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Abstract

In the present investigation, the effect of recombinant (BST) and pituitary-derived (bGH) bovine somatotrophin on progesterone and oxytocin release was examined. Individual copora lutea (CL) were obtained from cows at different stages of the oestrous cycle (days 5–7, 8–12 and 15–18) and also from early pregnancy (days 60–120) and were implanted with an in vitro microdialysis system (MDS).

Perfusion with BST for 60 min (005, 0·5 and 5 μmol/l) induced a dose-dependent stimulation of progesterone release. Release of oxytocin from CL was significantly stimulated by BST at all dose levels. BST (0·5 μmol/l) stimulated progesterone release most during the early and mid-luteal phases and oxytocin release especially during the early luteal stage (days 5–7) of the oestrous cycle. CL from early pregnancy (days 60–120) treated with BST showed a significant response in progesterone and oxytocin release. bGH showed comparable effects.

Our results suggest that somatotrophin acts directly on the secretory function of bovine CL in the MDS, specifically during the early luteal stage (days 5–7) of the oestrous cycle and early pregnancy (days 60–120). Somatotrophin may therefore have physiologically relevant effects associated with the development and maintenance of luteal function.

Journal of Endocrinology (1994) 143, 243–250

ABSTRACT

Oxytocin concentrations were measured radioimmunologically in sows on the day of standing oestrus over a 6-h period (controls, n=6) or 1 h before and 5 h after mating (n=5) or transcervical infusion of either 100 ml saline (0·9% (w/v) NaCl, n=7) or saline plus 10 μg oestradiol (simulation of seminal oestrogens, n=5).

In the controls, oxytocin was low, at around 1·0 pmol/l, throughout the investigation period. Similarly, saline infusion did not lead to a noticeable change in oxytocin concentrations in six out of seven sows. In one sow, however, infusion led to a maximum of 86 pmol/l at 1 min after infusion. Oestradiol led to no immediate increase in oxytocin concentrations. Later in the post-treatment period (2–5 h) they were only slightly increased (1 pmol/l vs 3 pmol/l). All mated sows reacted with a rapid and clear increase in oxytocin. Maximal concentrations (42·0±5·1 pmol/l; mean ± s.e.m.) appeared 2 min after the onset of ejaculation. Clearly increased concentrations were found for 40 min. It was concluded that mating specifically leads to a rise in oxytocin, probably due to both mechanical and pheromonal stimuli provided by the boar.

Journal of Endocrinology (1990) 126, 361–365

ABSTRACT

The neuroendocrine reflex theory of milk ejection was investigated in the horse under natural suckling conditions. To this end 12 lactating mares were provided with acute jugular catheters and with intramammary pressure (IMP) recording catheters. The foal had free access to the contralateral mammary complex. Intramammary pressure could thus be recorded while blood was drawn simultaneously for oxytocin analysis from the undisturbed animal. Suckling periods associated with a characteristic increase in IMP lasted significantly longer than unsuccessful nursing attempts. Elements of successful sucklings involved physical stimulation of the mammary gland, a quiet phase and a sudden increase in IMP. Successful suckling took place at about 20-min intervals with a wide range from < 5 min to > 100 min. Between 5 and 10 mU oxytocin i.v. were sufficient to evoke an increase in IMP identical in shape and duration to a naturally induced increase in IMP. Mean peak oxytocin levels reached 15·8 pmol/l plasma, with a maximal release of 39 pmol/l. In the majority of cases (> 80%) peak oxytocin release did not occur until after the increase in IMP; in some cases an oxytocin surge was not detectable at all, despite a milk ejection-associated increase in IMP. In three cases increase in IMP could be observed while the foals were away from the mother with no signs of any intention to suckle. The data indicate that in the horse some elements of the neuroendocrine reflex, such as tactile stimulation of the teat and a surge of oxytocin before an increase in IMP, are facultative and not essential for normal milk ejection.

J. Endocr. (1988) 119, 219–227

ABSTRACT

The effect of insulin-like growth factor-I (IGF-I), epidermal growth factor (EGF), fibroblast growth factor (FGF) and nerve growth factor (NGF) on production of oxytocin and progesterone by cultured bovine granulosa and luteal cells was studied. Secretion of oxytocin was stimulated, in a dose-dependent manner, by IGF-I at 48 and 120 h of culture to levels much higher than those after stimulation with LH, FSH, EGF, FGF or NGF. A similar effect of IGF-I was observed for progesterone but, in contrast to oxytocin, secretion of progesterone was not increased by EGF, NGF or FGF. During primary culture, for 4 h, of dispersed bovine luteal cells obtained from corpora lutea between days 4 and 10 of the oestrous cycle, all the growth factors tested failed to stimulate secretion of oxytocin or progesterone. The data suggest the relevance of growth factors (especially IGF-I) for ovarian physiology and their possible importance for differentiation of follicles and luteinization.

J. Endocr. (1988) 116, 97–100

ABSTRACT

Milking cows with a 1-min manual stimulation (treatment 1) and without any udder preparation (treatment 2) was compared by application of an improved, highly sensitive radioimmunoassay for oxytocin and recordings of milk-flow curves. Both treatments caused the release of oxytocin, but treatment 2 generally seemed to be less efficient. Milking characteristics supported the advantage of manual stimulation; milk yield and milk flow were significantly higher, while 'machine-on' time was shorter. This clearly indicates the importance of the right timing of release of oxytocin before commencement of milking. Substitution of stimulation by an i.v. injection of 0·5 i.u. oxytocin (treatment 3) resulted in milking parameters very similar to those of treatment 2. This implies that manual stimulation has other effects besides the secretion of oxytocin which are also responsible for optimal milk removal.

J. Endocr. (1984) 103, 355–361

ABSTRACT

Oxytocin secretion in dairy cows was measured during milking by means of a specific radioimmunoassay. Basal oxytocin concentrations before milking recorded from 147 milkings from 21 cows, where an assay method with enhanced sensitivity was employed, were 1·5 ± 0·6 pmol/l and there was no evidence for any conditioned release of oxytocin. A 1-min manual stimulation before milking evoked a variable but distinct increase in oxytocin concentrations in 188 out of 195 milkings performed using 29 cows. Despite the high variation in absolute concentrations five types of typical secretion pattern could be distinguished. There was no obvious relationship between patterns or absolute concentrations of oxytocin and milk-flow characteristics. Evidence is given that milk ejection seems to follow the threshold principle in that small releases of oxytocin up to a range of 3–5 pmol/l plasma are sufficient to evoke maximum milk ejection.

J. Endocr. (1984) 102, 337–343

Oxytocin was measured radioimmunologically during the oestrous cycle in six ewes of each of two local Moroccan breeds of sheep. Concentrations in both breeds approached the lower limit of the assay (3 pg/ml) from 2 days before oestrus, throughout heat and ovulation until day 2 of the cycle. Oxytocin concentrations then increased in both breeds, the resulting highest levels on days 5–7 were, on average, between 30 and 60 pg/ml in the D'man sheep and 13–31 pg/ml in the Timhadite breed. Oxytocin levels then decreased to about 7 pg/ml in the D'man and about 4–5 pg/ml in the Timhadite breed on days 14–15. After ovariectomy oxytocin concentrations remained at about the limit of detection for a further 19–20 days in both breeds.

Growth hormone was measured by radioimmunoassay in blood samples collected from ewes during the hormonal induction of lactation and during hand-milking post partum. Ovariectomized ewes were induced to lactate with injections of progesterone + oestradiol benzoate every 3 days for 30 days (priming phase) and then daily injections of dexamethasone for 5 days (trigger phase). The ewes were then milked daily. Immunoreactive GH levels fluctuated considerably but were generally in the range 1–15 ng/ml during all phases of lactation and were unaffected by bromocriptine treatment. Milk yield was unrelated to GH levels.

Growth hormone was also measured in blood plasma taken at frequent intervals around the time of milking in lactating ewes approximately 4 weeks post partum. Although a clear prolactin response to milking was observed, there was no indication of a GH response.

Although there is probably a minimum requirement for GH for lactation, a relationship between immunoreactive GH levels and milk yields was not established, perhaps because of limitations of the radioimmunoassay to detect all the biologically active GH.