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Summary. Plasma concentrations of β-endorphin and met-enkephalin were measured, with appropriate radioimmunoassays, in cows during gestation and at parturition and in newborn calves. During pregnancy β-endorphin immunoreactivity (IR) concentration increased, but values during the last month of gestation were not different from those at parturition. Highest met-enkephalin IR levels were obtained in cows during calving. A term Caesarean section caused an increase in plasma β-endorphin and metenkephalin IR concentrations, but no such increase occurred in cases of a preterm Caesarean section. In calves β-endorphin IR values were lower before umbilical cord rupture than immediately after birth. Values decreased continuously thereafter. This was also the case for met-enkephalin IR concentrations in calves born at term. In preterm calves met-enkephalin IR values were low immediately after delivery and increased during the first hour of life. A significant correlation existed between the degree of acidosis and plasma levels of both opioid peptides in the calves. We conclude that a direct stimulation of peripheral β-endorphin release by the pain or stress associated with calving does not seem to exist in cattle, whereas met-enkephalin seems to be more directly related to parturition. In calves the change to the extrauterine environment causes an immediate, increased release of both opioids.
Keywords: β-endorphin; met-enkephalin; cattle; parturition; neonate
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Summary.
Eight ewes each with an autotransplanted ovary received infusions of tritium-labelled pregnenolone (41 μCi/hr) for 8 hr into the artery supplying the ovary, together with prostaglandin (PG) F-2α (30 μg/hr) for 3 hr beginning 2 hr after the start of the pregnenolone infusion. All animals exhibited oestrus 2-3 days after the start of the experiment. During the PGF-2α infusion blood flow through the ovaries was increased by 13%, but subsequently returned to pre-infusion levels. Secretion rates of endogenous progesterone and 20α-hydroxypregn-4-en-3-one dropped rapidly 5 hr after the PGF-2α infusion had started from values of 250 μg/hr and 25 μg/hr to values below 60 μg/hr and 8 μg/hr, respectively. At this time the conversion of radioactive pregnenolone to progesterone was reduced by 50% of its initial value, but the secretion of endogenous pregnenolone and the formation of radioactive metabolites other than progesterone were not diminished. In 4 control animals, which received pregnenolone only, no changes in ovarian blood flow, steroid secretion rates, or in the conversion of labelled pregnenolone were observed. These results suggest a possible involvement of PGF-2α in the regulation of progesterone biosynthesis by an action on the 3β-hydroxysteroid oxidoreductase-Δ5−4 isomerase enzyme system.
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Concentrations of β-endorphin and oxytocin were measured in plasma of cows before, during and after parturition. The effect of the PGF2α analogue cloprostenol on β-endorphin and oxytocin release was investigated. During parturition, there were marked, parallel increases in β-endorphin and oxytocin concentrations. Both hormones were released in an episodic manner in conjunction with uterine and abdominal contractions. It is therefore likely that factors stimulating oxytocin release also enhance β-endorphin secretion. This suggests a role of labour or labour-associated hormones in stimulating peripheral β-endorphin release. Cloprostenol caused an immediate, pronounced increase in plasma β-endorphin and oxytocin concentrations.
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Effects of the opioid antagonist naloxone on concentrations of LH and FSH in plasma were measured in mares during different stages of the oestrous cycle. During the follicular phase of the cycle, naloxone (300 mg i.v.) had no discernible effects on basal concentrations of LH and FSH. A significant increase in plasma LH (P < 0.01) and FSH (P < 0.05) concentrations was observed after naloxone in mares during the luteal phase. This response was not different between suckled and non-suckled mares. The gonadotrophin-releasing hormone analogue buserelin (0.02 mg i.v.) caused a significant (P < 0.05) LH and FSH release irrespective of the stage of the oestrous cycle and a previous naloxone treatment. The results of this study indicate that endogenous opioid peptides are involved in the inhibition of LH and FSH release during the luteal phase of the oestrous cycle in mares and may partially mediate the suppressive influence of progesterone on gonadotrophin secretion. The opioid-mediated suppression of LH and FSH release does not seem to be affected by suckling.
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The aim of this study was to examine the progestin content and biosynthetic potential of the corpus luteum of the African elephant (Loxodonta africana). Luteal tissue was collected from nonpregnant and early, mid- and late pregnant elephants (n = 2 per group) shot in the Kruger National Park. Pieces of individual corpora lutea (2–3 per animal; 23 in total) were stored directly in ethanol before hormone analysis. Matching tissue pieces were incubated for 2 h with [3H]pregnenolone (2 × 105 c.p.m.), after which tissue plus medium were also stored in ethanol. Progesterone and 17α-hydroxyprogesterone immunoreactivity in tissue extracts were determined by enzymeimmunoassay and radioimmunoassay, respectively, before and after reverse phase HPLC. Progesterone immunoreactivity predominated over that of 17α-hydroxyprogesterone in all corpora lutea examined but concentrations of both hormones were very low (73–374 ng g−1 and 3–93 ng g−1, respectively after HPLC). There were no obvious differences in hormone concentrations in corpora lutea from animals at different reproductive stages. Progesterone and 17α-hydroxyprogesterone immunoreactivity assayed before HPLC was 10–30 times higher than that measured after chromatographic separation. HPLC consistently revealed two large immunoreactive peaks associated with relatively nonpolar compounds, which together accounted for most (at least 75%) of all progesterone immunoreactivity measured. Large amounts of radioactivity with the same retention times as these peaks were also detected after HPLC in samples incubated with [3H]pregnenolone. Analysis of conversion products from four corpus luteum incubations indicated that between 52% and 84% of [3H]pregnenolone had been converted; 19–33% was accounted for by progesterone, and 12–50% by the two substances represented by the unidentified peaks. Subsequent GCMS analysis identified the two immunoreactive peaks as 5α-pregnane-3α-ol-20-one and 5α-pregnane-3,20-dione (5α-dihydroprogesterone). These results indicate that the major progestins contained within and biosynthesized by corpora lutea of African elephants are 5α-reduced metabolites, and that progesterone and 17α-hydroxyprogesterone are quantitatively of minor importance.
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The shortening of the time interval between the onset of oestrus and ovulation in sows by the transcervical administration of seminal plasma was investigated in 23 German Landrace gilts, using the technique of single horn infusions (Mariensee model) in combination with the transcutaneous sonographic monitoring of ovaries. Preparative surgery comprised the detachment of the left uterine horn from the corpus, leaving the caudal end open to the peritoneal cavity but sealing the corpus wound. The left ovary was loosely tied to the ventral abdominal wall for better sonographic distinction. The animals were used in two to four consecutive cycles. After detection of oestrus by the teaser boar, the patent (right) horns were filled by transcervical infusion of 100 ml of a variety of test solutions. Ovulation was probed by transcutaneous sonography at intervals of 4 h thereafter. Native seminal plasma provoked ovulation in the ipsilateral ovary of the treated horn 10.7 h earlier than in the contralateral ovary. This effect was reduced to 7.3 h after charcoal treatment of seminal plasma; addition of 10 μg oestradiol restored the effect in full, while 10 μg of oestradiol in PBS shortened the time interval to only 3.3 h versus the control ovary. Little effect was seen with oestrone sulfate, none with prostaglandins in PBS or with PBS alone. The preliminary characterization of the nonsteroidal component of seminal plasma advancing ipsilateral ovulation after transcervical infusion suggests a proteinaceous nature. The activity resides in the 1–10 kDa fraction separated by ultrafiltration and is lost after treatment with pronase.
