Osmoregulation during pregnancy in the rat. Evidence for resetting of the threshold for vasopressin secretion during gestation

Vasopressin and cardiovascular autonomic adjustment in chronic hypertensive pregnancy

Chronic hypertensive pregnancy (CHP) is a growing health issue with unknown etiology. Vasopressin (VP), a nonapeptide synthesized in paraventricular (PVN) and supraoptic nucleus (SON), is a well-known neuroendocrine and autonomic modulator of the cardiovascular system, related to hypertension development. We quantified gene expression of VP and its receptors, V1aR and V1bR, within the PVN and SON in CHP and normal pregnancy, and assessed levels of secreted plasma VP. Also, we evaluated autonomic cardiovascular adaptations to CHP using spectral indices of blood pressure (BPV) and heart rate (HRV) short-term variability, and spontaneous baroreflex sensitivity (BRS). Experiments were performed in female spontaneously hypertensive rats (SHRs) and in normotensive Wistar rats (WRs). Animals were equipped with a radiotelemetry probe for continuous hemodynamic recordings before and during pregnancy. BPV, HRV and BRS were assessed using spectral analysis and the sequence method, respectively. Plasma VP was determined by ELISA whilst VP, V1aR, and V1bR gene expression was analyzed by real-time-quantitative PCR (RT-qPCR). The results show that non-pregnant SHRs exhibit greater VP, V1aR, and V1bR gene expression in both PVN and SON respectively, compared to Wistar dams. Pregnancy decreased VP gene expression in the SON of SHRs but increased it in the PVN and SON of WRs. Pregnant SHRs exhibited a marked drop in plasma VP concentration associated with BP normalization. This triggered marked tachycardia, heart rate variability increase, and BRS increase in pregnant SHRs. It follows that regardless of BP normalization in late pregnancy, SHRs exhibit cardiovascular vulnerability and compensate by recruiting vagal mechanisms. Pregnant SHR dams have reduced expression of VP in SON associated with increased V1bR expression, lower plasma VP, normal BP during late pregnancy and marked signs of enhanced sympathetic cardiac stimulation (increased HR and LFHR variability) and recruitment of vagal mechanisms (enhancement of BRS and HFHR variability).

The role of maternal hormones in regulating autonomic functions during pregnancy

Offspring development relies on numerous physiological changes that occur in a mother's body, with hormones driving many of these adaptations. Amongst these, the physiological functions controlled by the autonomic nervous system are required for the mother to survive and are adjusted to meet the demands of the growing foetus and to ensure a successful birth. The hormones oestrogen, progesterone, and lactogenic hormones rise significantly during pregnancy, suggesting they may also play a role in regulating the maternal adaptations linked to autonomic nervous system functions, including respiratory, cardiovascular, and thermoregulatory functions. Indeed, expression of pregnancy hormone receptors spans multiple brain regions known to regulate these physiological functions. This review examines how respiratory, cardiovascular, and thermoregulatory functions are controlled by these pregnancy hormones by focusing on their action on central nervous system circuits. Inadequate adaptations in these systems during pregnancy can give rise to several pregnancy complications, highlighting the importance in understanding the mechanistic underpinnings of these changes and potentially identifying ways to treat pregnancy-associated afflictions using hormones.

Vasopressin regulation of maternal body fluid balance in pregnancy and lactation: A role for TRPV channels?

Renal water reabsorption increases in pregnancy and lactation to expand maternal blood volume to cope with the cardiovascular demands of the developing fetus and new-born baby. Vasopressin (antidiuretic hormone) promotes renal water reabsorption and its secretion is principally stimulated by body fluid osmolality. Hence, lowered osmolality normally decreases vasopressin secretion. However, despite water retention profoundly reducing osmolality in pregnancy and lactation, vasopressin levels are maintained to drive blood volume expansion. Despite its importance for successful reproduction, the cellular mechanisms that maintain vasopressin secretion in the face of decreased osmolality during pregnancy and lactation are unknown. Vasopressin is secreted by neurons that are intrinsically osmosensitive through expression of N-terminal truncated-transient receptor potential vanilloid-1 channel, ΔN-TRPV1, which is mechanically activated by osmotically-induced cell shrinkage to increase vasopressin neuron activity. Vasopressin neurons also express TRPV4 but the role of TRPV4 in vasopressin neuron function is not well characterised. Here, we summarise our novel evidence showing that TRPV4 forms functional channels with ΔN-TRPV1 that have a greater single-channel conductance compared to channels with ΔN-TRPV1 alone. We propose that upregulation of TRPV4 heteromerisation with ΔN-TRPV1 might maintain vasopressin secretion in pregnancy and lactation to expand blood volume for successful reproduction.

Local kisspeptin excitation of rat oxytocin neurones in late pregnancy

Abstract

The hormone, oxytocin, is synthesised by magnocellular neurones of the supraoptic and paraventricular nuclei and is released from the posterior pituitary gland into the circulation to trigger uterine contractions during parturition. Kisspeptin fibre density increases around the supraoptic nucleus over pregnancy and intracerebroventricular kisspeptin excites oxytocin neurones only in late pregnancy. However, the mechanism of this excitation is unknown. Here, we found that microdialysis administration of kisspeptin into the supraoptic nucleus consistently increased the action potential (spike) firing rate of oxytocin neurones in urethane‐anaesthetised late‐pregnant rats (gestation day 18–21) but not in non‐pregnant rats. Hazard analysis of action potential firing showed that kisspeptin specifically increased the probability of another action potential firing immediately after each action potential (post‐spike excitability) in late‐pregnant rats. Patch‐clamp electrophysiology in hypothalamic slices showed that bath application of kisspeptin did not affect action potential frequency or baseline membrane potential in supraoptic nucleus neurones. Moreover, kisspeptin superfusion did not affect the frequency or amplitude of excitatory postsynaptic currents or inhibitory postsynaptic currents in supraoptic nucleus neurones. Taken together, these studies suggest that kisspeptin directly activates oxytocin neurones in late pregnancy, at least in part, via increased post‐spike excitability.

Key points

Oxytocin secretion is triggered by action potential firing in magnocellular neurones of the hypothalamic supraoptic and paraventricular nuclei to induce uterine contractions during birth.

In late pregnancy, kisspeptin expression increases in rat periventricular nucleus neurones that project to the oxytocin system.

Here, we show that intra‐supraoptic nucleus administration of kisspeptin increases the action potential firing rate of oxytocin neurones in anaesthetised late‐pregnant rats, and that the increased firing rate is associated with increased oxytocin neurone excitability immediately after each action potential.

By contrast, kisspeptin superfusion of hypothalamic slices did not affect the activity of supraoptic nucleus neurones or the strength of local synaptic inputs to supraoptic nucleus neurones.

Hence, kisspeptin might activate oxytocin neurons in late pregnancy by transiently increasing oxytocin neuron excitability after each action potential.

Adaptive Changes in single-nephron GFR, Tubular Morphology, and Transport in a Pregnant Rat Nephron: Modeling and Analysis

Normal pregnancy is characterized by massive increases in plasma volume and electrolyte retention. Given that the kidneys regulate homeostasis of electrolytes and volume, the organ undergoes major adaptations in morphology, hemodynamics, and transport to achieve the volume and electrolyte retention required in pregnancy. These adaptations are complex, sometimes counterintuitive, and not fully understood. In addition, the demands of the developing fetus and placenta change throughout the pregnancy. For example, during late pregnancy, K⁺ retention and thus enhanced renal K⁺ reabsorption is required despite many kaliuretic factors. The goal of this study is to unravel how known adaptive changes along the nephrons contribute to the ability of the kidney to meet volume and electrolyte requirements in mid- and late pregnancy. We developed computational models of solute and water transport in the superficial nephron of the kidney of a rat in mid- and late pregnancy. The mid-pregnant and late-pregnant rat superficial nephron models predict that morphological adaptations and increased activity of the sodium hydrogen exchanger 3 (NHE3) and epithelial sodium channel (ENaC) are essential for enhanced Na⁺ reabsorption observed during pregnancy. Model simulations showed that for sufficient K⁺ reabsorption, increased H ⁺-K ⁺-ATPase activity and decreased K⁺ secretion along the distal segments is required in both mid- and late-pregnancy. Furthermore, certain known sex differences in renal transporter pattern (e.g., the higher NHE3 protein abundance but lower activity in the proximal tubules of virgin female rats compared to male) may serve to better prepare the female for the increased transport demand in pregnancy.

Plasticity in Intrinsic Excitability of Hypothalamic Magnocellular Neurosecretory Neurons in Late-Pregnant and Lactating Rats

Oxytocin and vasopressin secretion from the posterior pituitary gland are required for normal pregnancy and lactation. Oxytocin secretion is relatively low and constant under basal conditions but becomes pulsatile during birth and lactation to stimulate episodic contraction of the uterus for delivery of the fetus and milk ejection during suckling. Vasopressin secretion is maintained in pregnancy and lactation despite reduced osmolality (the principal stimulus for vasopressin secretion) to increase water retention to cope with the cardiovascular demands of pregnancy and lactation. Oxytocin and vasopressin secretion are determined by the action potential (spike) firing of magnocellular neurosecretory neurons of the hypothalamic supraoptic and paraventricular nuclei. In addition to synaptic input activity, spike firing depends on intrinsic excitability conferred by the suite of channels expressed by the neurons. Therefore, we analysed oxytocin and vasopressin neuron activity in anaesthetised non-pregnant, late-pregnant, and lactating rats to test the hypothesis that intrinsic excitability of oxytocin and vasopressin neurons is increased in late pregnancy and lactation to promote oxytocin and vasopressin secretion required for successful pregnancy and lactation. Hazard analysis of spike firing revealed a higher incidence of post-spike hyperexcitability immediately following each spike in oxytocin neurons, but not in vasopressin neurons, in late pregnancy and lactation, which is expected to facilitate high frequency firing during bursts. Despite lower osmolality in late-pregnant and lactating rats, vasopressin neuron activity was not different between non-pregnant, late-pregnant, and lactating rats, and blockade of osmosensitive ΔN-TRPV1 channels inhibited vasopressin neurons to a similar extent in non-pregnant, late-pregnant, and lactating rats. Furthermore, supraoptic nucleus ΔN-TRPV1 mRNA expression was not different between non-pregnant and late-pregnant rats, suggesting that sustained activity of ΔN-TRPV1 channels might maintain vasopressin neuron activity to increase water retention during pregnancy and lactation.

H,K-ATPase type 2 regulates gestational extracellular compartment expansion and blood pressure in mice

The modifications of the hemodynamic system and hydromineral metabolism are physiological features characterizing a normal gestation. Thus, the ability to expand plasma volume without increasing the level of blood pressure is necessary for the correct perfusion of the placenta. The kidney is essential in this adaptation by reabsorbing avidly sodium and fluid. In this study, we observed that the H,K-ATPase type 2 (HKA2), an ion pump expressed in kidney and colon and already involved in the control of the K⁺ balance during gestation, is also required for the correct plasma volume expansion and to maintain normal blood pressure. Indeed, compared with WT pregnant mice that exhibit a 1.6-fold increase of their plasma volume, pregnant HKA2-null mice (HKA2KO) only modestly expand their extracellular volume (×1.2). The renal expression of the epithelial Na channel (ENaC) α- and γ-subunits and that of the pendrin are stimulated in gravid WT mice, whereas the Na/Cl^- cotransporter (NCC) expression is downregulated. These modifications are all blunted in HKA2KO mice. This impeded renal adaptation to gestation is accompanied by the development of hypotension in the pregnant HKA2KO mice. Altogether, our results showed that the absence of the HKA2 during gestation leads to an "underfilled" situation and has established this transporter as a key player of the renal control of salt and potassium metabolism during gestation.

Osmoregulation and electrolyte balance in a fully marine mammal, the dugong (Dugong dugon)

Dugongs (Dugong dugon) are fully marine mammals that live independently of fresh water so must balance water and electrolytes in a hyperosmotic environment. To investigate osmoregulation, matched plasma and urine from 51 live wild dugongs were analysed for osmolality, major electrolytes (Na⁺, Cl^-, K⁺), urea, creatinine, and glucose. Maximum urine osmolality (1468 mOsm kg ^-1) and Na⁺, K⁺, and Cl^- concentrations (757, 131.3, 677 mmol L^-1, respectively) indicate that dugongs are capable of concentrating urine above seawater and could potentially realise a net gain of free water from drinking seawater. However, mean urine osmolality of 925.4 (± 46.6) mOsm kg^-1 suggests that mariposia is unlikely to be an important osmoregulatory mechanism. Dugongs may obtain enough preformed water from their seagrass diet and metabolic oxidation to maintain homeostasis. Mean plasma osmolality of 339.6 (± 1.8) mOsm kg^-1 is higher than in the related manatees but within the range for fully marine cetaceans. Relatively high mean plasma Na⁺ (175.5 ± 1.7 mmol L^-1) and K⁺ (6.9 ± 0.1 mmol L^-1), as well as mean urinary Na⁺ (469.6 ± 22.5 mmol L^-1) and K⁺ levels (32.5 ± 4.5 mmol L^-1) may reflect a salt-rich seagrass diet. Pregnant females had higher mean plasma osmolality (355.3 ± 4.9 mmol L^-1) than non-pregnant females and males (337.9 ± 1.7 mOsm kg^-1), suggesting that fluid retention was not a feature of pregnancy. Further research on water intake and endocrinology will enhance our understanding of osmoregulation in dugongs.

Preeclampsia and the brain: neural control of cardiovascular changes during pregnancy and neurological outcomes of preeclampsia

Preeclampsia (PE) is a form of gestational hypertension that complicates ∼5% of pregnancies worldwide. Over 70% of the fatal cases of PE are attributed to cerebral oedema, intracranial haemorrhage and eclampsia. The aetiology of PE originates from abnormal remodelling of the maternal spiral arteries, creating an ischaemic placenta that releases factors that drive the pathophysiology. An initial neurological outcome of PE is the absence of the autonomically regulated cardiovascular adaptations to pregnancy. PE patients exhibit sympathetic overactivation, in comparison with both normotensive pregnant and hypertensive non-pregnant females. Moreover, PE diminishes baroreceptor reflex sensitivity (BRS) beyond that observed in healthy pregnancy. The absence of the cardiovascular adaptations to pregnancy, combined with sympathovagal imbalance and a blunted BRS leads to life-threatening neurological outcomes. Behaviourally, the increased incidences of maternal depression, anxiety and post-traumatic stress disorder (PTSD) in PE are correlated to low fetal birth weight, intrauterine growth restriction (IUGR) and premature birth. This review addresses these neurological consequences of PE that present in the gravid female both during and after the index pregnancy.

Magnocellular Neurons and Posterior Pituitary Function

The posterior pituitary gland secretes oxytocin and vasopressin (the antidiuretic hormone) into the blood system. Oxytocin is required for normal delivery of the young and for delivery of milk to the young during lactation. Vasopressin increases water reabsorption in the kidney to maintain body fluid balance and causes vasoconstriction to increase blood pressure. Oxytocin and vasopressin secretion occurs from the axon terminals of magnocellular neurons whose cell bodies are principally found in the hypothalamic supraoptic nucleus and paraventricular nucleus. The physiological functions of oxytocin and vasopressin depend on their secretion, which is principally determined by the pattern of action potentials initiated at the cell bodies. Appropriate secretion of oxytocin and vasopressin to meet the challenges of changing physiological conditions relies mainly on integration of afferent information on reproductive, osmotic, and cardiovascular status with local regulation of magnocellular neurons by glia as well as intrinsic regulation by the magnocellular neurons themselves. This review focuses on the control of magnocellular neuron activity with a particular emphasis on their regulation by reproductive function, body fluid balance, and cardiovascular status. © 2016 American Physiological Society. Compr Physiol 6:1701-1741, 2016.

The Hypo-Osmolality of Pregnancy Does Not Alter Baroreceptor Responses to Acute Changes in Osmolality

Reducing extracellular osmolality by 10-12 mOsm/L reduces baroreceptor firing in vitro in tissues from male rats. Pregnancy is associated with a similar reduction in plasma osmolality (pOsm) as well as with alterations in baroreceptor firing and reduced baroreceptor reflex activity. Mechanisms for reduced baroreflex activity are not yet understood, but they have important implications for maintenance of cardiovascular homeostasis in the pregnant female. Thus, this study was designed to test whether changes in plasma osmolality alter baroreceptor discharge during pregnancy. Late-gestation pregnant and virgin control rats were anesthetized, femoral vessels were cannulated for measurement of arterial pressure and drug infusion, and the aortic depressor nerve, containing baroreceptor afferents, was isolated and prepared for recording. Plasma osmolality was measured before and 30 min after graded intraperitoneal injections of NaCl (50-1500 mOsm/L). Arterial pressure, heart rate, and aortic depressor nerve activity (ADNA) were measured continuously before and after injections. A 50 mOsm/L NaCl injection significantly decreased pOsm but did not alter ADNA in either group. Likewise, 1200 mOsm/L NaCl injections significantly increased pOsm and had no effect on ADNA in either group. The 1500 mOsm salt load significantly increased pOsm and ADNA in pregnant rats, and in virgin animals, it increased pOsm but evoked a paradoxical decrease in ADNA. The authors’ studies indicate that pOsm has minimal effects on baroreceptor activity, which are not significantly different during gestation. Their findings suggest that in vitro data from male rats indicating a correlation between pOsm and baroreceptor discharge are not applicable in anesthetized intact female animals.

Increased levels of copeptin before clinical diagnosis of preeclampsia

Copeptin, a surrogate biomarker of vasopressin, has been associated with renal function decline and may serve as a useful early biomarker for preeclampsia. We measured serum copeptin using samples collected longitudinally during pregnancy among unaffected controls (n=136) and cases of preeclampsia (n=169), gestational diabetes mellitus (n=92), gestational hypertension (n=101), and preterm birth (n=86) in the Calcium for Preeclampsia Prevention trial (1992-1995). Preeclampsia and gestational hypertension were defined as having a diastolic blood pressure≥90 mm Hg on 2 occasions with and without proteinuria, respectively. The risk of pregnancy complications associated with copeptin was estimated by logistic regression adjusting for maternal age, race, body mass index, insurance status, marital status, current smoking, and clinical site. Baseline copeptin levels, at mean 16 weeks of gestation, were associated with increased preeclampsia risk (adjusted odds ratios and 95% confidence interval being 1.55 per log unit; 1.03-2.31) when compared with controls (P=0.03). The association was stronger among cases diagnosed before 37 weeks (1.86; 1.08-3.20) than those diagnosed later (1.45; 0.91-2.32). Copeptin levels rose with increasing gestational age in both cases and controls but remained significantly higher among those who were diagnosed with preeclampsia. Differences in levels of copeptin between cases and controls became more apparent closer to time of diagnosis. No significant associations were found for gestational hypertension without proteinuria, gestational diabetes mellitus, or preterm birth without preeclampsia. Copeptin levels are elevated in pregnant women before diagnosis of preeclampsia with elevation specific to this pregnancy complication rather than hypertension alone.

The role of osmoregulation in the pathophysiology and management of severe ovarian hyperstimulation syndrome

Severe ovarian hyperstimulation syndrome (OHSS), with an incidence of 1-2% of superovulation cycles, remains one of the most important complications of gonadotrophin use in assisted reproductive technologies because of its associated morbidity and rarely, mortality. Despite the wealth of scientific and clinical interest that this iatrogenic complication has generated, its pathophysiology is still not adequately elucidated and its management has thus remained empirical. Disorders of salt and water balance are two very important features that have been reported during severe OHSS. Some of the clinical and biochemical changes resulting from this disorder of salt and water balance are similar to those previously reported in pregnancy and liver cirrhosis. The pathophysiology of these clinical changes has been explained in part in pregnancy and liver cirrhosis by changes in osmoregulation function. It is this similarity in the clinical and biochemical changes in OHSS, pregnancy and liver cirrhosis that has prompted the investigation of the role of osmoregulation function in the pathophysiology of OHSS. The current article has been written to provide further details in support of recent excellent articles and guidelines, highlighting the physiological basis and rationale governing some aspects of, and the role of osmoregulation in the management of the OHSS syndrome.

Hormones and hemodynamics in pregnancy

CONTEXT

Normal pregnancy is associated with sodium and water retention, which results in plasma volume expansion prior to placental implantation. The explanation offered for these events is that pregnancy 'resets' both volume and osmoreceptors.

EVIDENCE ACQUISITION

The mechanisms for such an enigmatic 'resetting' in pregnancy have not previously been explained. However, recent human pregnancy studies have demonstrated that the earliest hemodynamic change in pregnancy is primary systemic arterial vasodilation. This arterial underfilling is associated with a secondary increase in cardiac output and activation of the neurohumoral axis, including stimulation of the renin-angiotensin-aldosterone, sympathetic, and non-osmotic vasopressin systems. Resistance to the pressor effects of angiotensin and sympathetic stimulation in pregnancy is compatible with an increase in endothelial nitric oxide synthase activity.

RESULTS

In contrast to the sodium and water retention which occur secondary to the primary arterial vasodilation in cirrhosis, glomerular filtration and renal blood flow are significantly increased in normal pregnancy. A possible explanation for this difference in arterial vasodilation states is that relaxin, an arterial vasodilator which increases during pregnancy, has a potent effect on both systemic and renal circulation. Endothelial damage in pregnancy is pivotal in the pathogenesis of preeclampsia in pregnancy.

CONCLUSIONS

Against a background of the primary arterial vasodilation hypothesis, it is obvious that reversal of the systemic vasodilatation in pregnancy, without subsequent activation of the renin-angiotensin-aldosterone system (78), will evoke a reversal of all the links in the chain of events in normal pregnancy adaptation, thus, it may cause preeclampsia. Namely, a decrease of renal vasodilation will decrease glomerular filtration rate.

Excess maternal salt intake produces sex-specific hypertension in offspring: putative roles for kidney and gastrointestinal sodium handling

Hypertension is common and contributes, via cardiovascular disease, towards a large proportion of adult deaths in the Western World. High salt intake leads to high blood pressure, even when occurring prior to birth – a mechanism purported to reside in altered kidney development and later function. Using a combination of in vitro and in vivo approaches we tested whether increased maternal salt intake influences fetal kidney development to render the adult individual more susceptible to salt retention and hypertension. We found that salt-loaded pregnant rat dams were hypernatraemic at day 20 gestation (147±5 vs. 128±5 mmoles/L). Increased extracellular salt impeded murine kidney development in vitro, but had little effect in vivo. Kidneys of the adult offspring had few structural or functional abnormalities, but male and female offspring were hypernatraemic (166±4 vs. 149±2 mmoles/L), with a marked increase in plasma corticosterone (e.g. male offspring; 11.9 [9.3–14.8] vs. 2.8 [2.0–8.3] nmol/L median [IQR]). Furthermore, adult male, but not female, offspring had higher mean arterial blood pressure (effect size, +16 [9–21] mm Hg; mean [95% C.I.]. With no clear indication that the kidneys of salt-exposed offspring retained more sodium per se, we conducted a preliminary investigation of their gastrointestinal electrolyte handling and found increased expression of proximal colon solute carrier family 9 (sodium/hydrogen exchanger), member 3 (SLC9A3) together with altered faecal characteristics and electrolyte handling, relative to control offspring. On the basis of these data we suggest that excess salt exposure, via maternal diet, at a vulnerable period of brain and gut development in the rat neonate lays the foundation for sustained increases in blood pressure later in life. Hence, our evidence further supports the argument that excess dietary salt should be avoided per se, particularly in the range of foods consumed by physiologically immature young.

The ouabain-binding site of the α2 isoform of Na,K-ATPase plays a role in blood pressure regulation during pregnancy

BACKGROUND

The cardiotonic steroid/ouabain-binding site of the α subunit of Na,K-ATPase is thought to play an important role in cardiovascular homeostasis. Previously, we demonstrated the cardiotonic steroid-binding site of the α2 Na,K-ATPase is involved in adrenocorticotropic hormone (ACTH)-induced hypertension by using gene-modified α2(R/R) mice in which the cardiotonic steroid-binding site is relatively resistant to ouabain compared to the ouabain-sensitive wild-type α2(S/S) mice. To further explore the importance of this site in the cardiovascular system, we investigated blood pressure regulation during pregnancy in mice with the α2(R/R) isoform.

METHODS

The systolic blood pressure (SBP) of the α2(S/S) and α2(R/R) mice was measured before and during pregnancy by tail-cuff. The expression of the α isoforms of Na, K-ATPase in various tissues and plasma endogenous ouabain contents were assessed prior to pregnancy as well as days 7 and 17 of gestation.

RESULTS

The α2(S/S) mice showed a gradual decrease in the SBP during the first two trimesters, followed by an increase above the preconceptional level in the third trimester. However, the α2(R/R) mice exhibited a lower blood pressure in the third trimester. The cardiac expression of the α2 Na,K-ATPase in the α2(S/S) mice was significantly less than that of the α2(R/R) mice throughout the pregnancy. The plasma endogenous ouabain concentration significantly increased by twofold at day 17 of pregnancy in the α2(R/R) mice but not in the α2(S/S) mice.

CONCLUSIONS

The cardiotonic steroid-binding site of the α2 Na,K-ATPase plays a role in maintaining normal SBP during pregnancy.

Dilemmas in human and rat pregnancy: proposed mechanisms relating to arterial vasodilation

There are several mysteries about the normal physiology of pregnancy (1). Renal sodium and water reabsorption occurs despite an increase in total blood volume. Moreover, the renin-angiotensin-aldosterone system (RAAS) is also stimulated in normal pregnancy, even though an increase in total blood volume normally suppresses the RAAS and increases sodium excretion in the nonpregnant state. Water retention also occurs and leads to hypoosmolality in normal pregnancy, even though total blood volume is expanded. On rare occasions, an extreme polyuria occurs in pregnancy that is unresponsive to arginine vasopressin (AVP). The receptors whereby AVP and oxytocin stimulate the principal cell of the collecting duct, vascular smooth muscle, glomerular mesangial and endometrial cells are in need of clarification. The present study proposes potential mechanisms relating to systemic arterial vasodilation to explain some of the apparent dilemmas of pregnancy.

Administration of D-Alanine-[Ang-(1-7)] (A-779) Prior to Pregnancy in Sprague Dawley Rats Produces Antidiuresis in Late Gestation

We previously demonstrated that angiotensin-(1-7) [Ang-(1-7)], which is increased in the kidney and urine during pregnancy, influences normal fluid expansion of pregnancy. These previous studies were completed by chronic administration of the Ang-(1-7) receptor antagonist D-Alanine-[Ang-(1-7)] (A-779) at a dose of 48 μg/kg/hr after the start of pregnancy (gestational days 11-19). To further explore the role of Ang-(1-7) on kidney function during early, middle, and late pregnancy, Sprague Dawley rats were chronically pretreated 8 days prior to pregnancy and throughout pregnancy (gestational days 0-19) with vehicle or A-779 at a dose of 24 μg/kg/hr. Metabolic studies were completed in virgin animals and throughout pregnancy (gestational days 4-5, 14-15, and 18-19). Chow consumption and water intake increased throughout pregnancy while the difference between intake and output (balance) was increased only at late (day 19) pregnancy with both vehicle and A-779 administration. Urine volume and urinary osmolality were significantly increased and decreased respectively throughout pregnancy in vehicle treated rats only. In late (19 day) pregnancy, A-779 administration significantly decreased chow consumption and water intake. In virgin animals, A-779 administration significantly increased urine volume, while during late pregnancy (19 day), urine volume was significantly decreased with A-779 administration. These studies using pretreatment with a lower dose of A-779 prior to pregnancy confirm results of higher dose A-779 administration after the start of pregnancy. These studies show that Ang-(1-7) produces antidiuresis in virgin rats and diuresis in late gestation. Ang-(1-7) also contributes to the enhanced water intake during pregnancy allowing maintenance of the normal volume expanded state despite diuresis.

Hemodynamic parameters during normal and hypertensive pregnancy in rats: evaluation of renal salt and water transporters

OBJECTIVE

To determine whether alterations in extracellular volume expansion observed during normal and hypertensive pregnancy run in parallel to changes in the mRNA expression of renal transporters.

METHODS

Wistar rats were divided into four groups: control (C, n = 5); pregnancy (P, n = 5); N(omega)-nitro-l-arginine methyl ester (L-NAME; 50 mg/kg/d)-treated control (H, n = 6); and pregnant rats (HP, n = 6). Hemodynamic studies were performed on day 14 of pregnancy, at which time we also analyzed of the sodium transporters (NHE3, Na/K/2Cl and Na/Cl), potassium channel (ROMK2) and water channel (AQP2).

RESULTS

As expected, P rats presented high cardiac output (CO) and normal blood pressure (BP), whereas H rats presented lower CO and elevated BP. A significant (threefold) increase in total vascular resistance and a decrease in stroke volume were observed in the HP group. Hypertension resulting from nitric oxide (NO) synthesis inhibition blunted systemic hemodynamic adaptations during pregnancy. Compared with C rats, mRNA expression of ROMK2 in P rats was lower, whereas that of AQP2 was higher. Expression of AQP2 was significantly higher in H than in C or HP groups. Expression of BSC and NHE3 was lower in the HP than in the P group. The NO inhibition also provoked renal transporter alterations in HP.

CONCLUSIONS

Our results suggest that tubule transporter variants may mediate the hemodynamic adaptations seen during pregnancy, although we cannot rule out the hypothesis that other factors are also mediating hemodynamic changes.

Angiotensin-(1-7) serves as an aquaretic by increasing water intake and diuresis in association with downregulation of aquaporin-1 during pregnancy in rats

We previously demonstrated that kidney and urine levels of angiotensin-(1-7) [ANG-(1-7)] were increased in pregnancy. To explore the role of ANG-(1-7) on fluid and electrolyte homeostasis during pregnancy, we evaluated the effect of the ANG-(1-7) antagonist d-alanine-[ANG-(1-7)] (A-779) on kidney function. Virgin and pregnant rats received infusion of vehicle or A-779 (48 μg·kg−1·h−1) for 8 days by osmotic minipumps. Metabolic studies were done on treatment day 7–8. Virgin and pregnant rats at day 15 and 19 were killed, and blood and kidneys were collected. Kidneys were prepared for Western blot analysis for aquaporin-1 (AQP1) and aquaporin-2. In virgin female rats, A-779 increased urine volume and decreased urinary osmolality and AQP1 with no change in water intake. In 19-day pregnant rats, A-779 significantly decreased water intake and urine volume and increased urinary osmolality and kidney AQP1 expression. Only in late gestation did A-779 treatment decrease the difference between intake and output (balance). A-779 treatment increased plasma vasopressin in late gestation but did not change vasopressin in virgins. In virgin and pregnant animals, A-779 administration had no effect on blood pressure, plasma volume, blood volume, or urinary electrolytes. These results suggest that ANG-(1-7) produces antidiuresis associated with upregulation of AQP1 in virgin rats, whereas ANG-(1-7) produces diuresis in late gestation with downregulation of AQP1. ANG-(1-7) contributes to the enhanced water intake during pregnancy, allowing maintenance of the normal volume-expanded state despite diuresis produced in part by decreased AVP and AQP1.

[The cardiovascular paradox of pregnancy]

Despite widespread accessibility to prenatal care, little is known on the mechanisms initiating early maternal adaptation to pregnancy. Moreover, preeclampsia and intrauterine growth retardation remain the most frequent and serious complications of pregnancy. Recent studies, both in humans and in laboratory animals, have shown that very early events in gestation may be important determinants for the continuation of healthy pregnancy. Certain of these early adaptations appear to be linked to the corpus luteum of pregnancy, as ovarian steroid hormones (especially progesterone) would set the basic hemodynamic conditions, more specifically, generalized vasodilation. This new hemodynamic setup initiates a vicious cycle in which the renin - angiotensin - aldosterone system is activated, together with the resetting of the control of antidiuretic hormone secretion relative to plasma osmolality. This leads to a gradual and substantial increase in plasma volume and a parallel increase in cardiac function (both heart rate and stroke volume) with the goal of maintaining blood pressure in the face of the generalised vasodilation. This includes the creation of a functional arterio-venous shunt represented by the utero-placental circulation. By the end of the first trimester, the decrease in peripheral vascular resistance is marked relative to the increase in cardiac output, resulting in a significant decrease in blood pressure that will be maintained until the third trimester. It is proposed that in preeclampsia, these very early events (vasodilation - increased plasma volume) fail to occur, resulting in an absence of the usual decrease in blood pressure, which is normally seen in the second trimester of pregnancy, and hypertension in the third trimester. Experimental animals, especially the rat, are suitable models to study this early maternal adaptation to pregnancy, since both endocrine and hemodynamic changes appear to be similar to humans.

Estrogen receptors: their roles in regulation of vasopressin release for maintenance of fluid and electrolyte homeostasis

Long standing interest in the impact of gonadal steroid hormones on fluid and electrolyte balance has led to a body of literature filled with conflicting reports about gender differences, the effects of gonadectomy, hormone replacement, and reproductive cycles on plasma vasopressin (VP), VP secretion, and VP gene expression. This reflects the complexity of gonadal steroid hormone actions in the body resulting from multiple sites of action that impact fluid and electrolyte balance (e.g. VP target organs, afferent pathways regulating the VP neurons, and the VP secreting neurons themselves). It also reflects involvement of multiple types of estrogen receptors (ER) in these diverse sites including ERs that act as transcription factors regulating gene expression (i.e. the classic ERalpha as well as the more recently discovered ERbeta) and potentially G-protein coupled, membrane localized ERs that mediate rapid non-genomic actions of estrogen. Furthermore, altered expression of these receptors in physiologically diverse conditions of fluid and electrolyte balance contributes to the difficulty of using simplistic approaches such as gender comparisons, gonadectomy, and hormone replacement to assess the role of gonadal steroids in regulation of VP secretion for maintenance of fluid and electrolyte homeostasis. This review catalogs these inconsistencies and provides a frame work for understanding them by describing: (1) the effect of gonadal steroids on target organ responsiveness to VP; (2) the expression of multiple types of estrogen receptors in the VP neurons and in brain regions monitoring feedback signals from the periphery; and (3) the impact of dehydration and hyponatremia on expression of these receptors.

Neuroendocrine regulation of fluid and electrolyte balance by ovarian steroids: contributions from central oestrogen receptors

Like other hormonally mediated mechanisms, maintenance of body fluid osmolality requires integrated responses from multiple signals at various tissue locales, a large number of which are open to modulation by circulating endocrine factors including the ovarian steroid, oestrogens (E(2)). However, the precise mechanism and the site of action of E(2) in regulating fluid osmolality are not properly understood. More importantly, the biological significance of this action is not clear and the physiological circumstances in which this modulation is engaged remain incomplete. The demonstration of oestrogen receptors (ER) in neural tissues that bear no direct relation to reproduction led us to examine and characterise the expression of ER in brain nuclei that are critical for the maintenance of fluid osmolality. In the rat, ERbeta is prominently expressed in the vasopressin magnocellular neuroendocrine cells of the hypothalamus, whereas ERalpha is localised extensively in the sensory circumventricular organ neurones in the basal forebrain. These nuclei are the primary brain sites that are engaged in defense of fluid perturbation, thus providing a neuroendocrine basis for oestrogenic influence on body fluid regulation. Plasticity in receptor expression that accompanies fluid disturbances at these central loci suggests the functional importance of the receptors and implicates E(2) as one of the fluid regulating hormones in water homeostasis.

Gender specificity of programmed plasma hypertonicity and hemoconcentration in adult offspring of water-restricted rat dams

OBJECTIVE

We studied the impact of maternal water-restriction during rat pregnancy on newborn plasma composition, and determined the persistence of plasma composition alterations in adult offspring.

METHODS

Maternal dams were water-restricted from 10 days of pregnancy until term (21 days) and throughout lactation to increase plasma sodium levels by approximately 6 mEq/L. At 21 days of age, offspring were weaned, and subsequently maintained on ad libitum food and water until 12 weeks of age. Daily water and food intake was monitored. Blood samples and organs were collected from 1-day- and 12-week-old offspring. Hematocrit, plasma osmolality, sodium, and arginine vasopressin (AVP) levels were analyzed. Because water-restriction led to concomitant reduction in maternal food intake (ie, dehydration anorexia), henceforth these dams and their offspring are referred to as "water-deprived/food-reduced" rats.

RESULTS

Water-deprived/food-reduced dams had significantly increased plasma sodium levels, reduced food intake, and lower body weight gain during pregnancy and lactation as compared to control dams. One-day-old newborns of water-deprived/food-reduced dams weighed 17% less and had increased plasma sodium levels, osmolality, and hematocrit. At 12 weeks of age, males exhibited 11% and females 19% reduction in body weight from controls. Notably, male offspring of water-deprived/food-reduced dams showed significantly elevated plasma sodium levels, osmolality, and hematocrit. Additionally, males demonstrated reduced adrenal growth and decreased water intake. Conversely, the female offspring had similar plasma osmolality with decreased sodium levels, though a persistently elevated hematocrit. No differences were evident in plasma AVP levels.

CONCLUSIONS

Maternal water deprivation/food reduction is associated with increased newborn plasma osmolality and sodium levels and long-term physiologic changes in the offspring. The gender-specificity of programmed hyperosmolality, though not hemoconcentration, implicates differing pathways/mechanisms for these phenotypic alterations. The contributions of pregnancy hypertonicity versus nutrient restriction in the mechanism for programmed offspring phenotype remain to be elucidated.

Influence of brain angiotensin on thermoregulation and hydromineral balance during pregnancy in rats

During mammalian pregnancy, body temperature decreases and there are changes in fluid and electrolyte balance. Angiotensin signaling mechanisms in the brain have been shown to influence thermoregulation and body fluid balance in the nonpregnant state. We hypothesized that brain angiotensin is also implicated in adjusting these physiological systems in the pregnant rat. We compared core temperature and fluid regulation in three groups of pregnant rats: untreated rats, rats receiving continuous infusion of an AT1 antagonist candesartan (5 μg·kg−1·day−1) into a lateral cerebral ventricle to block brain AT1 receptors, and rats receiving vehicle [artificial cerebrospinal fluid (aCSF)] vehicle. Untreated and aCSF-treated rats showed a decrease in colonic temperature (−0.5 and −0.8°C respectively) by day 20 of gestation. However, rats treated with candesartan had increased colonic temperature compared with baseline (+0.9°C), and their temperature was significantly higher on days 7 ( P < 0.05), 17 ( P < 0.05), and 20 ( P < 0.001) compared with the other groups (aCSF and untreated). Daily food and water intakes and body weight were not different between the three groups. Similarly, litter sizes and pup weights were equal in all groups. Finally, the expected decreases in plasma Na+ and osmolality during pregnancy were equivalent in all groups. This study suggests that brain angiotensin mediates the progressive decrease in body temperature that occurs during pregnancy. However, the changes in fluid balance associated with pregnancy are not dependent on brain angiotensin.

Sensory circumventricular organs: central roles in integrated autonomic regulation

Circumventricular organs (CVO) play a critical role as transducers of information between the blood, neurons and the cerebral spinal fluid (CSF). They permit both the release and sensing of hormones without disrupting the blood-brain barrier (BBB) and as a consequence of such abilities the CVOs are now well established to have essential regulatory actions in diverse physiological functions. The sensory CVOs are essential signal transducers located at the blood-brain interface regulating autonomic function. They have a proven role in the control of cardiovascular function and body fluid regulation, and have significant involvement in central immune response, feeding behavior and reproduction, the extent of which is still to be determined. This review will attempt to summarize the research on these topics to date. The complexities associated with sensory CVO exploration are intense, but should continue to result in valuable contributions to our understanding of brain function.

Maternal plasma hypertonicity is accentuated in the postterm rat

OBJECTIVE

In humans and rats, pregnancy-associated maternal plasma volume expansion and plasma hypotonicity may facilitate maternal-to-fetal water transfer. Although reduced amniotic fluid volume occurs commonly in postterm pregnancy, the mechanisms are unknown. We previously demonstrated a reversal of pregnancy-induced maternal plasma hypotonicity that occurs in the near term (20 days) pregnant rats. We sought to determine whether the relative maternal plasma hypertonicity continues in the postterm period.

STUDY DESIGN

Rat gestation (normal, 21 days) was prolonged with subcutaneous progesterone injection. Pregnant rats at gestation, 18 days, 21 days, and 24 days and nonpregnant rats were studied. Maternal and fetal hematocrit levels, plasma osmolality, electrolyte levels, and amniotic fluid volume were determined. In addition, maternal and fetal tissues were analyzed for water and electrolyte content.

RESULTS

Compared with term (21days), postterm pregnant rats (24 days) had a significant increase in maternal and fetal plasma osmolality (293.7+/-1.4 mOsm/kg vs 302.8+/-3.7 mOsm/kg and 301.0+/-2.0 mOsm/kg vs 310.3+/-3.2 mOsm/kg, respectively; P<.01) and sodium and chloride concentrations. Conversely, both maternal and fetal hematocrit levels decreased significantly in the postterm period. Postterm rats demonstrated an increased fetal mortality rate (24%) and a significantly reduced amniotic fluid volume (4.2+/-0.6 mL vs 6.6+/-0.6 mL, P<.01).

CONCLUSION

These results indicate that the near-term reversal of maternal plasma hypotonicity that has been observed previously is further accentuated in the postterm pregnancy. This continued hypertonicity may induce a fetal-to-maternal water flow and contribute to postterm oligohydramnios and increased fetal morbidity and mortality rates.

Aquaporins in the kidney: from molecules to medicine

The discovery of aquaporin-1 (AQP1) answered the long-standing biophysical question of how water specifically crosses biological membranes. In the kidney, at least seven aquaporins are expressed at distinct sites. AQP1 is extremely abundant in the proximal tubule and descending thin limb and is essential for urinary concentration. AQP2 is exclusively expressed in the principal cells of the connecting tubule and collecting duct and is the predominant vasopressin-regulated water channel. AQP3 and AQP4 are both present in the basolateral plasma membrane of collecting duct principal cells and represent exit pathways for water reabsorbed apically via AQP2. Studies in patients and transgenic mice have demonstrated that both AQP2 and AQP3 are essential for urinary concentration. Three additional aquaporins are present in the kidney. AQP6 is present in intracellular vesicles in collecting duct intercalated cells, and AQP8 is present intracellularly at low abundance in proximal tubules and collecting duct principal cells, but the physiological function of these two channels remains undefined. AQP7 is abundant in the brush border of proximal tubule cells and is likely to be involved in proximal tubule water reabsorption. Body water balance is tightly regulated by vasopressin, and multiple studies now have underscored the essential roles of AQP2 in this. Vasopressin regulates acutely the water permeability of the kidney collecting duct by trafficking of AQP2 from intracellular vesicles to the apical plasma membrane. The long-term adaptational changes in body water balance are controlled in part by regulated changes in AQP2 and AQP3 expression levels. Lack of functional AQP2 is seen in primary forms of diabetes insipidus, and reduced expression and targeting are seen in several diseases associated with urinary concentrating defects such as acquired nephrogenic diabetes insipidus, postobstructive polyuria, as well as acute and chronic renal failure. In contrast, in conditions with water retention such as severe congestive heart failure, pregnancy, and syndrome of inappropriate antidiuretic hormone secretion, both AQP2 expression levels and apical plasma membrane targetting are increased, suggesting a role for AQP2 in the development of water retention. Continued analysis of the aquaporins is providing detailed molecular insight into the fundamental physiology and pathophysiology of water balance and water balance disorders.

Altered osmotic thresholds for arginine vasopressin secretion and thirst during superovulation and in the ovarian hyperstimulation syndrome (OHSS): relevance to the pathophysiology of OHSS

OBJECTIVE

To test the hypothesis that decreases in and maintenance of a new steady state in plasma osmolality and sodium level in ovarian hyperstimulation syndrome (OHSS) are due to altered osmoregulation of arginine vasopressin secretion and thirst.

DESIGN

Prospective study.

SETTING

IVF-ET program in a university-based assisted reproductive treatment center.

PATIENT(S)

Eight women undergoing superovulation for IVF-ET and five women with normal menstrual cycles.

INTERVENTION(S)

Two-hour infusion of 5% saline on day 3 or 4 after hCG administration in patients undergoing IVF or in the early luteal phase in controls. A 5% saline infusion test was done on day 10 after hCG administration in one patient with OHSS and one patient without OHSS, both of whom were undergoing IVF.

MAIN OUTCOME MEASURE(S)

Comparison of changes in thresholds for thirst and plasma vasopressin to plasma osmolality. Changes in urine osmolality, plasma electrolytes, hemoglobin level, and hematocrit were assessed at baseline and during infusion of 5% saline.

RESULT(S)

The sensitivity of the changes in arginine vasopressin secretion and thirst after 5% saline infusion was similar in IVF patients on day 3 or 4 after hCG and controls. However, the osmotic threshold was significantly lower by 6 mOsm/kg in IVF patients. By day 10 after hCG, the lower osmotic thresholds for arginine vasopressin secretion and thirst persisted in OHSS, although the sensitivity to arginine vasopressin secretion was markedly reduced.

CONCLUSION(S)

The osmotic thresholds for arginine vasopressin secretion and thirst are reset to lower plasma osmolality during superovulation for IVF-ET. This new lower body tonicity is maintained until at least day 10 after hCG in OHSS. Decreases in plasma osmolality and plasma sodium levels in OHSS are due to altered osmoregulation rather than electrolyte losses; correction of apparent "electrolyte imbalance" in OHSS is therefore inappropriate.

Water-losing and water-retaining states: role of water channels and vasopressin receptor antagonists

Alterations in water metabolism are present in conditions such as diabetes insipidus, syndrome of inappropriate antidiuretic hormone secretion, cardiac failure, cirrhosis, and pregnancy. Recent advances in molecular biology have enhanced our understanding of disordered water metabolism in these conditions. This review examines the roles of central vasopressin synthesis and release and collecting duct vasopressin V2 receptor and aquaporin-2 water channel regulation in water-losing and water-retaining states.

The role of steroid hormones in the regulation of vasopressin and oxytocin release and mRNA expression in hypothalamo-neurohypophysial explants from the rat

Vasopressin and oxytocin release from the neural lobe, and the vasopressin and oxytocin mRNA contents of the supraoptic and paraventricular nuclei are increased by hypertonicity of the extracellular fluid. The factors regulating these parameters can be conveniently studied in perifused explants of the hypothalamo-neurohypophysial system that include the supraoptic nucleus (but not the paraventricular nucleus) with its axonal projections to the neural lobe. Vasopressin and oxytocin release and the mRNA content of these explants respond appropriately to increases in the osmolality of the perifusate. This requires synaptic input from the region of the organum vasculosum of the lamina terminalis. Glutamate is a likely candidate for transmitting osmotic information from the organum vasculosum of the lamina terminalis to the magnocellular neurones, because agonists for excitatory amino acid receptors stimulate vasopressin and oxytocin release, and because increased vasopressin release and mRNA content induced in hypothalamo-neurohypophysial explants by a ramp increase in osmolality are blocked by antagonists of both NMDA (N-methyl-D-aspartate) and non-NMDA glutamate receptors. Osmotically stimulated vasopressin release is also blocked by testosterone, dihydrotestosterone, oestradiol and corticosterone. Both oestrogen and dihydrotestosterone block NMDA stimulation of vasopressin release, and in preliminary studies oestradiol blocked AMPA stimulation of vasopressin release. Thus, steroid inhibition of osmotically stimulated vasopressin secretion may reflect inhibition of mechanisms mediated by excitatory amino acids. Recent studies have demonstrated numerous mechanisms by which steroid hormones may impact upon neuronal function. Therefore, additional work is warranted to understand these effects of the steroid hormones on vasopressin and oxytocin secretion and to elucidate the potential contribution of these mechanisms to regulation of hormone release in vivo.

Mechanism of volume adaptation in the awake early pregnant rat

The objective of the present study was to determine whether the increase in plasma volume (PV) during pregnancy is established by fluid retention or by a shift within the extracellular fluid volume (ECFV) from the interstitium toward the intravascular compartment. To this end, we simultaneously, measured total body water, (TBW), ECFV, and PV together with the hematocrit (Hct) and plasma osmolality 4, 8, and 12 days postsurgery in chronically instrumented pregnant (P) and nonpregnant (NP) rats. The P rats were instrumented with a catheter in the femoral artery on day 1 postconception. In the NP group, neither TBW nor ECFV and PV had changed consistently on days 8 and 12 postsurgery relative to day 4. In contrast, in the P animals, TBW, ECFV, and PV had increased by 16, 24, and 20%, respectively, by day 12 relative to day 4. To evaluate whether PV had increased in concert with an overall rise in TBW or as a result of a fluid shift at the cost of the interstitial fluid volume, we calculated the relative size of each fluid compartment on three consecutive measurement sessions. In the NP group, TBW, presented as percentage of maternal weight (%MW) as well as ECFV (%TBW) and PV (%ECFV) had not changed consistently throughout the measurement period. In the P animals, TBW (%MW) was slightly higher on day 12 compared with day 4, but ECFV (%TBW) and PV (%ECFV) had not changed significantly. Finally, in the NP group, Hct had not changed, whereas, in the P animals, Hct was 10% lower on days 8 and 12 compared with day 4. Plasma osmolality did not change consistently in either group during the course of the experimental period. The gradual synchronous increase in all fluid compartments, without consistent change in their relative distribution, suggests that, in normal rat pregnancy, PV expansion is primarily achieved by fluid retention rather than by a redistribution of the ECFV.

Upregulation of aquaporin 2 water channel expression in pregnant rats

Water retention is characteristic of pregnancy but the mechanism(s) of the altered water metabolism has yet to be elucidated. The collecting duct water channel, aquaporin 2 (AQP2), plays a pivotal role in the renal water regulation, and we hypothesized that AQP2 expression could be modified during pregnancy. Sprague-Dawley female rats were studied on days 7 (P7), 14 (P14), and 20 (P20) of pregnancy, and expression of AQP2 in papillae was examined. Nonpregnant (NP) littermates were used as controls. Plasma osmolalities were significantly lower in pregnant rats by day 7 of gestation (P7 283.8+/-1.82, P14 284.3+/-1.64, P < 0.001, P20 282. 4+/-1.32, P < 0.0001, vs. NP 291.8+/-1.06 mosmol/kgH2O). However, plasma vasopressin concentrations in pregnant rats were not significantly different than in nonpregnant rats (NP 1.03+/-0.14, P7 1.11+/-0.21, P14 1.15+/-0.21, P20 1.36+/-0.24 pg/ml, NS). The mRNA of AQP2 was increased early during pregnancy: AQP2/beta actin: P7 196+/-17.9, P14 200+/-6.8, and P20 208+/-15.5%, P < 0.005 vs. NP (100+/-11.1%). AQP2 protein was also increased during pregnancy: AQP2 protein: P7 269+/-10.0, P14 251+/-12.0, P < 0.0001, and P20 250+/-13.6%, P < 0.001 vs. NP (100+/-12.5%). The effect of V2 vasopressin receptor antagonist, OPC-31260, was then investigated. AQP2 mRNA was suppressed significantly by OPC-31260 administration to P14 rats (AQP2/beta actin: P14 with OPC-31260 39.6+/-1.7%, P < 0.001 vs. P14 with vehicle) and was decreased to the same level of expression as NP rats receiving OPC-31260. Similar findings were found with the analysis of AQP2 protein. The decreased plasma osmolality of P14 rats was not modified by OPC-31260. The results of the study indicate that upregulation of AQP2 contributes to the water retention in pregnancy through a V2 receptor-mediated effect. In addition to vasopressin, other factors may be involved in this upregulation.

Diagnosis and Management of Diabetes Insipidus During Pregnancy

OBJECTIVE

To provide an overview of diagnostic and treatment strategies in pregnant patients with diabetes insipidus (DI).

METHODS

We review the changes in osmoregulation during normal pregnancy, characterize the various types of DI that can occur during pregnancy, and discuss the recommended management.

RESULTS

The incidence of DI is 2 to 4 cases per 100,000 gestations. Central DI can precede pregnancy or manifest initially during gestation. With preexistent central DI, pregnancy usually aggravates the disorder, and the requirements for antidiuretic hormone (ADH) usually increase. Such an effect is less likely to be noted in ADH-independent nephrogenic forms of DI. Currently, the major type of DI associated with pregnancy is a transient syndrome that is resistant to arginine vasopressin (AVP) but responsive to desmopressin (dDAVP); such cases of DI are often associated with liver abnormalities or preeclampsia. This syndrome is explained by excess vasopressinase, a placental enzyme which degrades AVP but not dDAVP. A transient recurrent type of DI can occur during gestation in patients with limited ADH-secreting capacity and is responsive to both AVP and dDAVP. Latent central DI manifesting after complicated delivery and transient nephrogenic DI, resistant to both AVP and dDAVP, have also been reported.

CONCLUSION

The differential diagnosis of polyuric and polydipsic states during pregnancy is broad, and precise diagnosis may be difficult. The use of dDAVP to treat DI during gestation has proved effective and safe for both the mother and the fetus.

Osmolality- and angiotensin-induced responses in the AV3V neurons are affected by estrogen in the ovariectomized rat

The effect of hyperosmolality (300, 320 mosmol/kg H2O) and angiotensin II (A II, 10(-8) and 10(-6) M) was tested on a total of 64 neurons within the periventricular part of the anteroventral third ventricle (AV3V) region in brain slice preparations obtained from ovariectomized (OVX) rats with or without chronic treatment of estradiol-17 beta (E2). Hypertonic perfusion with a 320 mosmol/kg H2O but not a 300 mosmol/kg H2O medium caused a significant increase in the firing discharge rate in OVX animals. Perfusion with either hypertonic medium had no effect in E2-treated rats. The neuronal firing discharge rate of neurons in OVX rats was increased following perfusion with 10(-6) M A II, but not affected following perfusion with 10(-8) M A II. In E2-treated rats, perfusion with neither 10(-8) nor 10(-6) M A II had any effect. These data suggest a dynamic relationship between the ovarian endocrine function and the central mechanisms regulating dipsogenic behavior and/or release of vasopressin in the female rat.

Central administration of porcine relaxin stimulates drinking behaviour in rats: an effect mediated by central angiotensin II

Central injection of porcine relaxin into the lateral ventricle of water-replete rats caused a marked drinking response. Relaxin in 2 µL 0.9% saline caused a dose-dependent (range 10-50 ng), significant (P<0.01) dipsogenesis compared with saline-treated controls. There was no drinking response to <10 ng relaxin. At 10 ng relaxin ICV rats drank 4.2 ± 0.2 mL water within 15 min of injection. The amount of water taken increased with increasing dose and plateaued at 50 ng ICV (10.2 ± 1.3 mL) thereafter; increasing the dose of relaxin did not significantly increase the total volume of water consumed. In contrast, there was no significant increase in water consumed in rats treated with a deactivated form of porcine relaxin, or with insulin. Rats appeared to compensate for the period of hyperdipsia, as there was no significant difference in the water consumed in control (saline-injected) and relaxin-treated rats in the 23 h period after testing.The effect of blocking the central action of angiotensin II on the dipsogenic effects of relaxin was tested by infusing of a specific angiotensin II receptor antagonist into the lateral ventricle before treatment with relaxin. Antagonism of the central angiotensin II system, confirmed by lack of a dipsogenic response to ICV exogenous angiotensin II (10 ng), completely blocked the dipsogenic response of relaxin (50 ng in 1 µL) in female rats.These data demonstrate that exogenous porcine relaxin is dipsogenic in the rat and that the mechanism of action appears to be through the central angiotensin II system. It is possible that relaxin may affect water intake during pregnancy when relaxin levels are detectable in the plasma and the hormone may be implicated in the regulation of cardiovascular function in pregnancy.

Paradoxes of body fluid volume regulation in health and disease. A unifying hypothesis

The body's normal homeostasis is maintained by the integrity of the excretory capacity of the kidneys. In advanced cardiac failure, however, the avidity of the renal sodium and water retention contributes to the occurrence of pulmonary congestion and peripheral edema. In patients with advanced cirrhosis, the kidneys again fail to excrete the amounts of sodium and water ingested, thus leading to ascites and peripheral edema. The signals for this renal retention of sodium and water in a patient with cirrhosis must be extrarenal because when the same kidneys are transplanted into persons with normal liver function, renal sodium and water retention no longer occurs; rather, the kidneys maintain normal fluid and electrolyte balance. Excessive sodium and water retention by the kidneys also occurs during pregnancy despite a 30% to 50% increase in plasma volume, cardiac output, and glomerular filtration rate. What are the afferent and efferent signals whereby normal kidneys retain sodium and water so that total extracellular, interstitial, and intravascular volumes expand far beyond those limits observed in normal subjects? These dilemmas are the subject of this review, in which a "unifying hypothesis of body fluid volume regulation" is presented.

Renal tubular function in the gravid rat

Pregnancy in the rat is accompanied by enhanced reabsorption of salt and water throughout most, if not all, of the gestational period. Many mechanisms have been suggested but definitive answers are still awaited. The major area of controversy centres around the detection of changes at term. There is general agreement that, at least in mid-gestation, the increase in reabsorption can be attributed to increases in the proximal tubules, the loop of Henle and the collecting duct. The contribution of the proximal tubule to the increased reabsorption at term is still uncertain. Enhanced salt and water reabsorption is demonstrated in distal nephron segments irrespective of the stage of gestation. Micropuncture and microperfusion experiments have identified increased reabsorption of water, sodium and chloride in the loop of Henle, but it appears that there is net addition of glucose, urea and potassium to the tubular fluid in this segment which, at least for potassium and glucose, offsets to some extent increased reabsorption by the proximal tubule. Altered renal handling of other solutes (uric acid, calcium and magnesium) also occurs throughout pregnancy but the mechanisms responsible and nephron sites involved remain to be investigated. Attempts to attribute altered reabsorption to direct renal effects of changes in maternal hormones are inconclusive. Prolactin mimics some of the pregnancy-associated increases in reabsorption following chronic administration to male and non-pregnant female rats. These effects might be due to a direct renal action of the hormone or even to the volume expansion following its dipsogenic action.

The effects of chronic glucocorticoid excess, adrenalectomy and stress on neuropeptide Y in individual rat hypothalamic nuclei

Several lines of evidence indicate a role for neuropeptide Y (NPY) in the modulation of the corticotroph axis. In two separate studies reported here, the concentrations of NPY and noradrenaline (NA), as well as corticotropin-releasing factor (CRF) and arginine vasopressin (AVP), were measured in extracts of individual rat hypothalamic nuclei after various manipulations producing either a state of chronic glucocorticoid excess or depletion, and also following repeated restraint stress. Alterations induced in the activity of hypothalamic neurones were inferred from the respective changes in these concentrations. 12 days after bilateral adrenalectomy (ADX), NPY levels were decreased by 24% in the arcuate nucleus (ARC) and 23% in the paraventricular nucleus (PVN, p < 0.05 vs controls). Forced immobilization of the animals for 4 h each day for 9 consecutive days (repeated stress) also decreased NPY content of the ARC by 25% (p < 0.01 vs controls), an effect blocked by the administration of glucocorticoids. NA levels in both hypothalamic nuclei were unaffected by repeated stress or ADX. Administration of glucocorticoids in the first of these studies induced decreases in NA levels by 15% and 25% in the ARC and PVN respectively (p < 0.05 vs controls). However, in subsequent experiments no significant effect of glucocorticoids on NA was observed. Our results demonstrate that the activity of the hypothalamic NPY-ergic neurones is modulated by glucocorticoids and by chronic stress. They also suggest that brainstem catecholaminergic and hypothalamic NPY-ergic neurones are differentially affected by altered glucocorticoid concentrations or by chronic stress, possibly in a stimulus-specific way.