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C. A. McArdle

ABSTRACT

Oxytocin is synthesized in the granulosa-derived large cells of the ruminant corpus luteum from a gene which is dramatically up-regulated in the first few days after ovulation. In this work, the regulation of granulosa and luteal cells by prostaglandins and insulin (or insulin-like growth factor-I; IGF-I) has been explored by comparing their effects on oxytocin and progesterone production in cell culture. In granulosa cells, chronic exposure to insulin (17 nmol/l) stimulated luteinization as indicated by increased release of oxytocin and progesterone. Prostaglandin F (PGF) alone had little effect, but synergized with insulin (or IGF-I) to increase the release of both these hormones. In direct contrast, insulin-stimulated oxytocin production by luteal cells was inhibited by PGF. The half-maximal dose (EC50) for PGF action in both cell preparations was similar (10–100 nmol/l). Dose–response studies revealed that PGF increased the potency of insulin in granulosa cells (EC50 for insulin-stimulation of oxytocin release reduced from 141 to 13 nmol/l by 1 μmol PGF/l), but not in luteal cells. Insulin-stimulated oxytocin release from granulosa cells was also synergistically increased by PGE1, PGE2 and forskolin, suggesting this effect to be mediated by adenylate cyclase-coupled PGE receptors. The results reveal that the effects of prostaglandins on oxytocin release are dependent on both the developmental stage of the target tissue and on the presence of other regulators of cellular differentiation. Moreover, they suggest that the increase in responsiveness to insulin and IGF-I, which appears to accompany luteinization in the cow, may be an effect of prostaglandins produced locally during the peri-ovulatory period.

Journal of Endocrinology (1990) 126, 245–253

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J. A. Russell, R. G. Gosden, E. M. Humphreys, R. Cutting, N. Fitzsimons, V. Johnston, S. Liddle, S. Scott and J. A. Stirland

ABSTRACT

Oxytocin secretion is inhibited by opioids, and oxytocin is important in parturition. The effects on parturition of morphine, a relatively selective μ-opioid receptor agonist, were studied in the rat. Morphine or vehicle with or without the opiate antagonist naloxone were administered immediately after the birth of the second pup and the subsequent course of parturition was recorded in a total of 80 rats. Both s.c. morphine (10 mg/kg) and intracerebroventricular (i.c.v.) morphine (18 μg through a previously implanted cannula) interrupted parturition, delaying the birth of the sixth pup after treatment to 187·3 ± 35·9 (s.e.m.) min and 195·4 ± 19·5 min respectively, compared with 46·4 ± 3·7 and 66·1 ± 17·5 min after vehicle alone. The dose of morphine given i.c.v. had no effect when given s.c. Naloxone given concurrently prevented the effects of morphine. Eventually the rate of parturition in the morphine-treated groups recovered. Perinatal pup mortality rate was not increased when morphine was given to the mothers, but it did inhibit the expression of normal intrapartum maternal behaviour.

Pup mortality was increased 48 h post partum by morphine given during parturition, and it reduced the proportion of rats with normal maternal behaviour 24 h post partum. Morphine did not affect spontaneous or oxytocin-stimulated contractile activity of the parturient uterus in vitro. The concentration of oxytocin in trunk blood plasma was decreased 40 min after i.c.v. morphine (24·3 ± 3·9 vs 39·3± 6·5 pmol/l in controls), as was vasopressin (7·2 ± 1·5 vs 19·7 ± 4·5 pmol/l in controls). Intravenous infusion of oxytocin (2–5 mU/min for 144·3 ± 8·2 min; total infused 448·5 ± 61·9 mU) after i.c.v. morphine re-started parturition; all pups were born to these rats (mean time to pup 6, 110·3 ± 12·7 min) before the i.v. vehicle-infused rats given i.c.v. morphine re-started (mean time to pup 6, 406·3±125·2 min).

It is concluded that morphine given during parturition acts centrally through opioid receptors to inhibit oxytocin secretion, and impairs the expression of maternal behaviour. Reversal of the effects of morphine on parturition by i.v. oxytocin demonstrates the important role of oxytocin in fetus ejection and expulsion.

Journal of Endocrinology (1989) 121, 521–536

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A. Lindén, K. Uvnäs-Moberg, G. Forsberg, I. Bednar and P. Södersten

ABSTRACT

Intraperitoneal injection of 5 μg cholecystokinin octapeptide (CCK-8) into male rats deprived of food for 48 h produced a transient (less than 15 min) increase in plasma levels of CCK-8 but suppressed food intake for an extended period (45 min). Plasma concentrations of CCK-8 after i.p. injection of CCK-8 were raised to levels which were fairly comparable to those after feeding. Intracerebroventricular (i.c.v.) injection of the CCK antagonist proglumide (100 μg) reversed the effect of CCK-8 on food intake, while i.p. injection of proglumide (100 μg) did not have this effect. Feeding increased the plasma concentrations of somatostatin and gastrin but not of oxytocin, and somatostatin and oxytocin but not gastrin were released in response to i.p. injection of CCK-8. However, neither somatostatin nor oxytocin affected food intake, and their release in response to CCK-8 was unaffected by i.c.v. injection of proglumide. These results support the suggestion that CCK-8 is a physiological 'satiety' peptide, which can affect food intake in rats by mechanisms involving both peripheral and central CCK receptors.

Journal of Endocrinology (1989) 121, 59–65

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CB Brenninkmeijer, SA Price, A Lopez Bernal and S Phaneuf

There is evidence for hormonal receptor desensitisation in human myometrium, but little is known about the mechanisms involved in the loss of myometrial response to agonists such as beta(2)-adrenergic agonists, prostaglandin gamma and oxytocin. It is well known that the receptors for these hormones are coupled to G-proteins. The first step of receptor desensitisation is the phosphorylation of activated receptors by a G-protein-coupled receptor kinase (GRK). GRKs are members of a multigene family and the various subtypes differ in their localisation, regulation and mode of action. We have used Western blotting and reverse transcription PCR to identify the GRKs present in human myometrium from pregnant and non-pregnant women as well as in cultured human myometrial cells. We have found that human myometrium expresses the GRK subtypes 2, 4gamma, 5 and 6. On the other hand, GRK3 and the isoforms GRK4alpha, beta and delta were not found in myometrial tissue. Our data indicate that GRK2 is only expressed in pregnant term myometrium and is not found in non-pregnant tissue. Moreover, GRK6 appears to be expressed at a much higher level in pregnant term tissue than in non-pregnant myometrium. Our observations suggest that GRK2 and GRK6 may contribute to the regulation of uterine contractility at term. Further work is necessary to determine whether GRKs and receptor desensitisation play a role in disorders of uterine contractility.

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A. H. Taylor, G. St J. Whitley and S. S. Nussey

ABSTRACT

Binding of [3H]arginine vasopressin (AVP) and [3H]oxytocin to primary monolayer cultures of bovine adrenal chromaffin cells was time-dependent, and the binding sites for each peptide were specific and saturable. Studies with the V1 AVP antagonist d(CH2)5Tyr(Me)2-AVP, the V2 agonist 1-deamino-8-d-AVP and the V2 antagonist d(CH2)5 d-Leu2,Val4-AVP indicated that the AVP receptor was V1 in specificity. Scatchard plots showed that each ligand interacted with a single high-affinity, low-capacity binding site: oxytocin dissociation constant (K d) 0·29 ± 0·02 nmol/l, maximum binding capacity (Bmax) 7·6 ± 0·2 fmol/106 cells (or 4500 ± 102 sites/cell) (n = 3); AVP K d 0·09±0·02 nmol/l, Bmax 5·1±0·63 fmol/106 cells (or 3050 ± 318 sites/cell) (n = 3). Although forskolin in concentrations from 1 nmol/l to 1 mmol/l stimulated cyclic AMP (cAMP) production in isolated chromaffin cells, this did not result in detectable catecholamine release. Neither AVP nor oxytocin in concentrations between 10 pmol/l and 10 μmol/l stimulated cAMP production in these cells. Vasopressin in concentrations as low as 10 pmol/l stimulated a sixfold increase in total inositol phosphates; the dose–response curve was triphasic. Oxytocin had little effect on total inositol phosphate accumulation at low concentrations, but concentrations above micromolar stimulated total inositol phosphate production approximately fourfold. There was no measurable release of catecholamines in response to either peptide. The physiological consequences of these AVP-induced changes in inositol phosphate concentrations remain to be elucidated.

Journal of Endocrinology (1989) 121, 133–139

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S Jarvis, AB Lawrence, KA McLean, J Chirnside, LA Deans, SK Calvert, CL Gilbert, JA Goode and ML Forsling

Oxytocin plays an important role at parturition due to its involvement in uterine contractions, foetal expulsion and the onset of maternal behaviour. The role of the related neurohypophysial hormone, vasopressin, is less clear; however, there is some evidence that it is also involved in maternal behaviour and its role in osmotic regulation is well established. The aim of this study was to investigate the inhibitory effects of endogenous opioids on these hormones during the expulsive phase of parturition in the pig, and to examine how opioid restraint interacts with environmental restriction. The subjects of this study were 31 Large Whitex Landrace primiparous sows (gilts). An indwelling jugular catheter was implanted under general anaesthesia at 12 days before the expected parturition day (EPD). From 5 days before the EPD 15 of the gilts were individually housed in a restrictive parturition crate without straw and 16 were individually housed in a straw-bedded pen. Blood samples were taken with increasing frequency towards and during parturition through a catheter extension to reduce disturbance. At 7.5 min after the birth of the first piglet half of the gilts in each environment received a dose of the opioid receptor antagonist naloxone (1 mg/kg, i.v.) with the remaining gilts receiving saline as a control. Overall, there was no effect of environment on either circulating oxytocin or vasopressin. However, both oxytocin and vasopressin were inhibited by endogenous opioids during the expulsive phase. The inhibitory effects of opioids on these hormones did not appear to have any adverse effects on the progress of parturition as judged by cumulative piglet birth intervals. The regulation of the opioid inhibition of oxytocin and vasopressin during parturition is discussed in relation to other neurotransmitters and whether opioid inhibition of these neurohypophysial hormones is part of the 'normal' physiological response to parturition or whether it is stress-induced.

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D. K. O. CHAN and I. CHESTER JONES

SUMMARY

In the European eel, Anguilla anguilla L., neurohypophysial peptides caused prolonged, dose-dependent, increases in blood pressure in the ventral aorta but decreased that in the dorsal aorta. In order of potency they were: isotocin, oxytocin, arginine vasotocin and arginine vasopressin. Lysine vasopressin was without appreciable effects.

Depletion of adrenergic stores in the animal by pharmacological means or blockade of α-adrenergic receptors reduced or abolished the pressor response in the ventral aorta. Partial tachyphylaxis to the peptides could be restored by injection of noradrenaline. Blockade of cholinergic receptors abolished the postbranchial vasodepressor effect. Thioglycerol attenuated the pressor response. The possible mechanism of action of neurohypophysial peptides with special reference to the unifying concept of increases in membrane permeability is discussed.

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Gareth Leng, Rafael Pineda, Nancy Sabatier and Mike Ludwig

, which expresses greatly increased levels of oxytocin receptor at term, by either acutely blocking oxytocin release or slowing parturition ( Blanks & Thornton 2003 , Russell et al . 2003 ). The trigger for initiating parturition varies between species

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D. G. PORTER, SANDRA J. DOWNING and JANE M. C. BRADSHAW

Porcine relaxin (250 guinea-pig units/mg) infused intravenously into anaesthetized rats at 20 μg/h reversibly abolished spontaneous intra-uterine pressure cycles yet left the myometrium responsive to oxytocin in doses of 4–8 mu. The inhibition was found to be primarily of the frequency, rather than of the amplitude, of pressure cycles. Relaxin (5 or 10 μg) was capable of completely suppressing uterine activity driven by prostaglandin F infusion in oestrogen-treated ovariectomized rats. Whereas the β-adrenergic blocker, propranolol, had no effect on relaxin-induced inhibition, phentolamine, an α-blocker, significantly delayed the relaxin effect. It is unlikely, however, that relaxin operates through an α-inhibitory receptor. The results show that relaxin acts primarily as a frequency modulator and is capable of antagonizing an exogenous myometrial stimulant.

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S B Richardson, T Laya and M VanOoy

Abstract

Vasopressin (VP) elicits almost identical insulin-stimulatory dose responses in isolated mouse islets and hamster β (HIT) cells. We have further pharmacologically characterized HIT cell VP receptors by comparing the potencies of a series of VP agonists including the novel V1b agonist, desamino(d-3-(3′-pyridyl)-Ala2,Arg8)VP (d(d-3-Pal)VP), in stimulating insulin secretion and inositol phosphate (IP) production. The relative orders of potency of VP analogues were parallel in both respects: desamino-Arg-VP (dAVP)>Arg-vasotocin (AVT)= VP>oxytocin (OXY)>desamino-d-Arg-VP (ddAVP)> d(d-3-Pal)VP. dAVP, the most potent agonist tested, behaved as a V1 but non-V1a agonist. The potency of d(d-3-Pal)VP relative to VP was 1:134 in stimulating insulin secretion and 1:40 with respect to IP production. In HIT cell monolayers, the relative order of affinity of analogues in competition for binding with [3 H]AVP was: dAVP>AVT=VP>V1a antagonist>OXY>ddAVP>V2 antagonist=d(d-3-Pal)VP, in parallel with their biological activity. The relative orders of potency and affinity parallel those reported for corticotrophic V1b receptors. Binding studies with hamster liver membranes indicate that the hepatic VP receptor belongs to the V1a class. We conclude that VP activates phospholipase C and interacts with functional VP receptors of the V1 type, which do not belong to the V1a subclass and which are similar to V1b receptors.

Journal of Endocrinology (1995) 147, 59–65