone treatment of ovariectomized pigs resulted in a clear decrease in LH levels 40 min after a single i.v. dose.

ABSTRACT

β-Endorphin was measured in the plasma of pigs during late pregnancy and at different stages of the oestrous cycle. In pregnant animals, β-endorphin secretion from uteroplacental tissues into the maternal circulation and the possible effects of oxytocin and the prostaglandin F_2α (PGF_2α) analogue cloprostenol on β-endorphin release were determined. Plasma β-endorphin concentrations in pregnant sows were significantly higher than in non-pregnant pigs. However, there were no significant changes in β-endorphin values throughout the oestrous cycle. Because the increase in plasma β-endorphin concentrations had occurred before luteolysis and onset of labour it could not be attributed to the stress of parturition. The surgical intervention of a laparotomy increased β-endorphin release into peripheral plasma. Cloprostenol but not oxytocin caused an immediate increase in plasma β-endorphin concentrations. At parturition, endogenous PGF_2α may be involved in the regulation of β-endorphin secretion. Concentrations of β-endorphin in the jugular and uterine vein plasma were not significantly different, and so it would appear that β-endorphin in the plasma of pregnant sows is not of uteroplacental origin. In conclusion, changes in the concentration of β-endorphin in peripheral plasma, associated with pregnancy but not the oestrous cycle, exist in pigs. Hence a physiological function of peripheral opioid peptides in the periparturient sow is feasible.

Journal of Endocrinology (1993) 136, 199–206

†Division of Clinical and Experimental Endocrinology, University Women's Hospital, 34 Göttingen, and ‡Section of Biology of Reproduction, Department of Animal Science, 34 Göttingen, Federal Republic of Germany

(Received 4 September 1975)

To obtain information on the pituitary-gonadal system during sexual maturation in the pig, we have determined plasma luteinizing hormone (LH) and gonadal steroid levels in foetal, neonatal and pubertal animals of both sexes. Blood was collected from 45 male and 34 female miniature pig (Göttingen strain) foetuses between days 61 and 110 of foetal life (by cardiac puncture or from umbilical vessels). The piglets were removed 5–8 min after initiation of halothane anaesthesia in the sow. Blood (2–5 ml) was taken once by venipuncture of the jugular vessels from 79 male and 72 female unanaesthetized animals, 2 h (only females) to 12 weeks after birth.

Plasma LH and progesterone were measured by radioimmunoassay (Pomerantz, Ellendorff, Elsaesser, König & Smidt,

SUMMARY

Secretory patterns of LH and testosterone were characterized in the intact male miniature pig. All blood samples were taken from indwelling catheters. Hourly sampling was carried out over 24 h and during a morning period blood was collected for 2 h at 10 min intervals. No significant difference was detected in the plasma LH concentration on the basis of hourly sampling. Plasma testosterone was significantly (P < 0·05) lower during the evening and night when compared with morning values.

The second experiment was concerned with the pattern of plasma LH and testosterone concentrations before and after copulation. Blood sampling was performed at 10 min intervals. Plasma LH was significantly (P < 0·001) raised for 30 min after copulation when compared with any 30 min period (0–120 min) before copulation. Plasma testosterone was not significantly altered for any 30 min period of the experiment (0–270 min). The data are interpreted as a possible mechanism for endocrine control of testicular function.

SUMMARY

Plasma oxytocin concentrations were measured during late pregnancy, parturition and lactation in the miniature pig. Measurements were made of plasma oestradiol, oestrone and progesterone to determine whether there was any relationship between the concentrations of oxytocin and these steroids in the circulation.

Plasma oxytocin concentrations were low or undetectable in late pregnancy. Rises of up to 68·8 μu./ml were seen at the time of delivery of the foetuses and at the expulsion of the placenta. The only steroid that seemed to relate to oxytocin release was progesterone. Oxytocin release was consistently seen when progesterone concentrations had fallen to below 10 ng/ml but no increase in concentration was observed while oestrone and oestradiol increased to their maximum concentrations of 3·86–11·6 and 0·43–0·70 ng/ml respectively.

During lactation, when both oestrogen and progesterone concentrations were low, suckling caused the levels of oxytocin to increase to 7·4 μu./ml. These increases were greater during the first 2 weeks of lactation than later.