Reversal of pregnancy-mediated plasma hypotonicity in the near-term rat

The invention of aldosterone, how the past resurfaces in pediatric endocrinology

Sodium and water homeostasis are drastically modified at birth, in mammals, by the transition from aquatic life to terrestrial life. Accumulating evidence during the past ten years underscores the central role for the mineralocorticoid signaling pathway, in the fine regulation of this equilibrium, at this critical period of development. Interestingly, regarding evolution, while the mineralocorticoid receptor is expressed in fish, the appearance of its related ligand, aldosterone, coincides with terrestrial life, as it is first detected in lungfish and amphibian. Thus, aldosterone is likely one of the main hormones regulating the transition from an aquatic environment to an air environment. This review will focus on the different actors of the mineralocorticoid signaling pathway from aldosterone secretion in the adrenal gland, to mineralocorticoid receptor expression in the kidney, summarizing their regulation and roles throughout fetal and neonatal development, in the light of evolution.

Pregnancy complicated by central diabetes insipidus and oligohydramnios

This is the fourth reported case of central diabetes insipidus with oligohydramnios. Central diabetes insipidus does not adversely affect pregnancy; it can present with oligohydramnios, which can be improved by treating central diabetes insipidus.

Regulation of amniotic fluid volume: insights derived from amniotic fluid volume function curves

Recent advances in understanding the regulation of amniotic fluid volume (AFV) include that AFV is determined primarily by the rate of intramembranous absorption (IMA) of amniotic fluid across the amnion and into fetal blood. In turn, IMA rate is dependent on the concentrations of yet-to-be identified stimulator(s) and inhibitor(s) that are present in amniotic fluid. To put these concepts in perspective, this review 1) discusses the evolution of discoveries that form the current basis for understanding the regulation of AFV, 2) reviews the contribution of IMA to this regulation, and 3) interprets experimentally induced shifts in AFV function curves and amnioinfusion function curves in terms of the activity of the amniotic fluid stimulator and inhibitor of IMA. In the early 1980s, it was not known whether AFV was regulated. However, by the late 1980s, IMA was discovered to be a "missing link" in understanding the regulation of AFV. Over the next 25 years the concept of IMA evolved from being a passive process to being an active, unidirectional transport of amniotic fluid water and solutes by vesicles within the amnion. In the 2010s, it was demonstrated that a renally derived stimulator and a fetal membrane-derived inhibitor are present in amniotic fluid that regulate IMA rate and hence are the primary determinants of AFV. Furthermore, AFV function curves and amnioinfusion function curves provide new insights into the relative efficacy of the stimulator and inhibitor of IMA.

Regulation of amniotic fluid volume

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. Water flux across biologic membranes may be driven by osmotic or hydrostatic forces; existing data suggest that intramembranous flow in humans is driven by the osmotic difference between the amniotic fluid and the fetal serum. The driving force for placental flow is more controversial, and both forces may be in effect. The mechanism(s) responsible for regulating water flow to and from the amniotic fluid is unknown. In other parts of the body, notably the kidney, water flux is regulated by the expression of aquaporin water channels on the cell membrane. We hypothesize that aquaporins have a role in regulating water flux across both the amnion and the placenta, and present evidence in support of this theory. Current knowledge of gestational water flow is sufficient to allow prediction of fetal outcome when water flow is abnormal, as in twin-twin transfusion syndrome. Further insight into these mechanisms may allow novel treatments for amniotic fluid volume abnormalities with resultant improvement in clinical outcome.

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.

Amniotic fluid volume and composition in mouse pregnancy

OBJECTIVE

The current study was undertaken to determine simultaneous changes in amniotic fluid (AF) volume and composition across gestation in the pregnant mouse.

METHODS

Young adult mice (6 to 7 weeks old) of the CB6F1 strain were mated overnight. AF was collected on consecutive days from embryonic days 9.5 through 18.5 for measurements of volume and composition. Statistical analysis included one-factor analysis of variance (ANOVA).

RESULTS

AF volume increased from 18 +/- 4 (SE) microL on day 9.5 to a maximum of 147 +/- 4 microL on days 15.5 to 16.5 and decreased sharply to 17 +/- 3 microL on day 18.5. AF osmolality was unchanged except for a rise prior to delivery on day 19.5 to 20.5. AF sodium, calcium, and glucose concentrations increased and subsequently decreased as gestation progressed. AF potassium, chloride, and lactate concentrations initially decreased and then increased across gestation. Prior to day 9.5 and after day 18.5, AF volume was too small for volume or compositional determinations.

CONCLUSIONS

In the mouse, the rise in AF volume from mid gestation to a maximum late in gestation is similar to that in humans while the sharp fall prior to delivery is not. As observed in the fetal sheep, the changes in fluid volume are associated with AF osmolality and solute concentration changes that are correlated with advancing gestational age. These observations together with the feasibility of quantifying AF volume and composition in the mouse fetus demonstrate the possibility of using genetically altered mice as a model for future studies on the molecular mechanisms underlying the regulation of AF volume and composition.

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.

Life-long echoes--a critical analysis of the developmental origins of adult disease model

The hypothesis that there is a developmental component to subsequent adult disease initially arose from epidemiological findings relating birth size to either indices of disease risk or actual disease prevalence in later life. While components of the epidemiological analyses have been challenged, there is strong evidence that developmental factors contribute to the later risk of metabolic disease--including insulin resistance, obesity, and heart disease--as well as have a broader impact on osteoporosis, depression and schizophrenia. We suggest that disease risk is greater when there is a mismatch between the early developmental environment (i.e., the phase of developmental plasticity) versus that experienced in mature life (i.e., adulthood), and that nutritional influences are particularly important. It is also critical to distinguish between those factors acting during the developmental phase that disrupt development from those influences that are less extreme and act through regulated processes of epigenetic change. A model of the relationship between the developmental and mature environment is proposed and suggests interventional strategies that will vary in different population settings.

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.