Clearance studies of oxytocin in humans using radioimmunoassay measurements of the hormone in plasma and urine

Peripheral oxytocin in female baboons relates to estrous state and maintenance of sexual consortships

The neuro-hypophysial hormone oxytocin (OT) has been implicated in female reproductive and maternal behaviors and in the formation of pair bonds in monogamous species. Here we measure variation in urinary OT concentrations in relation to reproductive biology and socio-sexual behavior in a promiscuously breeding species, the chacma baboon (Papio hamadryas ursinus). Subjects were members of a habituated group of baboons in the Okavango Delta, Botswana. We collected behavioral data and urine samples from n=13 cycling females across their estrous cycles and during and outside short-term, exclusive sexual consortships. Samples were analyzed via enzyme immunoassay (EIA) and we used linear mixed models (LMM) to explore the relationship between peripheral OT and a female's estrous stage and consortship status, her previous reproductive experience and fertility. We also used a Pearson's correlation to examine the relationship between OT concentrations of consorting females and their extent of behavioral coordination with their consort partners. The results of the LMM indicate that only estrous stage had a significant influence on OT levels. Females had higher OT levels during their periovulatory period than during other stages of their estrous cycle. There were no differences in the OT levels between consorting and non-consorting periovulatory females. However, among consorting females, there was a significant positive relationship between urinary OT levels and the maintenance of close proximity between consort partners. Our results suggest that physiological and behavioral changes associated with the initiation and maintenance of short-term inter-sexual relationships in baboons correspond with changes in peripheral OT.

Effects of couple interactions and relationship quality on plasma oxytocin and cardiovascular reactivity: empirical findings and methodological considerations

Cardiovascular reactivity is a potential mechanism underlying associations of close relationship quality with cardiovascular disease. Two models describe oxytocin as another mechanism. The "calm and connect" model posits an association between positive relationship experiences and oxytocin levels and responses, whereas the "tend and befriend" model emphasizes the effects of negative relationship experiences in evoking oxytocin release. In this study of 180 younger couples, relationship quality had a small, marginally significant inverse association with plasma oxytocin levels, and neither positive nor negative couple interactions evoked change in plasma oxytocin. Negative couple interactions evoked significant cardiovascular reactivity, especially among women. Hence, in the largest study of these issues to date, there was little support for key tenets of the "calm and connect" model, and only very modest support for the "tend and befriend" model. However, findings were consistent with the view that CVR contributes to the effects of relationship difficulties on health.

Oxytocin and mutual communication in mother-infant bonding

Mother-infant bonding is universal to all mammalian species. In this review, we describe the manner in which reciprocal communication between the mother and infant leads to mother-infant bonding in rodents. In rats and mice, mother-infant bond formation is reinforced by various social stimuli, such as tactile stimuli and ultrasonic vocalizations (USVs) from the pups to the mother, and feeding and tactile stimulation from the mother to the pups. Some evidence suggests that mother and infant can develop a cross-modal sensory recognition of their counterpart during this bonding process. Neurochemically, oxytocin in the neural system plays a pivotal role in each side of the mother-infant bonding process, although the mechanisms underlying bond formation in the brains of infants has not yet been clarified. Impairment of mother-infant bonding, that is, deprivation of social stimuli from the mother, strongly influences offspring sociality, including maternal behavior toward their own offspring in their adulthood, implying a "non-genomic transmission of maternal environment," even in rodents. The comparative understanding of cognitive functions between mother and infants, and the biological mechanisms involved in mother-infant bonding may help us understand psychiatric disorders associated with mother-infant relationships.

Urinary oxytocin as a noninvasive biomarker of positive emotion in dogs

A reliable assay based on physiological parameters that does not require subjective input from the owners is required to assess positive emotions in dogs. In addition, when viewed from an animal welfare perspective, physiological parameters should be collected in a noninvasive manner. Oxytocin (OT) is a biomarker that may be associated with a calm, relaxed state, and positive emotion. We measured the time-lapse in the concentration of plasma OT relative to urinary OT using a radioimmunoassay with sufficient sensitivity and low variability, and examined the relationship between OT and cortisol. Six dogs were injected with exogenous OT intravenously to increase the blood OT concentration. As a result, the highest concentration of urinary OT occurred 1h after the injection, although there was little change in urinary cortisol. Moreover, to evaluate the influence of stimuli on urinary OT and cortisol, we provided three stimuli of eating food, exercising and stroking, all of which were assumed to inspire a positive emotion in dogs, and significantly increased urinary OT concentrations. Our findings indicate that urinary OT might be useful as a noninvasive and objective biomarker of positive emotion in dogs.

Oxytocin indexes relational distress following interpersonal harms in women

The hypothalamic neuropeptide oxytocin, known for its involvement in social affiliation and bonding in animals, has recently been associated with a host of prosocial behaviors that are beneficial for maintaining positive social relationships in humans. Paradoxically, however, people with high endogenous levels of oxytocin also tend to report relational distress and interpersonal difficulties in their everyday lives. To address these contradictory findings, oxytocin reactivity was measured in response to a well-defined laboratory task in young adult women following recent interpersonal harms. Elevated mean peripheral oxytocin reactivity (but not baseline levels of oxytocin or cortisol reactivity) was associated with increased post-conflict anxiety and decreased levels of forgiveness. These results corroborate previous research implicating oxytocin as a neuroendocrine marker of relational distress, but not general stress, and demonstrate the utility of studying oxytocin in response to naturally occurring relational events.

The role of oxytocin in mother-infant relations: a systematic review of human studies

BACKGROUND

Oxytocin is associated with the establishment and quality of maternal behavior in animal models. Parallel investigations in humans are now under way. This article reviews the current research examining the role of oxytocin in mother-infant relations, attachment, and bonding in humans.

METHODS

A systematic search was made of three electronic databases and other bibliographic sources for published research studies that examined oxytocin and mother-infant relations in humans, including attachment, maternal behavior, parenting, and mother-infant relations.

RESULTS

Eight studies were identified, all of which were unique in their methodologies, populations studied, and measures used. Seven studies found significant and strong associations between levels or patterns of oxytocin and aspects of mother-infant relations or attachment.

CONCLUSIONS

Oxytocin appears to be of crucial importance for understanding mother-infant relationships. The findings of this review suggest that the pioneering, but preliminary, research undertaken to date is promising and that replication with larger samples is needed. Research that draws on more robust measures of attachment and bonding, as well as improved measures of oxytocin that include both central and peripheral levels, will elucidate the role of oxytocin in human mother-infant relationships. As the production of oxytocin is by no means restricted to mothers, the extension of the oxytocin studies to fathering, as well as to alloparental caregiving, would be an intriguing next step.

Dog's gaze at its owner increases owner's urinary oxytocin during social interaction

Oxytocin (OT) has been shown to play an important role in social bonding in animals. However, it is unclear whether OT is related to inter-species social bonding. In this study, to examine the possibility that urinary OT concentrations of owners were increased by their "dog's gaze", perhaps representing social attachment to their owners, we measured urinary OT concentrations of owners before and after interaction with their dogs. Dog owners interacted with their dogs as usual for 30 min (interaction experiment) or were instructed not to look at their dogs directly (control experiment). We observed the behaviors of owners and their dogs during the experiments, and measured OT concentrations by radioimmunoassay in urine samples from the owners collected just before and 20 min after interaction with their dogs. Using a cluster analysis, owners could be divided into two groups: one received a longer duration of gaze from their dogs and reported a higher degree of relationship with their dogs (LG); the other received a shorter duration of gaze and reported a lower degree of relationship (SG). Urinary OT was higher in LG than SG after usual interaction with their dogs, but not in the control experiment. In the interaction experiment, a high correlation was found in LG between the frequency of behavioral exchanges initiated by the dog's gaze and the increase in urinary OT. We conclude that interactions with dogs, especially those initiated by the dog's gaze, can increase the urinary OT concentrations of their owners as a manifestation of attachment behavior.

Evidence of dysregulated peripheral oxytocin release among depressed women

OBJECTIVE

Oxytocin is a hypothalamic neuropeptide that plays a key role in mammalian female reproductive function. Animal research indicates that central oxytocin facilitates adaptive social attachments and modulates stress and anxiety responses. Major depression is prevalent among postpubertal females, and is associated with perturbations in social attachments, dysregulation of the hypothalamic-pituitary-adrenal stress axis, and elevated levels of anxiety. Thus, depressed women may be at risk to display oxytocin dysregulation. The current study was developed to compare patterns of peripheral oxytocin release exhibited by depressed and nondepressed women.

METHODS

Currently depressed (N = 17) and never-depressed (N = 17) women participated in a laboratory protocol designed to stimulate, measure, and compare peripheral oxytocin release in response to two tasks: an affiliation-focused Guided Imagery task and a Speech Stress task. Intermittent blood samples were drawn over the course of two, 1-hour sessions including 20-minute baseline, 10-minute task, and 30-minute recovery periods.

RESULTS

The 10-minute laboratory tasks did not induce identifiable, acute changes in peripheral oxytocin. However, as compared with nondepressed controls, depressed women displayed greater variability in pulsatile oxytocin release over the course of both 1-hour sessions, and greater oxytocin concentrations during the 1-hour affiliation-focused imagery session. Oxytocin concentrations obtained during the imagery session were also associated with greater symptoms of depression, anxiety, and interpersonal dysfunction.

CONCLUSIONS

Depressed women are more likely than controls to display a dysregulated pattern of peripheral oxytocin release. Further research is warranted to elucidate the clinical significance of peripheral oxytocin release in both depressed and nondepressed women.

Non-invasive measurement of small peptides in the common marmoset (Callithrix jacchus): a radiolabeled clearance study and endogenous excretion under varying social conditions

A non-invasive assay for measurement of oxytocin (OT) and vasopressin (AVP) in primates would enable researchers to study the relationship between the endocrine system and behavior without disturbing potentially endangered animals in their natural habitats. In order to test whether or not OT specifically would be measurable in the urine of a primate, 10 microCi of tritium-labeled OT were injected into the peripheral blood supply of four adult male common marmosets (Callithrix jacchus), with continuous urinary collection over 48 h. When urine was processed by HPLC separation and beta counting for radioactive clearance, the label was present in all samples in the fraction where OT elutes. Large amounts of OT were also seen in a fraction other than that containing the OT standard, indicating that OT is measurable but that it also undergoes substantial metabolic breakdown. In a second experiment, we isolated six common marmosets for 48 h and then exposed them to social contact to evaluate the effect of changing social stimuli on endogenous urinary measurement of both OT and AVP. Both were measured after HPLC separation to isolate the intact molecule and also to control for cross-reactivity with metabolites in subsequent RIA. Cortisol was also measured to objectively evaluate the stress response. A priori assumptions were that urinary OT and AVP would be lower during a period of isolation and higher during periods of social contact. These assumptions were met, leading us to conclude that peripheral OT and AVP are measurable via urinary assay and that such an assay is a valid means of evaluating social condition in this species.

Urinary oxytocin as a non-invasive biomarker for neurohypophyseal hormone secretion

The objective was to characterize the urinary oxytocin (OT) system with the goal of using it as a biomarker for neurohypophyseal peptide secretion. We studied urinary OT secretion in mice under three conditions: (1) in OT gene deletion mice (OT -/-) which lack the ability to produce the peptide; (2) after arterial vascular infusion of OT and (3) after physiological stimulation with consumption of 2% sodium chloride. OT was measured by radioimmunoassay (RIA) and Surface-Enhanced Laser Desorption Ionization Time of Flight Mass Spectroscopy (SELDI TOF MS). In OT -/- mice (n=25), urinary OT levels were not detectable, while in OT +/+ mice (n=23) levels were 250.2+/-35.3 pg/ml. To evaluate blood/urine transfer, mice with chronic carotid arterial catheters were infused with saline or OT (5 or 20 pmol/min). Peak urine OT levels were 89+/-11.5 and 844+/-181 ng/ml in the low and high OT groups, respectively. Proteomic evaluation showed MS peaks, corresponding to OT ( approximately 1009 Da) and a related peptide ( approximately 1030 Da) with highest levels in mice infused with OT. Salt loading (5 days of 2% NaCl as drinking water) increased plasma osmolality (3.3%), increased plasma and urinary vasopressin (AVP), but caused no changes in OT. Thus, using non-invasive urine samples, we document that urinary OT and AVP can be used to monitor changes in peptide secretion. Urinary OT and AVP, as well as other urinary peptides, may provide a viable biomarker for peptide secretion and may be useful in clinical studies.

Early experience in humans is associated with changes in neuropeptides critical for regulating social behavior

The formation of social attachments is a critical component of human relationships. Infants begin to bond to their caregivers from the moment of birth, and these social bonds continue to provide regulatory emotional functions throughout adulthood. It is difficult to examine the interactions between social experience and the biological origins of these complex behaviors because children undergo both brain development and accumulate social experience at the same time. We had a rare opportunity to examine children who were reared in extremely aberrant social environments where they were deprived of the kind of care-giving typical for our species. The present experiment in nature provides insight into the role of early experience on the brain systems underlying the development of emotional behavior. These data indicate that the vasopressin and oxytocin neuropeptide systems, which are critical in the establishment of social bonds and the regulation of emotional behaviors, are affected by early social experience. The results of this experiment suggest a potential mechanism whose atypical function may explain the pervasive social and emotional difficulties observed in many children who have experienced aberrant care-giving. The present findings are consistent with the view that there is a critical role for early experience in the development of the brain systems underlying basic aspects of human social behavior.

Antidiuretic action of oxytocin is associated with increased urinary excretion of aquaporin-2

BACKGROUND

The antidiuretic effect of oxytocin in humans is controversial. Urinary excretion of aquaporin-2 (AQP2) can be used as an index of the action of vasopressin on the kidney. We investigated whether exogenous oxytocin affects urinary concentration and urinary AQP2 excretion in human beings.

METHODS

Oxytocin was administered intravenously at a rate of 20 mU/min in 10 healthy volunteers, seven patients with central diabetes insipidus (CDI) and three patients with nephrogenic diabetes insipidus (NDI). On the next day, 2 micro g of 1-desamino-8-d-arginine vasopressin (dDAVP) was injected subcutaneously. Two-hour urine was collected before and after the administration of oxytocin and dDAVP, and urinary AQP2 was measured semi-quantitatively by western analysis.

RESULTS

Urine volume and free water clearance were decreased, and urine osmolality was increased by the administration of oxytocin or dDAVP in the normal volunteers and CDI patients. Urinary AQP2 excretion was increased by oxytocin infusion in the normal volunteers (from 34+/-12 to 326+/-120 densitometry unit (DU)/2 h) and in the CDI group (from 8+/-2 to 227+/-92 DU/2 h) (P<0.05), but not in the NDI group. dDAVP also had a similar but more potent effect on the urinary excretion of AQP2 in the normal and CDI groups.

CONCLUSIONS

Oxytocin has an antidiuretic effect and increases the urinary excretion of AQP2 in humans whose urinary concentration mechanism is preserved. These results suggest that AQP2 might have a regulatory role in the antidiuretic action of oxytocin in humans.

Preliminary research on plasma oxytocin in normal cycling women: investigating emotion and interpersonal distress

The neurohormone oxytocin is responsible for initiating childbirth and the let-down reflex in lactating women and is released during sexual orgasm. Oxytocin has been thought of as an affiliation hormone because research on nonhuman mammals has demonstrated that it plays a key role in the initiation of maternal behavior and the formation of adult pair bonds. It has been speculated that social stimuli may induce oxytocin release and that oxytocin may make positive social contact more rewarding. Data are presented from an initial study to examine change in plasma oxytocin in response to a standard imagery task that elicits emotion related to attachment. Twenty-five normal cycling, healthy women underwent imagery tasks and completed questionnaires on attachment and interpersonal problems. Blood draws (5 ml) were bone via an indwelling catheter before, during, and after three interventions (massage, positive emotion, and negative emotion) and to establish baselines. Overall, the data showed a tendency for oxytocin levels to be elevated in response to relaxation massage and decreased in response to sad emotion. There were individual differences in response to the interventions. Those who showed evidence of increased oxytocin levels for positive emotion and massage and who maintained oxytocin levels during negative emotion were less likely to report interpersonal problems associated with intrusiveness. Maintaining oxytocin levels during sadness was also correlated with lower anxiety in close relationships. Women who were in a couple relationship had greater increases in oxytocin in response to positive emotion. In contrast, higher basal levels of oxytocin were associated with greater interpersonal distress. These data suggest that peripheral secretion of oxytocin in response to emotional stimuli is associated with the individual's interpersonal characteristics.

Bioavailability and pharmacokinetics of sublingual oxytocin in male volunteers

The aim of this investigation was to assess the bioavailability and pharmacokinetics of oxytocin in six male subjects after a sublingual dose of 400 int. units (684 micrograms) and after an intravenous dose of 1 int. unit (1.71 micrograms). After intravenous administration, the pharmacokinetic profile could be described with a two-compartment model. The distribution half-life was 0.049 +/- 0.106 h, the elimination half-life was 0.33 +/- 0.23 h, the total body clearance was 67.1 +/- 13.4 L h-1 and the volume of distribution was 33.2 +/- 28.1 L. After sublingual administration, a poor bioavailability with a 10-fold variation between 0.007 and 0.07% was observed. The pharmacokinetic profile could be described with a one-compartment model. The lag time was subject-dependent and ranged between 0.12 and 0.30 h (40% CV). The absorption half-life was 0.45 +/- 0.29 h, and the apparent elimination half-life 0.69 +/ - 0.26 h. This study showed a very poor and interindividual variability in bioavailability. The sublingual route of administration with its 'long' lag time and 'long' absorption half-life would not seem a reliable route for accurate high dosing for immediate prevention of post-partum haemorrhage.

Oxytocin and desamino-oxytocin tablets are not stable under simulated tropical conditions

OBJECTIVES

This study is part of a programme on reduction of postpartum haemorrhage. Buccal oxytocin and desamino-oxytocin administration with a favourable effect on both blood loss and maternal morbidity and mortality were regarded as possible treatments for use in tropical countries. The stability of buccal oxytocin and desamino-oxytocin under tropical conditions was unknown and therefore tested in this study.

STUDY METHODS

The 'experimental shelf lives' of buccal oxytocin and desamino-oxytocin were examined by exposing the tablets to seven artificially controlled conditions. Samples were analysed by high performance liquid chromatography to determine the content of oxytocin and desamino-oxytocin at nine different times during the period of 1 year.

RESULTS

Oxytocin and desamino-oxytocin are fairly stable under refrigeration. Instability for both drugs was detectable after 20 weeks' storage under humid conditions, independent of temperature. Desamino-oxytocin is more sensitive to light exposure; its concentration declines to 55.6% of the stated amount after 1 year of exposure to light compared to 85% in the case of oxytocin. Oxytocin packaged as supplied by the manufacturer were stable for 21 weeks when exposed to simulated humid (75% relative humidity) conditions. At 40 degrees C and 25% relative humidity there is no difference in stability between tablets in sealed aluminium packs as supplied by the manufacturer and unpackaged tablets.

CONCLUSIONS

Tropical conditions make oxytocin and desamino-oxytocin tablets unstable, with humidity as the most adverse factor. The oxytocin tablets were partially protected from the harmful effect of humidity by sealed aluminium package.

Pharmacokinetic properties of the tocolytic agent [Mpa1, D-Tyr(Et)2, Thr4, Orn8]-oxytocin (antocin) in healthy volunteers

OBJECTIVE

The aim of this study was to study the pharmacokinetics of antocin, the tocolytic oxytocin antagonist [Mpa1, D-Tyr2(Et), Thr4, Orn8]-oxytocin.

DESIGN

Antocin was injected intravenously as a bolus dose (5 mumol). Blood samples were taken at intervals for 240 minutes. In addition, the binding of 125I-Tyr10-antocin to blood constituents was determined and compared with 125I-AVP and 125I-[Mpa1, D-Arg8]-vasopressin (desmopressin).

SUBJECTS

Eight healthy, non-smoking adults, three male and five female.

MEASUREMENTS

Antocin was measured using a specific radioimmunoassay after prior extraction of the plasma. Plasma binding was estimated using polyethyleneglycol precipitation.

RESULTS

The rate of plasma disappearance of antocin was best fitted by a biexponential curve. The clearance of antocin was 23.5 +/- 7.6 l/h, the volume of distribution was 13.1 +/- 3.8 l and the biological half-life was 39.0 +/- 4.1 minutes. A greater proportion of 125I-Tyr10-antocin bound to plasma proteins (33.5%) and red blood cells (13%) than did 125I-AVP, 125I-desmopressin and unlabelled desmopressin.

CONCLUSIONS

The half-life was longer and the clearance of antocin was less than that found in a previous study when a non-specific antiserum was used. This is most likely because of the extended blood sampling time period which revealed the biphasic decay pattern. The higher plasma clearance of antocin compared to oxytocin and desmopressin may be explained by its increased binding to blood constituents rather than by differences in enzymatic degradation of the molecules.

Metabolism of neurohypophysial hormones

Over the past decade several new routes of neurohypophysial hormone metabolism have been identified. These include nonhepatic splanchnic clearance and renal clearance in addition to filtration that appears to be receptor mediated. The intraluminal degradation of VP in the proximal tubule, and distal tubular secretion, at least in one species, has been identified. The brain has been identified as a site for VP and OT metabolism, and the amniotic sac may be a major site for VP clearance in the guinea pig fetus. There have been generalized findings regarding VP and OT metabolism. First, VP metabolism in the whole body and in the amniotic sac appears to increase with increasing concentrations of hormone; this does not appear to be the case with OT. Also, evidence has been presented that suggests a potential for the formation of biologically active metabolites. There have been several associations of pathophysiological states with altered VP or OT metabolism, sometimes with plasma levels remaining unchanged. Lastly, caution is emphasized when measuring these hormones by RIA, and differences in specificities of antisera toward hormone metabolites must be considered.

Hereditary vasopressin resistance in man and mouse

The DI +/+ Severe hereditary nephrogenic diabetes insipidus mouse is resistant to the antidiuretic action of vasopressin (VP) because of failure to accumulate cAMP and subsequent inability to form intramembranous particles on the apical (luminal) surface of kidney cells that normally respond to VP. The defect is primarily, if not exclusively, due to excessive activity of specific cAMP-phosphodiesterases. The abnormality can be overcome in vitro and in vivo by the phosphodiesterase inhibitor, rolipram. Most cases of hereditary NDI in man have sex-linked recessive inheritance, which appears to be due to an abnormality of the V2 receptor. The chromosomal locus of the defect is Xq28. Sporadic cases of congenital NDI have been described in females who appear to have a defect beyond the V2 receptor and the guanine nucleotide-binding stimulatory protein. There is no information on the biochemical defect in very rare cases with other types of inheritance patterns. No abnormalities of V1a and V1b receptor function have been found in patients with NDI. Mice and patients with NDI have evidence of increased AVP synthesis. AVP release in relation to plasma osmolality is increased in patients during infusion of hypertonic saline. This is the opposite of what has been described in patients with primary polydipsia (dipsogenic diabetes insipidus) who are chronically overhydrated. Together, these studies indicate that chronic dehydration and overhydration can cause up- and downregulation of the osmotic release of AVP.

Changes in the metabolic clearance of vasopressin and in plasma vasopressinase throughout human pregnancy

Metabolic clearance rates (MCR) of arginine vasopressin (AVP) were measured serially in five women starting before conception, during gestational weeks 7-8 (early), 22-24 (middle), and 36-38 (late pregnancy), and again 10-12 wk postpartum. Hormonal disposal rates were determined after water loading to suppress endogenous AVP release using a constant infusion method designed to achieve three different steady-state concentrations of plasma AVP (PAVP) on each test occasion. Dose schedules were altered in mid- and late pregnancy to obtain comparable AVP levels at each stage of the protocol. Prehydration decreased plasma osmolality sufficiently to suppress AVP release, as circulating AVP-neurophysin measured serially in three of the women was undetectable. The MCR of AVP was similar before conception (0.75 +/- 0.31, 0.79 +/- 0.34, and 0.76 +/- 0.28 liters/min at PAVP of 2.6 +/- 1.9, 4.7 +/- 2.4, and 8.3 +/- 3.9 pg/ml), in early pregnancy (0.89 +/- 0.34, 0.97 +/- 0.04, and 0.95 +/- 0.40 liters/min at PAVP of 2.2 +/- 2.1, 3.9 +/- 3.2, and 7.9 +/- 3.4 pg/ml), and postpartum (0.70 +/- 0.21, 0.69 +/- 0.24, and 0.75 +/- 0.20 liters/min at PAVP 3.5 +/- 1.8, 5.1 +/- 3.7, and 9.1 +/- 4.2 pg/ml). Values at mid-pregnancy (2.8 +/- 1.3, 3.0 +/- 1.2, and 2.7 +/- 1.2 liters/min at PAVP 2.3 +/- 2.2, 4.0 +/- 3.6, and 7.7 +/- 3.9 pg/ml) and late pregnancy (3.2 +/- 1.4, 3.3 +/- 1.4, and 2.9 +/- 1.2 liters/min at PAVP 1.9 +/- 2.0, 3.8 +/- 2.6, and 7.4 +/- 4.1 pg/ml) increased 3-4-fold (all P less than 0.01). Plasma vasopressinase, undetectable at 7-8 gestational wk, increased markedly by mid- and slightly more by late gestation. Finally, relationships between PAVP and urine osmolality were similar before, during, and after pregnancy. We conclude that marked increments in the MCR of AVP occur between gestational weeks 7 and 8 and mid-pregnancy, which parallel the period of greatest rise in both trophoblastic mass and plasma vasopressinase. There was no evidence of a renal resistance to AVP during gestation.

Degradation of radiolabelled arginine vasopressin (125I-AVP) by the human placenta perfused in vitro

The capacity of the human placenta to degrade 125I-labelled arginine vasopressin (125I-AVP) was studied in vitro using a dual circuit perfused lobule preparation. Seven placentas were perfused with the perfusate on the maternal side of the lobule containing 125I-AVP at the upper limit of the physiological range. On average, over a 30-min period, 48% of the 125I-AVP appeared to have been metabolized. With one exception, a patient whose labour was augmented with intravenous oxytocin, no 125I-AVP apparently crossed the placental lobule to the fetal circulation. These data indicate that the human placenta has a considerable capacity to degrade AVP.