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Departments of Physiology, *The Middlesex Hospital Medical School, Cleveland Street, London, W1P 6DB, and St Mary's Hospital Medical School, Praed Street, London, W2 1PG

(Received 13 October 1976)

Recently there has been increasing interest in the possible role of foetal neurohypophysial hormones in parturition (Alexander, Bashore, Britton & Forsling, 1974; Burton, Illingworth, Challis & McNeilly, 1974). The significance of plasma levels in the mother and foetus depends on the placental permeability to the hormone. It has been suggested that in the sheep there is no placental transfer of oxytocin (Forsling, Jack & Nathanielsz, 1975) or arginine-vasopressin (AVP) (Alexander et al. 1974). Burton and her co-workers, however, reported that the guinea-pig placenta allows passage of oxytocin. It is therefore of interest to know whether this placenta which is haemochorial in nature is also permeable to AVP.

The placentae of nine guinea-pigs in the third trimester of pregnancy were perfused by recirculation

In normally menstruating women plasma vasopressin concentrations vary with the stage of the cycle and are highest at the time of ovulation and lowest at the onset of menstruation. To determine whether this is the result of changes in the circulating concentrations of ovarian steroids, vasopressin concentrations were determined in six postmenopausal women given oestrogen and progestogen. An increase in plasma oestradiol concentrations to 299 ± 97·8 pmol/l augmented vasopressin release. Administration of medroxyprogesterone did not influence vasopressin concentrations but when given in combination with oestrogen a fall was observed. Thus it appears that ovarian steroids can modulate vasopressin release.

Department of Physiology, Middlesex Hospital Medical School, London W1P 6DB, and *Institut für Physiologie, Technische Universität München-Weihenstephan, West Germany

(Received 6 August 1974)

The release of both prolactin and oxytocin during lactation and machine milking in the cow is well established. Benson & Folley (1956) first suggested that oxytocin might initiate the release of prolactin. Bryant & Greenwood (1968), using a specific immunoassay for prolactin, were able to demonstrate this relationship. However, subsequent workers (Koprowski & Tucker, 1971; Schams, 1972) using exogenous oxytocin were unable to confirm these observations.

Experiments involving the use of exogenous oxytocin are inconclusive as it may not be possible to produce appropriate concentrations of oxytocin in the portal vessels to the hypophysis. Observations, therefore, were made on plasma samples taken frequently during milking and teat stimulation to elucidate the relationship between oxytocin and prolactin release. Release of the second neurohypophysial hormone, arginine-vasopressin (AVP), was also

Plasma vasopressin concentrations, determined by radioimmunoassay, were followed throughout the menstrual cycle in eight healthy women. The concentrations were found to depend on the day of the menstrual cycle. The mean concentration on day 1 was 0·5±0·08 (s.e.m.) μu./ml, while that on days 16–18 was 1·1±0·16 μu./ml. These values were significantly (P <0·02) different. Vasopressin release in women may thus depend on the hormonal changes during the menstrual cycle.

SUMMARY

A radioimmunoassay has been developed for plasma arginine-vasopressin in man and dog. The mean recovery of added arginine-vasopressin (AVP) was 60 ± 6·9 (s.d.)% and the lower threshold of detection 2·0 pmol/l. A close correlation was found between concurrent radioimmunoassay and bioassay values. The mean concentration found in peripheral venous blood in healthy men after overnight fasting was 5·3 pmol/l (range 4·6–6·2 pmol/l). In man, significant increases in plasma AVP occurred after dehydration (5·9–9·5 pmol/l) and significant decreases after oral water-loading (5·2–3·7 pmol/l). During i.v. infusion of graded doses of synthetic AVP in normal men, plasma levels were closely correlated with infusion rate. On stopping the infusion, plasma vasopressin fell exponentially with a half-life of between 7 and 8 min. In man, plasma AVP was unaffected by tilting head-up for 2 h, or by a non-hypotensive bleeding of 500 ml in 10 min. In the dog, haemorrhage of 5 ml/kg and over caused proportionate increases in AVP in the circulation. In normal men, plasma vasopressin was significantly correlated with concurrent urinary osmolality. Five patients with oat-cell bronchial carcinoma and hyponatraemia showed a marked increase of plasma vasopressin. Five patients with diabetes insipidus had significantly reduced, but detectable, levels of plasma AVP. The plasma concentration in these patients did not increase after water restriction.

The level of antidiuretic hormone (ADH) in the plasma of pigs was studied during hypoxia, anaesthesia and a combination of the two conditions. Hypoxia, caused by making conscious pigs breathe nitrogen, elicited a rise in the level of ADH without change in plasma osmolality; the hypoxia was accompanied in some cases by a slight lowering of arterial pressure which quickly returned to its original level after the period of hypoxic breathing. Pentobarbitone anaesthesia had no significant effect on the level of ADH but halothane anaesthesia elicited a rise in ADH.

Transient high levels of ADH were seen in animals which were exposed to hypoxia during halothane or pentobarbitone anaesthesia. These high levels of ADH were sometimes, but not invariably, accompanied by a fall in arterial pressure. No consistent changes in plasma osmolality or haematocrit were associated with the raised plasma ADH.

Changes in the neurohypophysial content of arginine vasopressin (AVP) and neurosecretory material in different states of hydration have been reported by many authors (see Jones & Pickering, 1969; Vilhardt, 1970). The present paper reports the effect of hydration on pituitary and plasma levels of AVP and neurophysin in the rat, and on the release of these two peptides in response to haemorrhage.

Four groups of male Wistar rats were studied over a period of 1 week. One group was maintained on an unrestricted water intake (control), another on a restricted water intake, the third on 1·8% sodium chloride solution and a fourth group was hydrated (water intake equivalent to 25% body weight/24 h). The rats used weighed 200 g and at least six animals were included in each group. After 1 week the animals were anaesthetized with sodium pentobarbitone (3 mg/100 g) and 0·8 ml blood removed for the determination

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,

SUMMARY

At 4°, the activity of exogenous 8-arginine vasopressin in rat plasma decreased to 50% in 2 days, whereas there was no loss of oxytocin under the same conditions after 3 days. At 37°, oxytocin was not inactivated in 9 hr., whereas 50% of 8-arginine vasopressin was lost in 2 hr.

Forty per cent of exogenous oxytocin in rat plasma was unable to pass through a cellophane (Visking 8/32 in.) membrane, when subjected to a pressure of 100 torr for 18 hr.

In rats under pentobarbitone anaesthesia, progressive haemorrhage of more than 2·0 ml./100 g. induced a secretion of both oxytocin and 8-arginine vasopressin, to produce maximum concentrations of 450 and 700 μ-u./ml., respectively, in the carotid plasma.

In the intact rat (weighing 200–250 g.) anaesthetized with pentobarbitone, 50% of oxytocin in the circulation disappeared within 2 min. after stopping an i.v. infusion of 250–6000 μ-u./100 g./min. The half-time was increased to 4 min. by sham operation, and to 6 or 7 min. by occluding either the renal, or the portal, coeliac and mesenteric vessels. In non-lactating animals, the half-time was 8 min. after clamping both the renal and splanchnic vessels, whereas it was 6 min. in the lactating rat, under the same conditions.

The apparent volume of distribution of oxytocin was 7·3 ml./100 g. in intact rats, and 10–13 ml./100 g. after operation.

The plasma clearance of oxytocin was unaltered in the sham-operated rat, but it decreased after the operative procedures.

Neurophysin is considered to act as a carrier protein for oxytocin and arginine vasopressin (AVP) in the neurohypophysis, and recent evidence has suggested that neurophysin is released into the blood together with the neurohypophysial hormones in response to various stimuli (Cheng & Friesen, 1970). Neurophysin, extracted from bovine pituitary posterior lobes, consists of two major components designated neurophysin I and II (Hollenberg & Hope, 1968) both of which bind oxytocin and AVP in vitro, although neurophysin II appears to be located specifically in neurosecretory granules containing AVP (Dean, Hope & Kazie, 1968).

We now report results relating to the release of neurophysin, oxytocin and vasopressin into the blood in response to hand-milking, mating and haemorrhage in the goat. Consecutive serial blood samples (approximately 25 ml each) were taken from an indwelling jugular cannula during hand-milking in one goat and during mating in four oestrous female goats (McNeilly & Folley, 1970).