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ABSTRACT
In urethane-anaesthetized lactating rats, intramammary pressure occasionally displayed recurrent variations or oscillations having a slow rise time, low amplitude, long duration and a periodicity of 1–4 min. These oscillations differed from changes in intramammary pressure characteristic of reflex milk ejections induced by suckling, and were also observed in unsuckled rats. They were suppressed by lesions of the pituitary stalk or by stimulating the septum, a structure that inhibits the activity of the magnocellular system. They could be induced by long-term low frequency stimulation of the pituitary stalk, lesions of the septum or long-term infusions of oxytocin at a low rate of 0·05–0·3 mu./min. We suggest that the recurrent oscillations in intramammary pressure constitute a particular mode of response of the mammary gland to a tonic release of oxytocin resulting from a moderate but sustained increase in the basal level of electrical activity of the oxytocin-secreting neurones.
J. Endocr. (1985) 107, 89–96
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ABSTRACT
The subcellular distribution of progesterone and oxytocin within the ovine corpus luteum was investigated using differential and density gradient centrifugation. Progesterone and oxytocin were associated with particles which sedimented to a density of 1·049–1·054 g/ml and 1·054–1·061 g/ml respectively. Particle-associated progesterone did not, however, display physical or biochemical characteristics consistent with its storage within secretory granules. When particle-associated progesterone was incubated in HEPES buffer at 37 °C, 70% of the total progesterone was recovered in the incubation medium. The remaining stable particle-associated progesterone was not affected by treatments which stimulated oxytocin release and which have been shown to cause the release of peptides and biogenic amines from secretory granules. These results suggest that particle-associated progesterone represents the intercalation of progesterone into cell membranes and they do not support the hypothesis that progesterone is stored, in a protein-bound form, in luteal secretory granules.
J. Endocr. (1986) 108, 109–116
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In most species there is a release of oxytocin during the expulsive phase of labour with little or no release during the earlier stages (see Chard, 1972). However, information is lacking on the release of arginine-vasopressin (AVP) during parturition. It appears to be released in the rat (Fuchs & Saito, 1971) but not in the goat (McNeilly, Martin, Chard & Hart, 1972). The present studies demonstrate the release of both oxytocin and AVP during parturition in the horse.
Thirty-one jugular blood samples (20 ml) were collected during labour in three Welsh ponies which produced live foals, 20 samples from three mares in early pregnancy and 25 samples from three mares after elective Caesarean operation. Plasma was separated immediately at 4 °C and stored at -20 °C to obviate enzymic destruction. Oxytocin was determined by radioimmunoassay (Chard, Boyd, Forsling, McNeilly & Landon, 1970) and AVP by bioassay (Forsling, Jones & Lee,
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SUMMARY
The effect of ethanol on parturition was studied in conscious unrestrained rabbits fitted with intra-uterine balloons. The onset of labour was predicted by an oxytocin sensitivity test.
With blood ethanol concentrations of 0·25–0·35 g./100 ml., the onset of labour was inhibited, typical contractions were absent, delivery was prolonged, abnormal and postponed for about 30 hr. With a blood concentration of 0·2 g./100 ml., labour contractions were less intense than normal and delivery was prolonged. At a lower concentration, 0·14 g./100 ml., parturition was apparently normal.
Uterine sensitivity to exogenous oxytocin was unimpaired by ethanol treatment. The interference with parturition by ethanol is attributed to central inhibition of the abrupt reflex release of oxytocin.
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Measurements of the contractions of mammary gland tissue are widely used for the assay of oxytocin both in vivo (Tindal & Yokoyama, 1962; Bisset, Clark, Haldar, Harris, Lewis & Silva, 1967) and in vitro (Mendez-Bauer, Cabot & Caldeyro-Barcia, 1960; Rydén & Sjöholm, 1962; Moore & Zarrow, 1965). It is generally accepted that this tissue is more selective than any other mammalian tissue currently used. The recently developed method of van Dongen & Hays (1966) for assaying oxytocin on rat mammary gland tissue in vitro is the most sensitive method so far devised. In this method the time of the beginning of milk expulsion is measured by means of microscopical observation of minute pieces of rat mammary gland. In our experience, however, the precision of this method is not suitable for quantitative measurements of relatively small changes of the oxytocin level in blood. In an attempt to measure the microscopically observed
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SUMMARY
On the 4th day after parturition lactating rats were hypophysectomized and their litters removed. Control rats were injected only with physiological saline for the next 10 days, and showed pronounced regression of the mammary parenchyma and no secretion. When oxytocin was injected together with prolactin and adrenocorticotrophic hormone (ACTH) for 10 days, mammary secretion, often with duct engorgement, was observed in twenty-two out of twenty-four rats, whereas only three out of fourteen rats given prolactin and ACTH showed slight secretory activity. Lobule-alveolar structure was significantly better preserved in the former rats. The combination of oxytocin, prolactin and cortisol acetate was also more effective than prolactin and cortisol acetate in maintaining secretory function and in retarding mammary involution. These results in hypophysectomized rats, as well as similar findings previously reported by us in intact rats, demonstrate that the favourable effects of oxytocin in maintaining secretory activity and retarding mammary involution are exerted directly on the mammary gland.
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Abstract
Oxytocin was administered to virgin female rats at doses of 25–200 pmol/min during 0·077 mol NaCl/l infusion at 150 μl/min on each day of the oestrous cycle. The resultant rates of urine flow, glomerular filtration (GFR) and electrolyte excretion were determined. Oxytocin caused significant increases in urine flow (P<0·001) and sodium excretion (P<0·001); both responses being dose-dependent (P<0·02 and P<0·01 respectively). Significant variations in the renal responsiveness to the hormone occurred over the 4 days of the oestrous cycle. On oestrus the lowest dose of 25 pmol oxytocin/min produced a significant increase in urine flow (from 139·5 ± 4·3 to 165·6 ± 7·1 μl/min, P<0·005) and a dose of 50 pmol/min produced a significant increase in sodium excretion (from 10·6 ± 0·1 to 14·5 ± 0·7 μmol/min, P<0·005). Significant increases in urine flow and sodium excretion were seen on pro-oestrus with hormone administration rates of 50 and 100 pmol/min respectively and on dioestrus day 2 with a rate of 100 pmol/min. On dioestrus day 1 no increase in urine flow or sodium excretion was seen over the dose range of oxytocin administration. A dose of 100 pmol oxytocin/min significantly increased GFR on pro-oestrus and dioestrus day 2, but not on the other 2 days of the cycle. The circulating hormone concentrations produced by oxytocin infusion were similar on each day of the cycle and so could not account for the differences seen. Therefore, these results suggest varying renal responsiveness to oxytocin during the reproductive cycle of the female rat.
Journal of Endocrinology (1997) 154, 347–353
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ABSTRACT
A detailed secretory profile of oxytocin during suckling and parturition was determined in unanaesthetized freely moving rats. Ten pups were reunited with their mothers after 12–15 h of separation. Unless the milk-ejection reflex occurred, there was no difference in serum oxytocin levels before separation and during the suckling of either four or five, or nine or ten pups. Serum oxytocin levels increased abruptly by 50·1 ± 4·2 (s.e.m.) pmol/l (n = 9) at milk ejection, and declined rapidly with a half-life of about 1·5 min. The peak concentration of blood oxytocin at each milk ejection was independent of the previous suckling period; values from the first three milk-ejection reflexes following the introduction of the pups and those observed 3–5 h after introduction were similar. The process of parturition was monitored by recording intra-uterine pressure with a balloon implanted in the uterus. On day 22 or 23 of pregnancy, continuous and rhythmical contractions of the uterus occurred (onset of parturition), but serum levels of oxytocin (21·1 ± 1·9 pmol/l; n = 13) did not alter until the expulsive phase. During the expulsive phase, fetuses were delivered after fetus-expulsion reflexes which were recorded as sudden large increases in intra-uterine pressure. Basal levels of oxytocin in the blood increased during this phase (32·5 ± 4·4 pmol/l; n = 13) and, in addition, rose by about 15 pmol/l and declined slowly after fetus-expulsion reflexes. The increase, however, was quite different from that seen at milk-ejection reflexes.
J. Endocr. (1986) 110, 251–256
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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
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ABSTRACT
Bovine granulosa cells secrete oxytocin when cultured in a serum-supplemented medium. The time-course of secretion is similar to that in the early corpus luteum in vivo, with a delay of 1 to 2 days followed by a peak and decline over the first 5 days of culture. We have investigated the basis of this time-course in vitro and studied the temporal characteristics of the stimulatory actions of ascorbic acid and adrenaline on this process.
Cells cultured on stirred microcarriers showed a similar pattern of secretion of oxytocin to those cultured on conventional flat plates, despite continuing and rapid mitosis. This indicated that the secretion profile in conventional culture was not an artifact related to the cessation of mitosis. Furthermore, secretion of oxytocin and progesterone by cells on microcarriers was stimulated without a corresponding change in mitotic rate, showing that the secretion per cell had been increased.
In conventional culture, addition of ascorbic acid to culture media (0·5 mmol/l) increased the secretion of oxytocin (up to 4·5-fold) but only if ascorbic acid was present during the first day of culture. The cells showed a progressive refractoriness to stimulation after 12 h. Since the time-course of secretion was unaltered by treatment, this resulted in a delay of 1 to 2 days before the action of the ascorbate was seen. The secretion of progesterone was similarly affected but with less stimulation and less consistency. In contrast, cells treated with adrenaline (10 μmol/l) secreted more oxytocin on the day of treatment and did so at any time during culture provided that there was sufficient basal secretion of hormone. Adrenaline also failed to alter the time-course of secretion but treated cells showed a persistent response, maintaining enhanced secretion for up to 3 days after the adrenaline had been removed.
Ascorbate and adrenaline were highly synergistic in their effects, provided that the ascorbate was present from the start of culture; the response to adrenaline strongly reflected the degree of ascorbate stimulation. We conclude that granulosa cells secrete oxytocin according to an inherent time-schedule and that there is a limited period during which they can respond to ascorbate. Since ascorbate is required for the biosynthesis of oxytocin, this suggests that the availability of ascorbate during corpus luteum formation may determine the amount of oxytocin which can be released subsequently in response to catecholamines.
J. Endocr. (1988) 116, 247–258
