The composition of maternal plasma and foetal urine after feeding and drinking in chronically catheterized ewes during the last two months of pregnancy

Preparing for Life After Birth: Introducing the Concepts of Intrauterine and Extrauterine Sensory Entrainment in Mammalian Young

Presented is an updated understanding of the development of sensory systems in the offspring of a wide range of terrestrial mammals, the prenatal exposure of those systems to salient stimuli, and the mechanisms by which that exposure can embed particular sensory capabilities that prepare newborns to respond appropriately to similar stimuli they may encounter after birth. Taken together, these are the constituents of the phenomenon of "trans-natal sensory continuity" where the embedded sensory capabilities are considered to have been "learnt" and, when accessed subsequently, they are said to have been "remembered". An alternative explanation of trans-natal sensory continuity is provided here in order to focus on the mechanisms of "embedding" and "accessing" instead of the potentially more subjectively conceived outcomes of "learning" and "memory". Thus, the mechanistic concept of "intrauterine sensory entrainment" has been introduced, its foundation being the well-established neuroplastic capability of nervous systems to respond to sensory inputs by reorganising their neural structures, functions, and connections. Five conditions need to be met before "trans-natal sensory continuity" can occur. They are (1) sufficient neurological maturity to support minimal functional activity in specific sensory receptor systems in utero; (2) the presence of sensory stimuli that activate their aligned receptors before birth; (3) the neurological capability for entrained functions within specific sensory modalities to be retained beyond birth; (4) specific sensory stimuli that are effective both before and after birth; and (5) a capability to detect those stimuli when or if they are presented after birth in ways that differ (e.g., in air) from their presentation via fluid media before birth. Numerous beneficial outcomes of this process have been reported for mammalian newborns, but the range of benefits depends on how many of the full set of sensory modalities are functional at the time of birth. Thus, the breadth of sensory capabilities may be extensive, somewhat restricted, or minimal in offspring that are, respectively, neurologically mature, moderately immature, or exceptionally immature at birth. It is noted that birth marks a transition from intrauterine sensory entrainment to extrauterine sensory entrainment in all mammalian young. Depending on their neurological maturity, extrauterine entrainment contributes to the continuing maturation of the different sensory systems that are operational at birth, the later development and maturation of the systems that are absent at birth, and the combined impact of those factors on the behaviour of newborn and young mammals. Intrauterine sensory entrainment helps to prepare mammalian young for life immediately after birth, and extrauterine sensory entrainment continues this process until all sensory modalities develop full functionality. It is apparent that, overall, extrauterine sensory entrainment and its aligned neuroplastic responses underlie numerous postnatal learning and memory events which contribute to the maturation of all sensory capabilities that eventually enable mammalian young to live autonomously.

Diurnal variation of urinary sodium-to-potassium ratio in free-living Japanese individuals

High sodium-to-potassium ratios are associated with elevated blood pressure levels and an increased risk of cardiovascular diseases. We aimed to determine whether urinary sodium-to-potassium ratios fluctuate diurnally during the day to understand measured values of casual urinary sodium-to-potassium ratios. A total of 13,277 casual urine specimens were collected under free-living conditions from 122 Japanese normotensive and hypertensive individuals. Participants collected all casual urine samples in aliquot tubes, reported urine volumes and the time at each voiding for 10-22 days. Then, specimens were classified into hourly data. Diurnal patterns of urinary sodium-to-potassium ratios and urinary concentrations of sodium and potassium were evaluated. Overall mean values of hourly urinary sodium-to-potassium ratios were highest (4.1-5.0) in the early morning, lower (3.3-3.8) in the daytime and higher (4.0-4.4) toward evening hours. The mean urinary sodium and potassium concentrations were the lowest (90-110 and 24-32 mmol l-1, respectively) during the early morning and higher (110-140 and 35-43 mmol l-1, respectively) after mid-morning. Diurnal variability of potassium concentrations was larger than for sodium concentrations. Diurnal variations in urinary sodium-to-potassium ratios were comparable between normotensive and hypertensive individuals, between hypertensive individuals with and without antihypertensive medications, and among age and gender-specific subgroups. Overall mean hourly urinary sodium-to-potassium ratios fluctuated diurnally under free-living conditions and were higher during the morning and evening and lower during the daytime compared with 24-h urinary sodium-to-potassium ratios. Diurnal variation in urinary sodium-to-potassium ratios should be considered to understand actual daily dietary levels and avoid over- and under-estimation in clinical practice.

Daily rhythms in renal blood flow and urine production rate in the near-term sheep fetus

Daily rhythmicity of renal blood flow (RBF) and urine flow (UF) was studied in fetal sheep between 121-125 d of gestation. Fetal arterial blood pressure, heart rate, UF, and right RBF were measured continuously for 24-h periods in 10 sheep. Rhythmic variations during a 24-h period were found for all variables studied. The rhythms of arterial blood pressure and heart rate were highly correlated, whereas an inverse correlation was found between arterial blood pressure with RBF and between arterial blood pressure with UF. These findings indicate that fetal RBF is not blood pressure dependent. Furthermore, fetal UF appears not to be mediated by pressure-dependent diuresis.

Effects of inhibition of prostaglandin synthesis on flow and composition of fetal urine, lung liquid, and swallowed fluid in sheep

OBJECTIVE

Our aim was to determine the effects of blocking prostaglandin synthesis, by infusion of indomethacin into the fetal circulation, on factors regulating amniotic fluid volume and on plasma composition in the mother and fetus.

STUDY DESIGN

Indomethacin was administered to fetal sheep during 8 hours at 124.0 +/- 1.2 days of gestation (n = 7) and at 134.7 +/- 0.8 days of gestation (n = 7) (term approximately 147 days). Vehicle infusions were performed at 128.8 +/- 1.4 days of gestation (n = 5).

RESULTS

Fetal urine production was significantly reduced and both fetal urine osmolality and plasma arginine vasopressin concentrations were significantly elevated in response to indomethacin infusions at both gestational ages. Fetal blood and urine lactate concentrations were elevated in response to indomethacin. Later in gestation, fetal lung liquid flow was significantly changed from a net efflux to a net influx. There was no significant effect of indomethacin on the volume of fluid swallowed by the fetus.

CONCLUSION

Inhibition of fetal prostaglandin synthesis profoundly reduces the production of the two major fetal fluids contributing to amniotic fluid, namely, urine and lung liquid. These findings may explain why indomethacin reduces amniotic fluid volume.

Amniotic fluid composition changes during urine drainage and tracheoesophageal occlusion in fetal sheep

OBJECTIVE

Recently an intramembranous pathway was reported in the ovine fetus as a route for the rapid exchange of water, ions, and molecules between the amniotic fluid and the fetal blood that perfuses the fetal surface of the placenta and the fetal membranes. Our study was designed to test the hypothesis that the amniotic fluid composition would gradually equilibrate with fetal plasma when the major flows to and from the amniotic compartment were eliminated.

STUDY DESIGN

Eleven near-term fetal sheep underwent ligation of the urachus to eliminate the allantoic fluid. An inflatable cuff was placed around the esophagus and trachea, and catheters were placed in the fetal urinary bladder, fetal circulation, and maternal circulation. At > or = 5 days after surgery the animals were subjected to either a control experiment or a continuous urine drainage plus tracheoesophageal occlusion for 8 hours.

RESULTS

During the urine drainage plus occlusion study, amniotic fluid osmolality (p < 0.0001), Na+ (p < 0.0001), K+ (p < 0.01) Cl- (p < 0.001), and lactate (p < 0.001) increased compared with the control experiment. These corresponded to 50% reductions in the gradients for osmolality and Na+ between fetal plasma and amniotic fluid; the K+ gradient increased, and the Cl- gradient reversed. The percentage increases in amniotic Na+, K+, Cl-, and lactate were all 10% at 8 hours.

CONCLUSION

These observations suggest that water is absorbed from the amniotic fluid through the intramembranous pathway into the fetal circulation at a rate of 1.25% of the total amniotic volume per hour or approximately 240 ml/day.

Dehydration increases the renal response to atrial natriuretic peptide in fetal sheep

OBJECTIVE

In sheep, maternal water deprivation results in urinary natriuresis in spite of suppression of plasma atrial natriuretic factor levels. Near-term fetal sheep also have a urinary natriuresis without change in plasma atrial natriuretic factor during maternal dehydration. This study was designed to explore the role of plasma atrial natriuretic factor levels in fetal dehydration-natriuresis.

STUDY DESIGN

Eight chronically instrumented preterm (113 +/- 1 days) ovine fetuses received two atrial natriuretic factor infusions (3 and 15 ng/kg/min) in a euhydrated state and after 48 +/- 1 hours of maternal water deprivation.

RESULTS

Dehydration significantly increased maternal plasma osmolality (302 +/- 2 to 313 +/- 2 mOsm/kg water), sodium (148.1 +/- 0.8 to 154.3 +/- 0.4 mEq/L), chloride (112.4 +/- 0.6 to 116.8 +/- 0.9 mEq/L), and arginine vasopressin (4.2 +/- 1.2 to 23.0 +/- 4.0 pg/ml) and significantly decreased plasma atrial natriuretic factor (36 +/- 6 to 19 +/- 4 pg/ml) concentrations. Fetal plasma osmolality (296 +/- 1 to 308 +/- 2 mOsm/kg), atrial natriuretic factor (128 +/- 16 to 241 +/- 36 pg/ml), and arginine vasopressin (3.5 +/- 0.8 to 12.3 +/- 4.8 pg/ml) concentrations and urine osmolality (170 +/- 10 to 253 +/- 10 mOsm/kg), osmolar clearance (0.80 +/- 0.02 to 0.14 +/- 0.02 ml/kg/min), and fractional sodium excretion (3.3% +/- 1.7% to 8.5% +/- 2.1%) increased significantly with dehydration, whereas the plasma atrial natriuretic factor clearance decreased from 127 +/- 27 to 63 +/- 10 ml/kg/min. Dehydration had no effect on fetal hematocrit, vascular pressures, glomerular filtration rate, urine flow, or free water clearance. In euhydrated fetuses plasma atrial natriuretic factor increased from 128 +/- 16 to 287 +/- 46 pg/ml with sequential atrial natriuretic factor infusion, and no significant increases were observed in urine flow, fractional sodium excretion, and glomerular filtration rate. In contrast, atrial natriuretic factor infusion to dehydrated fetuses significantly increased urine flow (0.17 +/- 0.03 to 0.32 +/- 0.07 ml/kg/min), osmolar clearance (0.14 +/- 0.02 to 0.28 +/- 0.06 ml/kg/min), and fractional sodium excretion (8.5% +/- 2.1% to 14.8% +/- 4.0%).

CONCLUSION

These results demonstrate that in the fetus at 113 days' gestation plasma atrial natriuretic factor levels increase with dehydration, probably a result of decreased plasma atrial natriuretic factor clearance, and the fetal renal responsiveness to atrial natriuretic factor infusion increases during maternal dehydration.

Maternal dehydration-rehydration: fetal plasma and urinary responses

Pregnant women may be exposed to exercise, thermal, or gastrointestinal (hyperemesis) water loss, all of which commonly induce a greater than 10 mosmol increase in plasma osmolality. Although fetal osmolality is dependent on maternal osmolality, the impact of maternal dehydration and subsequent maternal rehydration on the fetus has not been explored. Five pregnant ewes with singleton fetuses (136 +/- 1 day) were water deprived for 36 h resulting in a significant increase in plasma osmolality (298 +/- 3.4 to 313 +/- 5.0 mosmol). In response to maternal dehydration, fetal plasma osmolality (297.0 +/- 4.1 to 309.3 +/- 4.1 mosmol), arginine vasopressin (AVP) levels (1.5 +/- 0.2 to 7.9 +/- 1.0 pg/ml), hematocrit (35.1 to 38.6%), and urine osmolality (161.3 +/- 10.7 to 348.9 +/- 21.9 mosmol) significantly increased. Subsequently, ewes were rehydrated over 4 h with intravenously infused 0.45% saline (20 ml.kg-1.h-1). In response to maternal rehydration, maternal and fetal plasma osmolality decreased to basal values (298.9 +/- 3.2 and 300.1 +/- 3.8 mosmol, respectively) and fetal glomerular filtration rate (1.72 +/- 0.30 to 3.08 +/- 0.66 ml/min) and urine volume significantly increased (0.33 +/- 0.02 to 0.71 +/- 0.13 ml/min). However, fetal hematocrit (37.4%), plasma AVP (3.1 +/- 0.9 pg/ml), and urine osmolality (255.4 +/- 28.8 mosmol) did not return to basal levels during the observation period. These results demonstrate fetal hyperosmolality, blood volume contraction, AVP secretion, and altered urine production in response to maternal dehydration. Despite maternal rehydration and normalization of maternal and fetal plasma osmolality, fetal endocrine and fluid responses are prolonged.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma vasopressin levels during hypoxaemia and the cardiovascular effects of exogenous vasopressin in foetal and adult sheep

1. Fotetal plasma vasopressin levels were measured by bio-assay in chronically catheterized sheep from 110 to 145 days gestation. 2. In foetuses in good condition resting circulating vasopressin concentrations were generally undetectable (less than 5 micromicron./ml.). In 15% of the samples low concentrations (5-10 micromicron./ml.) were observed. 3. Hypoxaemia in the foetus was caused by allowing the ewe to breathe 9% O2-3% CO2 in N2 for 1 hr. Plasma vasopressin levels rose in the foetus to 119 +/- 32 micromicron./ml., whereas the hormone levels in the ewe were not routinely increased. In the foetus, the rise in plasma vasopressin levels was significantly related to the fall in pH and Pa, O2 during the hypoxia. 4. In foetuses in which the cervical vagosympathetic trunks were cut, the rise in plasma vasopressin levels (to 48 +/- 25 micromicron./ml.) during hypoxaemia was less than in intact foetuses. The increase was related only to the fall in arterial pH and the regression coefficient was less than in intact foetuses. 5. During hypoxaemia arterial pressure rose and heart rate fell in the normal foetuses. The rise in arterial pressure was greatest when the plasma vasopressin concentration was highest. 6. Spontaneous episodes of hypoxaemia (Pa, O2 less than 15 mmHg) and/or acidaemia (pH less than 7.30) occurred in four intact foetuses and four foetuses in which the cervical vagosympathetic trunks were cut; all of the latter lambs died in utero. Plasma vasopressin levels were elevated and the concentrations were inversely related to arterial pH. 7. Intravenous infusions of vasopressin to foetuses increased plasma vasopressin levels to 6-202 micromicron./ml.; the rate of clearance of the hormone was three times that in adult ewes. There was a large increase in arterial pressure and bradycardia. The hypertensive effects of vasopressin were relatively much greater in the foetus than in adult ewes.

Active sodium uptake by the skin of foetal sheep and pigs

1. Simultaneous measurements of unidirectional sodium fluxes across foetal skin incubated in vitro with identical solutions ([Na] = 150 mM) bathing either side showed a flux ratio (influx/efflux) of 1-40+/-0-08 in twenty-seven sheep skins, which was significantly different from unity (P less than 0-001). The gestational ages ranged from 47 to 98 days (term = 147 days). Similar experiments on eight foetal pig skins at 58 days gestation (term = 114-118 days) gave a mean flux ratio of 1-10 +/- 0-03 (P less than 0-02). 2. Unidirectional sodium fluxes measured with dilute Ringer solution on the outside (mucosal) surface ([Na]0 = 100mM) gave influx to efflux ratios of 0-86 +/- 0-09 in seventeen sheep (P less than 0-05) and 1-07 +/- 0-26 in five foetal pigs; the value predicted for passive movement was 0-67. 3. Incubation with inhibitors, ouabain (10-4 M) or dinitrophenol (DNP) (10-4 M) gave a flux ratio for sodium which was not significantly different from unity in the absence of a gradient, or from 0-67 when the concentration gradient was applied. 4. Sequential measurement of unidirectional diffusional fluxes of tritiated water across foetal skin gave flux ratios of 0-98 +/- 0-02 in six sheep skins and 1-06 +/- 0-11 for four pig skins in control conditions. When the outside solution was diluted to give an osmotic gradient of 100 m-osmole. kg-1 across the skin a flux ratio of 0-95 +/- 0-07 was obtained for seven sheep and was not measured in pig skin. Hormones and inhibitors had no effect on the diffusional flux ratio for water in the presence or absence of an osmotic gradient. 5. Lysine vasopressin (ADH) (200 mu./ml.) increased influx and efflux of water in the presence and, to a lesser extent in the absence of an osmotic gradient in sheep skin. In pig skin prolactin (1 u./ml.) increased both influx and efflux, but ADH had no effect on diffusional water fluxes. 6. ADH increased sodium influx in sheep skin slightly but vasotocin (5-5 mu./ml.) was more potent, particularly in the presence of an opposing diffusion gradient. Vasotocin (55 mu./ml.) reduced sodium influx in pig skin ADH had no effect on influx or efflux and prolactin reduced sodium influx and efflux. Ouabain and DNP generally reduced permeability to both sodium and water in sheep skin but had no effect in pig skin.