Osmotic threshold and sensitivity for vasopressin release and fos expression by hypertonic NaCl in ovine fetus

Cross-cultural variation in thirst perception in hot-humid and hot-arid environments: Evidence from two small-scale populations

OBJECTIVES

Thirst is an evolved central homeostatic feedback system that helps regulate body water for survival. Little research has examined how early development and exposure to extreme environments and water availability affect thirst perception, particularly outside Western settings. Therefore, we compared two indicators of perceived thirst (current thirst and pleasantness of drinking water) using visual scales among Tsimane' forager-horticulturalists in the hot-humid Bolivian Amazon and Daasanach agro-pastoralists in hot-arid Northern Kenya.

METHODS

We examined how these measures of perceived thirst were associated with hydration status (urine specific gravity), ambient temperatures, birth season, age, and population-specific characteristics for 607 adults (n = 378 Tsimane', n = 229 Daasanach) aged 18+ using multi-level mixed-effect regressions.

RESULTS

Tsimane' had higher perceived thirst than Daasanach. Across populations, hydration status was unrelated to both measures of thirst. There was a significant interaction between birth season and temperature on pleasantness of drinking water, driven by Kenya data. Daasanach born in the wet season (in utero during less water availability) had blunted pleasantness of drinking water at higher temperatures compared to those born in the dry season (in utero during greater water availability).

CONCLUSIONS

Our findings suggest hydration status is not a reliable predictor of thirst perceptions in extreme-hot environments with ad libitum drinking. Rather, our findings, which require additional confirmation, point to the importance of water availability during gestation in affecting thirst sensitivity to heat and water feedback mechanisms, particularly in arid environments. Thirst regulation will be increasingly important to understand given climate change driven exposures to extreme heat and water insecurity.

Birth elicits a conserved neuroendocrine response with implications for perinatal osmoregulation and neuronal cell death

Long-standing clinical findings report a dramatic surge of vasopressin in umbilical cord blood of the human neonate, but the neural underpinnings and function(s) of this phenomenon remain obscure. We studied neural activation in perinatal mice and rats, and found that birth triggers activation of the suprachiasmatic, supraoptic, and paraventricular nuclei of the hypothalamus. This was seen whether mice were born vaginally or via Cesarean section (C-section), and when birth timing was experimentally manipulated. Neuronal phenotyping showed that the activated neurons were predominantly vasopressinergic, and vasopressin mRNA increased fivefold in the hypothalamus during the 2–3 days before birth. Copeptin, a surrogate marker of vasopressin, was elevated 30-to 50-fold in plasma of perinatal mice, with higher levels after a vaginal than a C-section birth. We also found an acute decrease in plasma osmolality after a vaginal, but not C-section birth, suggesting that the difference in vasopressin release between birth modes is functionally meaningful. When vasopressin was administered centrally to newborns, we found an ~ 50% reduction in neuronal cell death in specific brain areas. Collectively, our results identify a conserved neuroendocrine response to birth that is sensitive to birth mode, and influences peripheral physiology and neurodevelopment.

Diabetes Mellitus and Hypertension-A Case of Sugar and Salt?

The majority of patients with diabetes mellitus (DM) have hypertension (HTN). A specific mechanism for the development of HTN in DM has not been described. In the Zucker, Endothel, und Salz (sugar, endothelium, and salt) study (ZEuS), indices of glucose metabolism and of volume regulation are recorded. An analysis of these parameters shows that glucose concentrations interfere with plasma osmolality and that changes in glycemic control have a significant impact on fluid status and blood pressure. The results of this study are discussed against the background of the striking similarities between the regulation of sugar and salt blood concentrations, introducing the view that DM is probably a sodium-retention disorder that leads to a state of hypervolemia.

Molecular and genetic aspects of guanylyl cyclase natriuretic peptide receptor-A in regulation of blood pressure and renal function

Natriuretic peptides (NPs) exert diverse effects on several biological and physiological systems, such as kidney function, neural and endocrine signaling, energy metabolism, and cardiovascular function, playing pivotal roles in the regulation of blood pressure (BP) and cardiac and vascular homeostasis. NPs are collectively known as anti-hypertensive hormones and their main functions are directed toward eliciting natriuretic/diuretic, vasorelaxant, anti-proliferative, anti-inflammatory, and anti-hypertrophic effects, thereby, regulating the fluid volume, BP, and renal and cardiovascular conditions. Interactions of NPs with their cognate receptors display a central role in all aspects of cellular, biochemical, and molecular mechanisms that govern physiology and pathophysiology of BP and cardiovascular events. Among the NPs atrial and brain natriuretic peptides (ANP and BNP) activate guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA) and initiate intracellular signaling. The genetic disruption of Npr1 (encoding GC-A/NPRA) in mice exhibits high BP and hypertensive heart disease that is seen in untreated hypertensive subjects, including high BP and heart failure. There has been a surge of interest in the NPs and their receptors and a wealth of information have emerged in the last four decades, including molecular structure, signaling mechanisms, altered phenotypic characterization of transgenic and gene-targeted animal models, and genetic analyses in humans. The major goal of the present review is to emphasize and summarize the critical findings and recent discoveries regarding the molecular and genetic regulation of NPs, physiological metabolic functions, and the signaling of receptor GC-A/NPRA with emphasis on the BP regulation and renal and cardiovascular disorders.

The role of hyperosmotic stress in inflammation and disease

Hyperosmotic stress is an often overlooked process that potentially contributes to a number of human diseases. Whereas renal hyperosmolarity is a well-studied phenomenon, recent research provides evidence that many non-renal tissues routinely experience hyperosmotic stress that may contribute significantly to disease initiation and progression. Moreover, a growing body of evidence implicates hyperosmotic stress as a potent inflammatory stimulus by triggering proinflammatory cytokine release and inflammation. Under physiological conditions, the urine concentrating mechanism within the inner medullary region of the mammalian kidney exposes cells to high extracellular osmolarity. As such, renal cells have developed many adaptive strategies to compensate for increased osmolarity. Hyperosmotic stress is linked to many maladies, including acute and chronic, as well as local and systemic, inflammatory disorders. Hyperosmolarity triggers cell shrinkage, oxidative stress, protein carbonylation, mitochondrial depolarization, DNA damage, and cell cycle arrest, thus rendering cells susceptible to apoptosis. However, many adaptive mechanisms exist to counter the deleterious effects of hyperosmotic stress, including cytoskeletal rearrangement and up-regulation of antioxidant enzymes, transporters, and heat shock proteins. Osmolyte synthesis is also up-regulated and many of these compounds have been shown to reduce inflammation. The cytoprotective mechanisms and associated regulatory pathways that accompany the renal response to hyperosmolarity are found in many non-renal tissues, suggesting cells are commonly confronted with hyperosmotic conditions. Osmoadaptation allows cells to survive and function under potentially cytotoxic conditions. This review covers the pathological consequences of hyperosmotic stress in relation to disease and emphasizes the importance of considering hyperosmolarity in inflammation and disease progression.

Altered dipsogenic responses and expression of angiotensin receptors in the offspring exposed to prenatal high sucrose

The present study determined water and salt intake as well as expression of AT(1) and AT(2) receptors in the brain and kidney in the adult offspring rats prenatally exposed to high sucrose. Following the exposure during pregnancy, water intake and salt intake at baseline levels were not changed in the adult offspring. However, after 24h water deprivation, consumption of water and salt was significantly increased compared to that of the control. Plasma sodium and osmolality levels remained the same between the offspring in the control and the exposed groups, while hematocrit was higher in the offspring exposed to prenatal high sucrose immediately following water deprivation. Density of renal AT(1) receptor protein was the same between the control and the exposed group, while AT(2) receptor protein in the kidney was significantly increased in the offspring exposed to prenatal high sucrose in association of thicker basal membrane of glomerular. In the forebrain, both AT(1) and AT(2) receptor levels were significantly increased in the offspring with history of prenatal high sucrose. In addition, water deprivation induced more c-fos expression in the central dipsogenic areas, including the paraventricular and supraoptic nuclei in the offspring exposed to prenatal high sucrose. The results suggested that prenatal high intake of sucrose may affect development of pathways in regulation of dipsogenic behavior in face of dehydration, which was associated with altered expression of AT(1) or/and AT(2) receptors in the kidney and brain.

Central angiotensin I increases fetal AVP neuron activity and pressor responses

Angiotensin (Ang) II plays a critical role in cardiovascular homeostasis and neuroendocrine regulation. Little is known about whether central angiotensin-converting enzyme (ACE) is functional in the fetal brain. We investigated cardiovascular and neuroendocrinological responses to intracerebroventricular (icv) application of Ang I in the chronically prepared near-term ovine fetus in utero and examined the action sites marked by c-fos expression in the fetal hypothalamus. ACE mRNA was detected in the specific central areas. Intracerebroventricular Ang I significantly increased fetal blood pressure and c-fos expression in the supraoptic nuclei (SON) and the paraventricular nuclei (PVN) in the hypothalamus, accompanied by an increase of fetal plasma arginine vasopressin (AVP). Double labeling demonstrated that AVP neurons in the fetal SON and PVN were expressing c-fos. Captopril, an inhibitor of ACE, significantly suppressed fetal pressor responses and plasma AVP. Double labeling experiments showed colocalization of AT(1) receptor (AT(1)R) and c-fos expression in both SON and PVN following icv Ang I. The results indicate that central endogenous ACE has been functional at least at the last third of gestation and the endogenous brain renin-angiotensin system-mediated pressor responses and AVP release via AT(1)Rs by acting at the sites consistent with the cardiovascular network in the hypothalamus.

Ovine fetal hormonal and hypothalamic neuroendocrine responses to maternal water deprivation at late gestation

Angiotensin II (Ang II), aldosterone, and arginine-vasopressin (AVP) are three major neuropeptides or hormones that are important in the control of body fluid regulation. Dehydration during pregnancy induces alterations in maternal-fetal fluid homeostasis. It is still not clear about effects and mechanisms of maternal water deprivation on fetal neuroendocrine and hormonal responses. The present study deprived water from pregnant sheep at near-term for 24 h and 48 h, and determined maternal and fetal blood osmolality and sodium levels before and immediately after water deprivation. Fetal renal excretion and plasma hormones were measured. Fetal forebrain was analyzed for cellular activation marked with Fos and Fos-B. The results showed that maternal and fetal blood osmolality and sodium were increased by water deprivation. Maternal and fetal Ang II, aldosterone, and AVP levels were elevated by 24-h and 48-h water deprivation, while fetal plasma Ang I levels were increased only under the condition of 48-h water deprivation. Intensive Fos and Fos-B expression was detected in the median preoptic nuclei and paraventricular nuclei in the fetal brain following exposure to maternal water deprivation. Double labeling demonstrated that many Fos-positive cells were AVP-containing neurons in the fetal paraventricular nucleus. Together, the results suggest that neuroendocrine and hormonal regulatory mechanisms play a role in the control of body fluid homeostasis, and relatively matured and functional at the last third of gestation, as well as the fetal hypothalamus is functional in the control of the neuropeptide in response to maternal dehydration.

Central cholinergic signal-mediated neuroendocrine regulation of vasopressin and oxytocin in ovine fetuses

BACKGROUND

The hypothalamic-neurohypophysial system plays a fundamental role in the maintenance of body fluid homeostasis by secreting arginine vasopressin (AVP) and oxytocin (OT) in response to a variety of signals, including osmotic and nonosmotic stimuli. It is well established that central cholinergic mechanisms are critical in the regulation of cardiovascular responses and maintenance of body fluid homeostasis in adults. Our recent study demonstrated that intracerebroventricular (i.c.v.) injection of carbachol elicited an increase of blood pressure in the near-term ovine fetuses. However, in utero development of brain cholinergic mechanisms in the regulation of the hypothalamic neuropeptides is largely unknown. This study investigated AVP and OT neural activation in the fetal hypothalamus induced by central carbachol.

RESULTS

Chronically prepared near-term ovine fetuses (0.9 gestation) received an i.c.v. carbachol (3 microg/kg). Fetal blood samples were collected for AVP and OT assay, and brains were used for c-fos mapping studies. I.c.v. carbachol significantly increased fetal plasma AVP and OT concentrations. Intense FOS immunoreactivity (FOS-ir) was observed in the fetal supraoptic nuclei (SON) and paraventricular nuclei (PVN) in the hypothalamus. Double labeling demonstrated that a number of AVP- and OT-containing neurons in the fetal SON and PVN were expressing c-fos in response to central carbachol.

CONCLUSION

The results indicate that the central cholinergic mechanism is established and functional in the regulation of the hypothalamic neuropeptides during the final trimester of pregnancy. This provides evidence for a functional link between the development of central cholinergic mechanisms and hypothalamic neuropeptide systems in the fetus.

Amniotic fluid water dynamics

Water arrives in the mammalian gestation from the maternal circulation across the placenta. It then circulates between the fetal water compartments, including the fetal body compartments, the placenta and the amniotic fluid. Amniotic fluid is created by the flow of fluid from the fetal lung and bladder. A major pathway for amniotic fluid resorption is fetal swallowing; however in many cases the amounts of fluid produced and absorbed do not balance. A second resorption pathway, the intramembranous pathway (across the amnion to the fetal circulation), has been proposed to explain the maintenance of normal amniotic fluid volume. Amniotic fluid volume is thus a function both of the amount of water transferred to the gestation across the placental membrane, and the flux of water across the amnion. Membrane water flux is a function of the water permeability of the membrane; available data suggests that the amnion is the structure limiting intramembranous water flow. In the placenta, the syncytiotrophoblast is likely to be responsible for limiting water flow across the placenta. In human tissues, placental trophoblast membrane permeability increases with gestational age, suggesting a mechanism for the increased water flow necessary in late gestation. Membrane water flow can be driven by both hydrostatic and osmotic forces. Changes in both osmotic/oncotic and hydrostatic forces in the placenta my alter maternal-fetal water flow. A normal amniotic fluid volume is critical for normal fetal growth and development. The study of amniotic fluid volume regulation may yield important insights into the mechanisms used by the fetus to maintain water homeostasis. Knowledge of these mechanisms may allow novel treatments for amniotic fluid volume abnormalities with resultant improvement in clinical outcome.

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.

Prenatal programming of hypernatremia and hypertension in neonatal lambs

Maternal water restriction and the accompanying dehydration-induced anorexia may induce long-term physiological changes in offspring. We determined the impact of prenatal hypertonicity (Pre-Dehy) on offspring cardiovascular and osmoregulatory function. Pre-Dehy lambs were exposed to in utero hypernatremia (8- to 10-meq increase; 110–150 days of gestation) induced by maternal water restriction. Control lambs were born to ewes provided ad libitum water and food throughout gestation. After delivery, all ewes were provided ad libitum water and all newborns were allowed ad libitum nursing. Lambs were prepared with vascular and bladder catheters at 15 ± 2 days of age and studied at 21 ± 2 days. After a 2-h basal period, lambs received an infusion of hypotonic (0.075 M) NaCl (0.15 ml·kg−1·h−1 iv) for 2 h. Lamb arterial blood pressure was monitored, and blood samples were obtained before, during, and after infusion. During the neonatal basal period, Pre-Dehy lambs had significantly increased plasma osmolality (302 ± 1 vs. 294 ± 1 mosmol/kgH2O, P < 0.01), sodium levels (144 ± 1 vs. 140 ± 1 meq/l, P < 0.01), hematocrit (28 ± 1% vs. 25 ± 1%, P < 0.05), and mean arterial blood pressure (79 ± 2 vs. 68 ± 1 mmHg, P < 0.001) compared with control lambs. Despite the infusion of hypotonic saline, Pre-Dehy lambs maintained relative hypertonicity, hypernatremia, and hypertension. However, plasma arginine vasopressin, glomerular filtration rate, and urinary osmolar and sodium excretion and clearance (per kg body wt) were similar in the groups. Offspring of prenatally water-restricted ewes exhibit hypernatremia, hypertonicity, and hypertension, which persist despite hypotonic saline infusion. In utero hypertonicity and perhaps maternal nutrient stress may program offspring osmoregulation and systemic arterial hypertension.

Gestational and early postnatal dietary NaCl levels affect NaCl intake, but not stimulated water intake, by adult rats

We examined body fluid regulation by weanling (21–25 days) and adult (>60 days) male rats that were offspring of dams fed chow containing either 0.1, 1, or 3% NaCl throughout gestation and lactation. Weanling rats were maintained on the test diets until postnatal day 30 and on standard 1% NaCl chow thereafter. Ad libitum water intake by weanlings was highest in those fed 3% NaCl and lowest in those fed 0.1% NaCl. Adult rats maintained on standard NaCl chow consumed similar amounts of water after overnight water deprivation or intravenous hypertonic NaCl (HS) infusion regardless of early NaCl condition. Moreover, baseline and HS-stimulated plasma Na+ concentrations also were similar for the three groups. Nonetheless, adult rats in the early 3% NaCl group consumed more of 0.5 M NaCl after 10 days of dietary Na+ deprivation than did rats in either the 1% or 0.1% NaCl group. Interestingly, whether NaCl was consumed in a concentrated solution in short-term, two-bottle tests after dietary Na+ deprivation or in chow during ad libitum feeding, adult rats in the 3% NaCl group drank less water for each unit of NaCl consumed, whereas rats in the 0.1% NaCl group drank more water for each unit of NaCl consumed. Thus gestational and early postnatal dietary NaCl levels do not affect stimulated water intake or long-term body fluid regulation. Together with our previous studies, these results suggest that persistent changes in NaCl intake and in water intake associated with NaCl ingestion reflect short-term behavioral effects that may be attributable to differences in NaCl taste processing.

Programming of hypertonicity in neonatal lambs: resetting of the threshold for vasopressin secretion

Lambs exposed in utero to maternal hypertonicity demonstrate plasma hypertonicity and arterial hypertension. To determine whether hypertonicity is due to an altered osmoregulatory set point, we examined arginine-vasopressin and cardiovascular responses to hypertonic saline infusion in these offspring. Study lambs [dehydrated (Dehy)] were exposed to maternal hypernatremia (8-10 mEq/liter increase; 110-150 d gestation) induced by water restriction. Control singleton and Control twins were born to ewes provided ad libitum water. We anticipated reduced birth weight due to maternal dehydration-induced anorexia and therefore included a Control group of twin gestations to approach a similar birth weight near term. After delivery, ewes from all three groups were provided ad libitum water, and their newborns were allowed ad libitum nursing. At 15 +/- 2 d of age, lambs were prepared with bladder and vascular catheters. At 23 +/- 2 d, after a 2-h basal period, neonatal lambs were iv infused with hypertonic 0.83 m NaCl (0.075 ml/kg x h) for 2 h, followed by a 2-h recovery. Neonatal mean arterial pressure and urine flow were continuously monitored, and blood samples were obtained before, during, and after infusion. During the basal period, Dehy neonates and Control twins demonstrated significantly increased plasma sodium levels and mean arterial pressure than Control singletons. In addition, the Dehy neonates had significantly increased plasma osmolality compared with Control singletons and twins. In response to hypertonic infusion, the Dehy offspring continued to exhibit hypertonicity and hypertension. Importantly, plasma tonicity and blood pressure were greatest in Dehy singletons, lowest in singleton controls, and intermediate in twin controls. Furthermore, the plasma osmolality threshold for AVP secretion was significantly higher in Dehy singletons (290 +/- 2 mOsm/kg) than Control twins (285 +/- 1 mOsm/kg) and Control singletons (280 +/- 2 mOsm/kg), indicating in utero programming of an altered set point for systemic osmolality and blood pressure regulation. Because both twin gestation and dehydration-anorexia incur potential fetal nutritional stress, the results suggest that both in utero hypertonicity and nutrition reduction contribute to offspring programming. We postulate that the nutritional stress associated with twins (as well as dehydration-induced anorexia) contributes to increased plasma sodium levels, whereas the increased plasma osmolality is due to in utero hypertonicity.

Glutamatergic input in the PVN is important in renal nerve response to elevations in osmolality

Elevations in plasma osmolality elicit reflex humoral and neural responses. The hypothalamic paraventricular nucleus (PVN) is important in humoral responses. We have investigated whether the PVN contributed to the renal nerve reduction that is normally elicited by increased plasma osmolality in the conscious rabbit. Renal sympathetic nerve activity (RSNA) was monitored after an intravenous infusion of hypertonic saline (1.7 M NaCl, 2 ml/min for 7 min). The responses were examined in animals microinjected with muscimol (10 nmol) into, and outside, the PVN to acutely inhibit neuronal function or with kynurenate (25 nmol) to block glutamate receptors. Compared with vehicle, the maximum reduction in RSNA elicited by hypertonic saline was significantly less with muscimol or kynurenate pretreatment into the PVN. A similar study with kynurenate was also performed in sinoaortically denervated rabbits, and similar effects were observed. The effect was specific to the PVN because microinjections of the drugs outside the PVN had no effect on the response. The findings suggest that excitatory inputs into the PVN may be important in the neural responses elicited by elevations in plasma osmolality.

Prolonged prenatal hypernatremia alters neuroendocrine and electrolyte homeostasis in neonatal sheep

Arginine vasopressin (AVP) is a neuroendocrine hormone synthesized in the hypothalamus, and is stored and secreted by the posterior pituitary gland in response to stimuli such as plasma hypertonicity and hypotension. The primary physiologic roles of AVP include plasma osmolality and blood pressure regulation. We have previously demonstrated that chronic prenatal plasma hypertonicity alters the AVP regulatory pathway in newborn lambs. The objectives of the present study were to evaluate prolonged effects of antenatal plasma hypertonicity on neonatal plasma osmoregulation. Pregnant ewes at 119 +/- 3 days of gestation were water restricted to achieve and maintain hypertonicity until normal-term delivery. After delivery, ewes were provided food and water ad libitum and lambs were allowed maternal nursing. At the age of 28 days, blood samples were obtained for the analysis of plasma osmolality, electrolytes, and AVP levels from study (n= 5) and age-matched control (n= 6) lambs. Subsequently, lambs were euthanized, and the pituitary and hypothalamus were processed for the determination of pituitary AVP content by radioimmunoassay, and AVP gene expression by Northern analysis. In response to water restriction, maternal plasma osmolality significantly increased (306 +/- 1.1 to 326 +/- 1.2 mOsm/kg, P< 0.001). At the age of 28 days, plasma sodium level was higher in study (prenatally dehydrated) than control lambs (144.6 +/- 0.4 vs 142.6 +/- 0.3,P< 0.05). Study lambs had higher plasma AVP concentrations than the control lambs (4.1 +/- 0.4 vs 1.7 +/- 0.4 pg/ml,P< 0.05). Similarly, total pituitary AVP content was higher in thein utero hypertonic lambs than in the control lambs (6.5 +/- 1.0 vs 2.8 +/-1.2 microg, P< 0.05). However, there was no difference in hypothalamic AVP mRNA levels between the two groups. The present study demonstrates that chronic maternal and fetal plasma hypertonicity has prolonged effects on pituitary and plasma AVP, as well as plasma sodium in neonatal lambs, providing further evidence suggesting prenatal imprinting of osmoregulation through at least 1 month of age.

Chronic in utero plasma hyperosmolality alters hypothalamic arginine vasopressin synthesis and pituitary arginine vasopressin content in newborn lambs

OBJECTIVE

Arginine vasopressin is synthesized in the hypothalamus and secreted by the posterior pituitary gland in response to plasma hypertonicity. Previous studies suggest that in utero and neonatal exposure of rat pups to prolonged alterations of plasma osmolality may permanently alter (imprint) arginine vasopressin synthesis and secretion, thus adult responses to osmotic challenges. Little is known, however, of the potential for imprinting of neuroendocrinologic systems in precocial species. In view of the frequent occurrence of altered maternal and fetal plasma tonicity (eg, maternal dehydration, hyperemesis), we sought to determine the effect of prolonged maternal hypertonicity on arginine vasopressin gene expression and pituitary gland content in neonatal sheep.

STUDY DESIGN

Pregnant ewes at 119 +/- 3 days of gestation were water restricted to achieve and maintain plasma hypertonicity (10-20 mOsm/kg above baseline level) until normal term delivery. Newborns were provided maternal nursing ad libitum. Within 24 hours after birth, study neonatal lambs (n = 6) and age-matched control neonatal lambs (n = 5) were killed, and the pituitary gland and hypothalamus were removed and frozen immediately. Pituitary arginine vasopressin content was determined by radioimmunoassay, and hypothalamic arginine vasopressin gene expression was quantified with Northern blot. Differences in pituitary arginine vasopressin content and hypothalamic arginine vasopressin gene expression (arginine vasopressin/ beta-actin ratio) between study and control newborn lambs were analyzed by unpaired t test.

RESULTS

In response to maternal water restriction, maternal plasma osmolality increased from 307 +/- 0.9 mOsm/kg to 325 +/- 1.3 mOsm/kg, and plasma sodium increased from 147 +/- 1.3 mEq/L to 156 +/- 1.2 mEq/L. The maternal plasma hyperosmolality and hypernatremia were maintained until normal term delivery. At the time of death, study (in utero dehydrated) lambs had higher plasma sodium (150 +/- 0.4 mEq/L vs 146.5 +/- 1.5 mEq/L; P <.05) and chloride (112.8 +/- 1.0 mEq/L vs 108.5 +/- 1.5 mEq/L; P <.05) levels, and lower potassium (4.5 +/- 0.2 mEq/L vs 5.5 +/- 0.3 mEq/L; P <.05) concentrations than control newborn lambs. Both newborn groups had similar plasma osmolality (320.0 +/- 1.3 mOsm/kg vs 318.0 +/- 3.4 mOsm/kg). Total pituitary arginine vasopressin content was significantly greater in the study than in the control newborn lambs (8.3 +/- 2.8 microg vs 1.6 +/- 1.3 microg; P <.05). Conversely, hypothalamic arginine vasopressin messenger RNA levels were lower in the study newborn lambs than in the control newborn lambs (arginine vasopressin/beta-actin ratio: 0.29 +/- 0.01 vs 0.68 +/- 0.15; P <.05).

CONCLUSION

Despite the presence of plasma hypernatremia, prolonged elevation of fetal plasma tonicity increases newborn pituitary arginine vasopressin content yet decreases hypothalamic arginine vasopressin gene expression. The present study suggests that prolonged prenatal exposure to plasma hypertonicity may imprint the hypothalamic-pituitary arginine vasopressin regulatory system.

Intact osmoregulatory centers in the preterm ovine fetus: Fos induction after an osmotic challenge

We previously demonstrated a functional systemic dipsogenic response in the near-term fetal sheep (128-130 days; 145 days = full-term) with swallowing activity stimulated in response to central and systemic hypertonic saline. Preterm fetal sheep (110-115 days) do not consistently demonstrate swallowing in response to hypertonic stimuli, and it is unclear whether this is due to immaturity of osmoreceptor mechanisms or neuronal pathways activating swallowing motor neurons. To determine whether osmoreceptive regions in the preterm fetus are activated by changes in plasma tonicity, we examined Fos expression with immunostaining in these neurons in response to an osmotic challenge. Nine preterm fetal sheep [five hypertonic saline-treated fetuses (Hyp) and four isotonic saline-treated fetuses (Iso)] were prepared with vascular and intraperitoneal catheters. Seventy-five minutes before tissue collection, hypertonic (1.5 M) or isotonic saline was infused (12 ml/kg) via an intraperitoneal catheter to fetuses. Brains were examined for patterns of neuronal activation (demonstrated by Fos protein expression). Hyp demonstrated increases in plasma osmolality (~10 mosmol/kg H(2)O) and Na concentrations (5 meq/l). Increased Fos expression was detected in Hyp in the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), median preoptic nucleus (MnPO), supraoptic (SON), and paraventricular nuclei (PVN) compared with Iso animals. Neuronal activation within the OVLT, SFO, and MnPO indicates intact osmoregulatory mechanisms, whereas activation of the SON and PVN suggests intact fetal neural pathways to arginine vasopressin neurons. These results suggest that preterm fetal swallowing insensitivity to osmotic stimuli may be due to immaturity of integrated motor neuron pathways.