Allantoic fluid compositional changes during acute urine drainage in fetal sheep

OBJECTIVE: This study examines the effects of ovine fetal urine drainage, which prevents urine inflow into the allantoic and amniotic cavities, on allantoic and amniotic fluid osmolalities and electrolyte concentrations in an attempt to study solute movement out of the allantoic cavity. METHODS: Nine chronically catheterized fetal sheep at 132 +/- 2 (mean +/- standard error) [SE] days' gestation, with allantoic and amniotic cavity catheters, were studied. Seven animals were studied for 9 hours, with maternal and fetal blood drawn hourly, and amniotic and allantoic fluid sampled for osmolality and electrolyte concentrations. On the second day, seven animals (five from the above group) were studied in the same fashion, except that fetal urine was drained after the first hour. RESULTS: Allantoic and amniotic fluid osmolality increased significantly (13.2 +/- 1.1 (SE) mOsm/kg, P < .0001, 7.5 +/- 1.5, P < .03, respectively) after fetal urine drainage but not sufficiently enough to account for the large amount of fetal urine diverted (217 mL per 8 hours). Fetal (3.1 +/- 0.6, P < .04) and maternal plasma (2.4 +/- 0.5, P < .03) osmolalities increased significantly, and this increase was consistent with the amount of fetal urine drained over the 8-hour drainage period. Allantoic (4.0 +/- 1.7 mEq/L, P < .003) and amniotic sodium (4.2 +/- 1.5 mEq/L, P < .03) and amniotic chloride (3.1 +/- 1.6 mEq/L, P < .04) increased significantly as compared with control animals. CONCLUSION: During the fetal urine drainage period, the increases in allantoic and amniotic fluid osmolalities and electrolyte concentrations confirmed that water left these compartments via the intramembranous pathway. If only water (and not solutes) had left these compartments, then the increase in osmolalities would have been too small to account for the large volume of diverted fetal urine. Therefore, solutes, in addition to water, must leave the allantoic cavity through the intramembranous pathway as no other pathway exists. Furthermore, solute movement through the intramembranous pathway may explain the ability of the fetus to maintain molecular and solute gradients between fluid compartments. Finally, the intramembranous pathway may play an important role in amniotic and allantoic fluid volume regulation and composition.     

Ovine intramembranous pathway permeability: use of solute clearance to determine membrane porosity.

OBJECTIVE: The contribution of the fetal chorioamniotic membranes (i.e. the intramembranous pathway) to the regulation and maintenance of amniotic fluid (AF) volume and composition has yet to be completely understood. Knowledge of membrane permeability properties is vital to understanding how the intramembranous pathway contributes to the overall maintenance of AF homeostasis. Although there are significant data regarding the regulation of intramembranous water flow, there is little understanding of the regulation of intramembranous solute flow. In the present study, we sought to determine the effect of molecular weight or size of non-polar compounds on intramembranous solute movement in the ovine model. METHODS: Five singleton ovine fetuses (117 +/- 3 days) were chronically prepared with bladder, tracheal, amniotic cavity and femoral arterial and venous catheters and an esophageal occluder. The allantoic membranes were excised. After 5 days' recovery, AF volume was calculated by intraamniotic injection of 99Tc-labelled red blood cells (time -6 to 0 h). At time 0, AF exchange routes were limited to the intramembranous pathway by inflation of the esophageal occluder and external drainage of fetal urine and lung fluid. Following intra-amniotic injection of creatinine (Cr, 1 g, MW 11 000 Da, 4 A) and [125I]albumin (RISA, 250 microCi, MW 69 000 Da, 36 A), maternal and fetal plasma and AF samples were collected at timed intervals during the subsequent 5 h. AF solute clearance (Cl(x)) was determined by the changes in AF total solute content. RESULTS: Cr and RISA disappeared from the AF with a corresponding increase in fetal, though not maternal, plasma levels. The mean Cl(Cr) was significantly greater than Cl(RISA) (2.0 +/- 0.3 ml/min vs. 1.0 +/- 0.2 ml/min; p < 0.04). CONCLUSION: Solute clearance from the amniotic cavity is inversely proportional to solute molecular weight/size. Although the membrane comprising the ovine intramembranous pathway is size restrictive, membrane pores allow passage of non-polar solutes up to 36 A. Knowledge of membrane permeability characteristics is essential for the utilization of the intramembranous pathway for fetal therapeutics.     

Ovine intramembranous pathway permeability: use of solute clearance to determine membrane porosity

Objective: The contribution of the fetal chorioamniotic membranes (i.e. the intramembranous pathway) to the regulation and maintenance of amniotic fluid (AF) volume and composition has yet to be completely understood. Knowledge of membrane permeability properties is vital to understanding how the intramembranous pathway contributes to the overall maintenance of AF homeostasis. Although there are significant data regarding the regulation of intramembranous water flow, there is little understanding of the regulation of intramembranous solute flow. In the present study, we sought to determine the effect of molecular weight or size of non-polar compounds on intramembranous solute movement in the ovine model. Methods: Five singleton ovine fetuses (117 &#45 3 days) were chronically prepared with bladder, tracheal, amniotic cavity and femoral arterial and venous catheters and an esophageal occluder. The allantoic membranes were excised. After 5 days' recovery, AF volume was calculated by intraamniotic injection of 99 Tc-labelled red blood cells (time &#109 6 to 0 h). At time 0, AF exchange routes were limited to the intramembranous pathway by inflation of the esophageal occluder and external drainage of fetal urine and lung fluid. Following intra-amniotic injection of creatinine (Cr, 1 g, MW 11 000 Da, 4 Å) and [ 125 I]albumin (RISA, 250 &#119 Ci, MW 69 000 Da, 36 Å), maternal and fetal plasma and AF samples were collected at timed intervals during the subsequent 5 h. AF solute clearance (Cl x) was determined by the changes in AF total solute content. Results: Cr and RISA disappeared from the AF with a corresponding increase in fetal, though not maternal, plasma levels. The mean Cl Cr was significantly greater than Cl RISA (2.0 &#45 0.3 ml/min vs. 1.0 &#45 0.2 ml/min; p < 0.04). Conclusion: Solute clearance from the amniotic cavity is inversely proportional to solute molecular weight/size. Although the membrane comprising the ovine intramembranous pathway is size restrictive, membrane pores allow passage of non-polar solutes up to 36 Å. Knowledge of membrane permeability characteristics is essential for the utilization of the intramembranous pathway for fetal therapeutics.     

Effect of esophageal ligation on amniotic fluid volume and urinary flow rate in fetal sheep

OBJECTIVE: Although the fetus normally swallows large volumes of amniotic fluid each day, it is unclear whether amniotic fluid volume increases after fetal esophageal obstruction or whether fetal urine production changes. Our objective was to determine the effects of fetal esophageal ligation on amniotic fluid volume and urinary flow rate over time. STUDY DESIGN: Seven late-gestation fetal sheep underwent esophageal ligation, and 7 served as time control animals. The urachus was ligated to eliminate urine flow to the allantoic cavity. On days 1, 3, 5, 7, and 9 after surgery, we measured the composition of amniotic fluid, fetal urine, and fetal and maternal blood, as well as amniotic fluid volume and fetal urinary flow rate. A 3-factor analysis of variance was used for statistical analysis. RESULTS: Amniotic fluid volume did not change with time in the control group, averaging 876 +/- 142 mL (mean +/- SEM), and it decreased in the esophageal ligation group (P =.020), averaging 309 +/- 75 mL on day 9. Fetal urinary flow rate was lower (P =.0063) in the esophageal ligation group (431 +/- 27 mL/d) than in the control group (631 +/- 54 mL/d). There were no differences in fetal or maternal blood compositions between the two groups. Amniotic fluid sodium and chloride increased in the ligated animals. CONCLUSION: Polyhydramnios did not occur after esophageal ligation, even though the fetuses excreted approximately 4000 mL of urine over the 9-day study period. This suggests that intramembranous absorption is substantially increased. With only small changes in amniotic solute concentrations, intramembranous solute absorption must occur simultaneously with water, suggesting a near-zero reflection coefficient for solutes. We speculate that fetal urine, lung secretions, or both contain a factor that increases intramembranous permeability.     

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.     

Technetium Tc 99m rapidly crosses the ovine placenta and intramembranous pathway

OBJECTIVE: This study examined the movement of the soluble ion technetium Tc 99m across the ovine placenta and intramembranous pathway. STUDY DESIGN: Nineteen fetal sheep at 131 ± 1 (SE) days' gestation were studied. After a 1-hour control period technetium Tc 99m was injected into either a fetal vein (n = 7), the amniotic cavity (n = 5), or a maternal vein (n = 5). Maternal and fetal blood, fetal urine, and amniotic and allantoic fluid were sampled during the control period and for 8 hours after the injection. Fetal urine was drained externally throughout the experiment. In five animals technetium Tc 99m was injected intraamniotically after the fetus was killed with air emboli and sampled as described. RESULTS: Intrafetally injected technetium Tc 99m rapidly crossed the placenta; then it entered and was concentrated in the amniotic cavity. Intraamniotically injected technetium Tc 99m rapidly entered into the fetal circulation. The maternally injected technetium Tc 99m rapidly crossed the placenta into the fetus, suggesting a half-time for placental exchange of <50 minutes. The technetium Tc 99m injected into the dead fetus group demonstrated significantly less maternal absorption than in the live fetus group. CONCLUSIONS: The soluble ion technetium Tc 99m demonstrated a much more rapid movement in both directions across the ovine placenta then previously demonstrated for the smaller ion sodium. Technetium Tc 99m rapidly crossed the intramembranous pathway bidirectionally, suggesting a high permeability of the intramembranous pathway. Minimal maternal absorption of technetium Tc 99m in the dead fetus group suggests little transmembranous absorption by the mother. (Am J Obstet Gynecol 1996;175:1557-62.)     

Renal effects of ovine fetal arginine vasopressin secretion in response to maternal hyperosmolality

Acute increases in maternal plasma osmolality can increase amniotic fluid osmolality. Amniotic fluid is primarily derived from fetal urine production, and arginine vasopressin infusion can affect both fetal urine production and amniotic fluid osmolality. To assess the effect of short-term changes in maternal osmolality on fetal arginine vasopressin secretion and renal function, six ewes of 126 +/- 1 days' gestation received intravenous infusions of 20% mannitol (500 ml/10 min). In response to mannitol infusion, both maternal and fetal plasma osmolality increased significantly (302 +/- 3 to 326 +/- 2 and 300 +/- 1 to 309 +/- 2 mosm, respectively). Increased fetal plasma and urine arginine vasopressin concentrations were associated with significant increases in fetal urine osmolality (146 +/- 12 to 262 +/- 30 mosm) and sodium concentration (35.8 +/- 2.8 to 76.5 +/- 20 mu Eq/ml), but fetal urine production rates did not change (0.68 +/- 0.11 to 0.62 +/- 0.15 ml/min). These conclusions were reached: Acute increases in maternal osmolality can affect fetal arginine vasopressin secretion; arginine vasopressin-induced increases in fetal urine osmolality may contribute to increased amniotic fluid osmolality in response to maternal hyperosmolality.     

Fetal plasma and renal responses to ruminal fluid

Amniotic fluid homeostasis is dependent on a balance of fetal fluid production and absorption. The fetal gastrointestinal tract is believed to resorb 500 to 1000 ml of amniotic fluid per day during 7 to 10 bouts of swallowing activity. However, the impact of ruminal fluid on fetal plasma composition and fluid homeostasis is largely unknown. Seven ovine fetuses (120 +/- 1 day) received intraruminal infusions of 0.9% or 3% saline solution on alternate days. In response to successive 40-minute intraruminal infusions of 0.9% saline solution (0.5 and 1.0 ml/kg/min), there was no change from basal levels of fetal plasma osmolality (295.7 +/- 2.9 mosm), plasma arginine vasopressin (1.45 +/- 0.29 pg/ml), urine osmolality (150 +/- 8 mosm), or urine volume (0.49 +/- 0.10 ml/min). In response to the 3% saline solution infusion, significant increases were noted in fetal plasma osmolality (295.4 +/- 3.1 to 302.6 +/- 2.6 mosm), plasma arginine vasopressin (1.77 +/- 0.31 to 4.84 +/- 0.79 pg/ml), and urine osmolality (157 +/- 13 to 342 +/- 25 mosm), whereas fetal urine volume significantly decreased (0.35 +/- 0.05 to 0.15 +/- 0.06 ml/min). These results indicate that hypertonic, but not isotonic, saline solution infusion into the fetal gastrointestinal tract may affect fetal plasma composition and urine production. Under conditions of significant plasma to luminal osmotic gradients, fetal gastrointestinal water and electrolyte transfer may be more rapid than can be compensated by either fetal renal function or placental equilibration.     

The missing link in amniotic fluid volume regulation: intramembranous absorption

Although fetal urine output and swallowing are major contributors to amniotic fluid regulation, other pathways for fluid movement must be involved in the regulation of amniotic fluid volume because many studies report fetal urine output to be greater than swallowing. This study was designed to examine the possibility of fluid transfer between the amniotic cavity and the fetal blood that perfuses the fetal membranes and surface of the placenta in the ovine fetus. We injected warmed distilled water into the amniotic fluid in three groups of chronically catheterized fetal sheep. In normal fetuses, there was rapid absorption of the water into the fetal circulation, resulting in highly significant decreases in fetal osmolality, plasma electrolytes, and heart rate as well as increases in arterial pressure and fetal hemolysis. Concomitantly, there was a small, delayed fall in maternal osmolality. In a second group of fetuses with ligated esophagi, these same responses occurred except that the fetal osmolality and electrolyte changes occurred earlier and were significantly greater. In a third group of fetuses killed just before the water injection, maternal osmolality was unchanged. These data suggest the intramembranous pathway as a major route of amniotic fluid absorption in the ovine fetus. In addition, esophageal ligation appears to augment the conductance of this pathway, as evidenced by a significantly greater estimated filtration coefficient and rate of water absorption in the ligated animals than in controls. Finally, the transmembranous pathway directly to the mother does not appear to be a major route.     

Fetal blood volume, urine flow, swallowing, and amniotic fluid volume responses to long-term intravascular infusions of saline

The purpose of this study was to determine the effects of a long-term infusion into the fetal circulation on fetal and amniotic fluid dynamics. In 10 chronically catheterized fetal sheep averaging 130 +/- 1 (SE) days' gestation, a balanced, isotonic electrolyte solution (Isolyte S) was infused continuously for 5 days into a fetal vein at a rate of 0, 1, 2, 4, and 0 L/day, respectively. During the infusion, fetal blood volume increased by a maximum of 6.4 +/- 2.0% (p less than 0.001), and the daily swallowing of amniotic fluid doubled (p less than 0.001). Fetal urine flow increased (p less than 0.0001) above preinfusion rates by a volume equal to the infusion rate plus the increase in swallowing, whereas renal excretion of sodium and chloride increased by the amount infused. The increase in the plasma concentration of atrial natriuretic factor (p less than 0.0001) and the decrease in arginine vasopressin (p less than 0.05) were not linearly related to urine flow changes. Amniotic fluid volume increased (p less than 0.0001) by 20% of the infused volume. All values returned to normal on day 5 except amniotic fluid volume, which remained elevated. Estimated allantoic fluid volume at the end of day 5 was 800 ml above normal. Thus it appears that on a long-term basis, the ovine fetus eliminates infused water and electrolytes through its kidneys rather than across the placenta. Although all of the infused volume left the fetus through its kidneys, only 30% of the infused volume remained in the amniotic and allantoic fluid compartments, suggesting transfer to the mother by unknown mechanisms.     

Swallowing, urine flow, and amniotic fluid volume responses to prolonged hypoxia in the ovine fetus

OBJECTIVE: Four days of hypoxia produce an extensive fetal polyuria with little change in amniotic fluid volume in the ovine fetus. We hypothesized that fetal swallowing and intramembranous absorption would increase with prolonged hypoxia to offset the polyuria. STUDY DESIGN: After a 24-hour normoxic period, nine ovine fetuses were subjected to 4 days of hypoxia induced by lowering maternal inspired oxygen content. Seven fetuses were monitored for 5 days as normoxic time controls. Measurements included fetal swallowed volume by a computerized system with Transonic flow probes, urine production by gravity drainage, and amniotic fluid volume by an indicator dilution technique. Data were averaged over 12-hour intervals, and a three-factor repeated-measures analysis of variance was used for statistical testing. RESULTS: During days 2 to 5, arterial oxygen tension was 20.7+/-1.1 (SE) mm Hg in the normoxic and 13.9+/-0.8 mm Hg in the hypoxic fetuses (P<.0001). Urine flow was unchanged over time in the normoxic fetuses and increased gradually from 693+/-88 to 2189+/-679 mL per day during hypoxia (P<.0001). The prehypoxia swallowed volume was similar in the two groups, averaging 447+/-95 mL per day. Although transiently decreased in eight of nine hypoxic fetuses, the 12-hour average swallowed volumes were not significantly different at any time in the hypoxic versus normoxic fetuses (P=.62). Amniotic fluid volume increased in the hypoxic fetuses relative to that in the normoxic fetuses (520+/-338 mL vs -226+/-136 mL, P<.01), although the increase was small (P<.01) relative to the excess volume of urine (4269+/-1306 mL). Estimated intramembranous absorption increased from 209+/-95 mL per day during normoxia to average 1032+/-396 mL per day during hypoxia. CONCLUSIONS: The current study supports the concept that prolonged hypoxia produces a progressive fetal polyuria with relatively small changes in amniotic fluid volume. Concomitantly, hypoxia does not induce prolonged changes in fetal swallowing; rather, intramembranous absorption greatly increases, thereby preventing severe polyhydramnios.     

Rapid intramembranous absorption into the fetal circulation of arginine vasopressin injected intraamniotically

Recently an intramembranous pathway was reported in the ovine fetus as a route for the movement of a significant volume of water from the amniotic cavity directly into the fetal blood, which perfuses the fetal membranes and fetal surface of the placenta. To test whether this pathway could be an avenue for the movement of arginine vasopressin from the amniotic cavity into the fetal circulation, we injected 1 to 25 micrograms of arginine vasopressin into the amniotic cavity of two groups of chronically catheterized fetal sheep: a control group of seven animals and a group of seven animals with surgical ligation of the fetal esophagus. We found similar and highly significant increases of arginine vasopressin concentrations in both control and surgically ligated fetuses in amniotic fluid (p less than 0.00001), fetal plasma (p less than 0.0001), and fetal urine (p less than 0.0001). Both groups had similar increases in arterial (p less than 0.0001) and venous (p less than 0.003) pressures with simultaneous decreases in urine flow (p less than 0.001) and heart rate (p less than 0.0001) after the intraamniotic injection of arginine vasopressin. We conclude that amniotic arginine vasopressin can be rapidly absorbed in its biologically active form directly into the fetal circulation through the intramembranous pathway. Furthermore, the observation that esophageal ligation did not alter this absorption suggests that the intramembranous pathway may be important in the regulation of amniotic fluid volume and composition.     

Comparison of amniotic and intramembranous unidirectional permeabilities in late-gestation sheep

OBJECTIVE: Amniotic fluid volume is regulated by the intrinsic modulation of intramembranous absorption. However, neither the mechanisms nor the rate-limiting barriers of this transport are known. We tested the hypothesis that the amnion is the rate-limiting barrier of intramembranous absorption by comparing unidirectional permeabilities of the amnion in vitro and the intramembranous pathway in vivo. STUDY DESIGN: Unidirectional permeabilities to 99m technetium pertechnate or [14 C]inulin of fresh ovine amnion were measured in vitro in a Ussing chamber; the permeability-surface area products were calculated by the multiplication of the permeabilities by gestational age-specific amniotic surface areas. Unidirectional permeabilities of the intramembranous pathway of the 2 tracers were calculated from solute fluxes between amniotic fluid and fetal blood in chronically catheterized late-gestation fetal sheep. Statistical comparisons included t -tests, least squares regression, analysis of variance, and analysis of covariance. RESULTS: In the isolated amnion in vitro, the ratio of permeabilities in the amniotic fluid to chorionic direction and the reverse direction was not significantly different from unity for 99m technetium pertechnate (1.03+/-0.10 [SE]; n=7) or [14 C]inulin (1.10+/-0.17; n=7). In contrast, in the in vivo preparation, the ratio of intramembranous permeabilities outward from the amniotic fluid and the reverse direction was greater than unity for 99m technetium pertechnate (2.10+/-0.34; n=8; P=.014) and [14 C]inulin (4.68+/-1.24; n=7; P=.025). The permeability-surface area product of 99m technetium pertechnate (2.18+/-0.79 mL/min) of the isolated amnion was similar to the in vivo intramembranous permeability (n=8) in the amniotic fluid to fetal blood direction (1.42+/-0.34 mL/min) and greater than that in the reverse direction (0.84+/-0.25 mL/min; P=.046). The permeability-surface area product of [14 C]inulin of the amnion (0.53+/-0.20 mL/min) was similar to intramembranous permeability (n=7) in the amniotic fluid to fetal blood (0.68+/-0.15 mL/min) direction and greater than that in the reverse direction (0.22+/-0.06 mL/min; P=.0097). CONCLUSION: Solute transport across the ovine amnion depends on solute size and appears to be limited only by the amnion's passive diffusional properties. In vivo intramembranous transport similarly depends on solute size but is not exclusively a passive diffusional process because it is primarily unidirectional outward from the amniotic fluid. Although it is a major barrier, the amnion is not the only barrier and does not appear to be responsible for the unidirectional nature of intramembranous absorption. Thus, unidirectionality appears to be imparted by nonpassive mechanisms in non-amnion tissues, which most likely includes vesicular transport within the endothelial cells of the intramembranous microvessels.     

Transplacental, amniotic, urinary, and fetal fluid dynamics during very-large-volume fetal intravenous infusions

With rapid intravenous infusion of very large volumes of isotonic saline solutions into the fetus, the fluid could stay within the fetal body, thereby creating hydrops fetalis, be transferred into the amniotic fluid through the fetal kidneys, thereby creating polyhydramnios, or be transferred across the placenta into the maternal circulation. This study was designed to explore these possibilities. After a 1-hour control period, 10 near-term chronically catheterized ovine fetuses were infused intravenously with 4 L (greater than 100% of fetal weight) of either isotonic saline solution or lactated Ringer's solution over 4 hours. Fetal arterial pressure was significantly elevated by 7 mm Hg throughout the infusion (p less than 0.00001). Venous pressure underwent a transient rise (4.8 mm Hg) at 20 minutes of infusion and remained elevated (2.7 mm Hg) during the rest of the infusion (p less than 0.00001). Fetal urine flow increased by an average of 5.7 +/- 0.4 ml/min throughout the infusion (p less than 0.00001) and accounted for 34.1% +/- 2.6% of the infused volume. Estimated fetal extracellular fluid volume increased by 17.7% +/- 1.8% of the infused volume. Because fetal fluid retention, urine flow, and amniotic fluid volume changes accounted for only half of the infused fluid, the remainder of the infused volume must have crossed the placenta and entered the maternal circulation. Given the above changes in vascular pressures, this requires a filtration coefficient of the placenta 50 to 100 times the previously reported values. Thus we conclude that relatively small changes in fetal vascular pressures dramatically alter the filtration capacity of the ovine placenta and transplacental volume flow.     

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.     

Ovine hourly fetal urine production: relation to fetal electrocortical activity

OBJECTIVE: Ultrasound studies of hourly urine production rate in human fetuses have suggested that a fall in urine production occurs in state 2F (fetal quiet sleep) secondary to a state-dependent decrease in renal blood flow. We sought to ascertain the relationship between fetal hourly urine production rate and behavioral state in the near-term ovine fetus, a model in which urine production and fetal brain activity can be directly measured. METHODS: Six ewes with singleton pregnancies were prepared with vascular and amniotic fluid catheters. Fetuses were prepared with hindlimb vascular catheters, a bladder catheter, and biparietal ECoG electrodes. After at least 5 days of recovery (ga 130 +/- 2 days; term = 145-150 days), each animal was monitored for a 6-h period. Urine production was measured by draining the bladder catheter through a drop counter and fetal ECoG was continuously recorded (sampling rate of 50 Hz). ECoG activity was analyzed using power spectral analysis and periods of active and quiet sleep identified using both signal amplitude and corresponding 85% spectral edge frequency. RESULTS: Basal fetal arterial pH (7.36 +/- 0.01), pO2 (22.0 +/- 1.2 mmHg) and pCO2 (47.0 +/- 1.6 mmHg) and plasma (295 +/- 2 mOsm/kg) and urine (179 +/- 3 mOsm/kg) osmolalities were within normal ranges. Active and quiet sleep comprised 50 +/- 2 and 43 +/- 1% time, respectively. There was no difference in hourly urine production rate in active sleep (21.4 +/- 9.7 ml/h) and quiet sleep (18.8 +/- 7.7 ml/h). CONCLUSIONS: 1) Hourly fetal urine production rate is independent of ECoG activity state in the near-term ovine fetus. 2) Assuming only minor species differences, ultrasound measurement of human fetal hourly urine production rate can be performed without concern for fetal neurobehavioral state changes.     

Regulation of amniotic fluid volume by intramembranous absorption in sheep: role of passive permeability and vascular endothelial growth factor

OBJECTIVE: During long-term intravascular fluid infusion in the ovine fetus, a large increase in fetal urinary flow rate occurs while amniotic fluid volume increases only slightly because of increased intramembranous absorption. The current study tested the hypotheses that passive intramembranous permeability increases in response to fetal intravascular saline solution infusion and that the increased intramembranous absorption occurs in parallel with an increase in vascular endothelial growth factor gene expression in the amnion, chorion, and placenta. STUDY DESIGN: Chronically catheterized fetal sheep that average 126 +/- 1 (SE) days of gestation either were infused intravascularly with 7 L of normal saline solution over 3 days (n = 8 sheep) or served as time controls (n = 6 sheep). Amniotic fluid volume and fetal urinary flow rate were measured daily. Intramembranous diffusional permeability was estimated daily as being equal to the clearance of intra-amniotically injected technetium 99m. Vascular endothelial growth factor messenger RNA abundance in the amnion, chorion, and placenta was determined by Northern blot analysis. Statistical analyses included analysis of variance. RESULTS: In the infused fetuses, amniotic fluid volume and urinary flow increased (P <.01) by 891 +/- 144 mL and 3488 +/- 487 mL per day, respectively, on infusion day 3 compared with no changes over time in the control fetuses. In the infused fetuses, estimated intramembranous absorption increased by 4276 +/- 499 mL during the 3-day infusion. Intramembranous technetium 99m permeability was similar over time in the two groups. In the infused group, vascular endothelial growth factor messenger RNA levels in the amnion, chorion, and placenta increased 2- to 4-fold compared with the control group (P <.001). CONCLUSION: The up-regulation of vascular endothelial growth gene expression may mediate the increase in the intramembranous absorption that is induced by volume-loading diuresis; however, this does not occur by passive mechanisms. We speculate that vascular endothelial growth mediates the increased intramembranous absorption by increasing vesicular transport.     

Ovine fetal swallowing: expression of preterm neurobehavioral rhythms

OBJECTIVE: Fetal swallowing contributes importantly to amniotic fluid volume regulation and fetal gastrointestinal maturation. Near-term ovine fetal swallowing occurs in discrete bouts of activity (at approximately 30-min intervals) in association with fetal electrocortical voltage changes. Thus, swallowing rhythms have been hypothesized to be entrained to fetal neurobehavioral states. In the preterm ovine fetus, electrocortical activity does not demonstrate differentiation into high- and low-voltage periods until 120-130 days' gestation. We sought to quantify patterns of preterm (114 days, 0.75 gestation) ovine fetal swallowing activity and volume, and, in view of the lack of electrocortical pattern changes, to explore whether swallowing activity was regulated by an independent central pacemaker. METHODS: Six singleton ovine pregnancies were chronically prepared with fetal and maternal femoral artery and vein catheters. Biparietal electrocortical electrodes were placed on the fetal skull. Following a minimum 5-day recovery period, fetuses were studied at 114 +/- 1 days. Patterns of fetal swallowing behavior were quantified by computer analysis of laryngeal-esophageal electromyography (EMG) and thoracic esophageal fluid flow during a 12-h period. RESULTS: Esophageal fluid flow was bidirectional, although antegrade flow predominated, leading to an average fluid acquisition rate of 13 +/- 3 ml/h (7.3 +/- 1.8 ml/h per kg) during the 12-h study (302 +/- 87 ml/day). Propagated esophageal EMG activity, representing coordinated 'swallows', averaged 56 +/- 6 swallows/h and correlated well with net esophageal fluid flow. 'Bouts' of swallowing activity (> or = 3 swallows/min) averaged 9 +/- 1 swallows/bout, lasted 1.8 +/- 1.4 min and accounted for 31 +/- 4% of the swallowed volume. Despite the absence of fetal electrocortical high-voltage/low-voltage transitions, there was a 26.1 +/- 3.9-min interval between periods of swallowing bout activity. CONCLUSIONS: Preterm (0.75 gestation) ovine fetal volume swallowed (302 ml/day) and volume swallowed for body weight (175 ml/day per kg) was significantly less than that previously noted at 0.85 gestation (831 ml/day, 274 ml/day per kg, respectively; p < 0.05) although the rates of swallowing activity were similar. The presence of swallowing bout activity at periodic intervals, in the absence of electrocortical differentiation, suggests an intrinsic central pacemaker regulating preterm fetal neurobehavior.     

Fetal absorption of intra-amniotic aldosterone: effects on urine composition.

OBJECTIVES: Amniotic fluid (AF) volume and composition are maintained by a balance of fetal fluid production and resorption. Ovine fetal resorption of peptide hormones (e.g., arginine vasopressin) from the amniotic cavity has been demonstrated, with resultant effects on fetal urine production. The present study was undertaken to determine whether intra-amniotically administered steroid hormones could be absorbed from the amniotic cavity into fetal plasma and whether intra-amniotic aldosterone administration would affect fetal renal sodium and potassium excretion. METHODS: Seven singleton fetuses (132 +/- 2 days) were prepared with bladder, vascular, and amniotic cavity catheters. After a 5-day recovery period, a bolus of aldosterone was injected into the amniotic cavity. Fetuses were monitored for an additional 24 hours during which time maternal, fetal, and AF samples were collected at timed intervals. RESULTS: After intra-amniotic aldosterone injection, AF aldosterone concentrations increased at 30 minutes and remained elevated for 4 hours after the aldosterone bolus. In response to increased AF aldosterone, fetal plasma aldosterone levels significantly increased by 30 minutes, peaked at 1 hour (17 +/- 4 to 758 +/- 160 pg/mL), and remained elevated for a minimum of 4 hours. Fetal urine sodium excretion significantly decreased and potassium excretion increased. Maternal plasma aldosterone levels increased significantly (25 +/- 10 to 401 +/- 56 pg/mL) but to levels below fetal values. Amniotic fluid and fetal and maternal aldosterone concentrations and fetal urine sodium and potassium excretion returned toward basal levels by 24 hours. CONCLUSION: The steroid hormone aldosterone can be absorbed from the amniotic cavity into the fetal circulation and can alter fetal urine electrolyte excretion. These results suggest that the amniotic cavity is a potential route of in utero pharmacologic fetal therapy.     

Increased urinary flow without development of polyhydramnios in response to prolonged hypoxia in the ovine fetus

OBJECTIVE: In the ovine fetus subjected to 24 hours of hypoxia, urinary flow is normal within a few hours from the onset of hypoxia and there is a maintained inhibition of swallowing. We hypothesized that 4 days of fetal hypoxia would lead to polyhydramnios. STUDY DESIGN: Five late-gestation fetal sheep were subjected to hypoxia for 4 days and 7 other late-gestation fetal sheep served as time control animals. Fetal hypoxia was produced on postsurgical days 5 through 9 by continuous intratracheal nitrogen insufflation to the ewe. On days 3, 5, 7, and 9 after surgery, amniotic fluid volume, fetal urinary flow rate, and the compositions of maternal and fetal blood, amniotic fluid, and fetal urine were measured. A 3-factor analysis of variance was used for statistical analysis. RESULTS: During the period of experimental hypoxia the mean (+/-SE) fetal PaO(2) was 16.0 +/- 0.6 mm Hg, versus 21.2 +/- 0.7 mm Hg in control sheep (P <.001). Fetal hypoxia was associated with increased urinary flow on days 7 and 9, averaging 1410 +/- 310 and 2101 +/- 345 mL/d, respectively, versus 585 +/- 92 and 699 +/- 78 mL/d, respectively, in control animals (P <.001). Amniotic fluid volume was unchanged with time and averaged 960 +/- 159 mL in hypoxic fetuses on postsurgical days 7 through 9 and 851 +/- 130 mL in control animals (P =.60). Fetal blood lactate increased in the hypoxic animals, averaging 3.4 +/- 2.1 mmol/L versus 1.6 +/- 0.3 mmol/L in control animals (P =.02). Fetal urinary excretions of sodium, potassium, chloride, and lactate increased significantly during hypoxia, by 170% to 400%. CONCLUSION: Four days of nitrogen-induced hypoxia in the ovine fetus resulted in excess fetal urinary flow approximating 1000 mL/d greater than normal without the development of polyhydramnios. Because amniotic fluid volume did not change and hypoxia is a known inhibitor of fetal swallowing, we speculate that intramembranous absorption of amniotic water, electrolytes, and lactate increased.