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.     

Prolonged hypoxia upregulates vascular endothelial growth factor messenger RNA expression in ovine fetal membranes and placenta

OBJECTIVE: In ovine fetuses, 4 days of hypoxia resulted in a large increase in urine flow, without the development of polyhydramnios, which suggests that intramembranous absorption of the amniotic fluid was enhanced. Because vascular endothelial growth factor is speculated to be a regulator of intramembranous absorption through increases of membrane vascularity and fluid transport, we hypothesized that hypoxia upregulated vascular endothelial growth factor gene expression in the fetal membranes. STUDY DESIGN: Five near-term ovine fetuses that were subjected to 4 days of hypoxia and 5 age-matched time controls were studied. On day 4, the amnion, chorion, and placenta were collected for cellular localization and quantification of vascular endothelial growth factor messenger RNA and for the determination of vascular endothelial growth factor molecular forms that were expressed. The data were analyzed statistically with the use of t tests and 2-factor analyses of variance. RESULTS: Vascular endothelial growth factor messenger RNA was expressed in the fetal membranes localized to the amniotic epithelium and chorionic cytotrophoblast, and to the villous cytotrophoblast of the placenta. In hypoxic fetuses, vascular endothelial growth factor messenger RNA levels in these cell layers were significantly increased compared with the controls. Five vascular endothelial growth factor molecular forms were identified with vascular endothelial growth factor(164) being the most abundant form expressed. The pattern of expression of the forms was not altered by hypoxia. CONCLUSION: In the near-term ovine fetus, hypoxia induced vascular endothelial growth factor messenger RNA expression in the amnion, chorion, and placenta. This was associated with an increase in intramembranous absorption of amniotic fluid. We speculate that the increased intramembranous absorption was mediated by a vascular endothelial growth factor-induced increase in the transport of amniotic fluid into the fetal membranes.     

Regulation of amniotic fluid volume: intramembranous solute and volume fluxes in late gestation fetal sheep

OBJECTIVE: Recent studies suggest that amniotic fluid volume is regulated by the rate of intramembranous absorption of amniotic fluid into fetal blood. The purpose of the present study was to determine the simultaneous intramembranous solute and water fluxes to gain insight into the intramembranous transport and amniotic fluid volume regulatory mechanisms. STUDY DESIGN: All major amniotic inflows and outflows, except intramembranous flow, were eliminated in 10 fetal sheep over 8 hours by occlusion of the fetal trachea and esophagus; the fetal urine was drained to the exterior. Amniotic fluid composition and volume were measured before and at the end of the 8 hours. Solute and volume fluxes through the intramembranous pathway were calculated from amniotic fluid concentration and volume changes. Statistical analyses included t-tests, linear regression, and analyses of variance. RESULTS: Amniotic fluid volume decreased by 128 +/- 24 (SE) mL over 8 hours (P < .001), which was correlated only marginally with the fetal to amniotic fluid osmotic gradient (r=0.59; P = .072). Amniotic fluid sodium, chloride, calcium, and bicarbonate concentrations increased (P < .0001), even though there were net outward fluxes of these solutes; these outward fluxes occurred against concentration gradients; and the clearances of these solutes were the same despite widely differing amniotic fluid concentrations and fetal blood to amniotic fluid concentration gradients. With the use of multivariate regression, intramembranous solute fluxes separated into 2 components, which were a primary outward flux that correlated with the volume flux and a minor inward component that correlated with the fetal plasma to amniotic fluid concentration gradient for sodium, chloride, calcium (P < .001), and bicarbonate (P < .02). The concentration-dependent fluxes averaged approximately one third of the bulk fluxes and were in the opposite direction. CONCLUSION: The poor correlation of amniotic fluid volume reduction with the fetal-to-amniotic fluid osmotic gradient shows that the primary mechanism that mediates intramembranous volume flow is not passive osmosis in the normal fetus under basal conditions. The strong correlations of solute fluxes simultaneously with volume flux and concentration gradients suggest that intramembranous solute fluxes are mediated by both bulk flow and passive diffusion. The small size of the passive component relative to the size of the bulk component suggests that intramembranous solute transfer is mediated primarily by bulk flow with a smaller and usually oppositely directed contribution by diffusion down concentration gradients. Bulk flow by vesicular transport is the only known physiologic transport mechanism that is compatible with these data, but it is not known whether this occurs in the amnion or intramembranous blood vessels or both.     

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.     

Hypoxia modulation of caveolin-1 and vascular endothelial growth factor in ovine fetal membranes

During normal pregnancy, amniotic fluid is absorbed from the amniotic compartment into fetal blood through the intramembranous blood vessels in the fetal membranes. It has been hypothesized that this transport process is mediated by transcytosis of caveolae-like vesicles. Because fetal hypoxia increases intramembranous absorption, the authors explore the effects of hypoxia on the gene expression of caveolin-1, a structural protein of caveolae, in ovine fetal membranes and cultured amnion cells. Near-term ovine fetuses were rendered hypoxic for 4 days. Caveolin-1 mRNA and protein levels were significantly reduced in the amnion and chorion but not in the placenta. In cultured ovine amnion cells incubated in 2% oxygen for 24 hours, hypoxia did not significantly alter caveolin-1 mRNA or protein expression. Vascular endothelial growth factor mRNA levels were increased in response to hypoxia in the fetal membranes as well as in cultured amnion cells. The results indicate that hypoxia does not augment but instead down-regulates or has no effect on caveolin-1 gene expression in the amnion and chorion, suggesting that caveolin-1 may play a role as a negative regulator of amnion transport function under hypoxic conditions.     

水通道蛋白-1和血管内皮生长因子的表达与羊水量的关系

目的:研究水通道蛋白-1(AQP1)、血管内皮生长因子(VEGF)与羊水量的关系。方法:应用免疫组织化学SP法分别检测20例羊水过少、22例羊水正常、17例羊水过多的胎盘、胎膜组织中AQP1、VEGF的表达,并对其表达强度行半定量分析。同时每组随机抽取10例用Western blot分别检测胎盘、胎膜组织中AQP1、VEGF蛋白表达量。结果:免疫组化显示AQP1、VEGF在3组胎盘、胎膜中均有阳性表达,胎膜中随着羊水量增加,其表达量相应上调,且具有显著性差异(P<0.05),胎盘中AQP1、VEGF表达量均无显著差异。Western blot检测蛋白的表达也有相同结果。且胎膜上AQP1和VEGF表达强度呈正相关。结论:AQP1和VEGF均与羊水量异常有关,AQP1和VEGF上调可能促进了羊水的跨膜转运。     

Cellular localization of vascular endothelial growth factor in ovine placenta and fetal membranes

To further understand the role of vascular endothelial growth factor (VEGF) in mediating angiogenesis and vascular permeability during development in the sheep placenta and fetal membranes, we examined the localization of VEGF mRNA and protein in placental, chorionic and amniotic tissues by in situ hybridization and immunohistochemistry in ovine fetuses at 62, 102 and 141 days gestation (term=150 days). In the placenta, VEGF mRNA expression and VEGF protein immunostaining were strong in cytotrophoblasts surrounding the villi. In addition, VEGF protein was localized in smooth muscle cells around fetal and maternal blood vessels and in the maternal epithelium. There was no apparent difference in placental VEGF mRNA or protein levels associated with advancing gestation. In the fetal membranes, VEGF mRNA was detected in the amniotic epithelium and the chorionic cytotrophoblastic cell layer. The intensity of the hybridization signals in both amnion and chorion appeared low at 62 days, moderate at 102 days and high at 141 days gestation. VEGF protein was detected in amniotic epithelium and chorionic cytotrophoblasts at all gestational ages studied. The increase in VEGF gene expression in fetal membranes as term approaches suggests that during fetal development VEGF may promote the vascularity and permeability of the microvessels which perfuse the fetal membranes, as well as permeability of the amniotic membrane itself. Thus VEGF may participate in the regulation of amniotic fluid volume.     

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.     

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.     

Expression of aquaporin 9 in human chorioamniotic membranes and placenta

OBJECTIVE: Aquaporin 9 (AQP9) is one of the recently identified water channels that is also permeable to neutral solutes including urea. To investigate the molecular mechanism of intramembranous pathway of amniotic fluid regulation, we sought to determine whether AQP9 is expressed, and the cellular localization of AQP9 expression in human fetal membranes. STUDY DESIGN: Fetal membranes from 5 normal term human pregnancies were studied. Northern analysis was used to determine the tissue AQP9 messenger RNA (mRNA) expression. In situ hybridization and immunohistochemical staining with specific anti-AQP9 antibody was used for cellular AQP9 localization in the human fetal membranes. RESULTS: Northern analysis detected AQP9 mRNA expression in human amnion, chorion, and placenta. In situ hybridization revealed AQP9 mRNA expression in epithelial cells of the amnion, chorion cytotrophoblasts, and syncytiotrophoblasts and cytotrophoblasts of placenta. Further immunohistochemical study confirmed the AQP9 protein expression in these cell types of fetal membranes. CONCLUSION: This study demonstrated the expression of AQP9 mRNA and protein in human chorioamniotic membranes and placenta. The AQP9 expression in fetal membranes suggests that AQP9 may be an important water channel in intramembranous amniotic fluid water regulation.     

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.     

Developmental expression of vascular endothelial growth factor (VEGF) receptors and VEGF binding in ovine placenta and fetal membranes

The receptor tyrosine kinases, kinase-insert domain-containing receptor (KDR) and fms-like tyrosine kinase (Flt-1), and their ligand vascular endothelial growth factor (VEGF) are essential for the development and maintenance of placental vascular function during pregnancy. To further understand the role of VEGF in mediating angiogenesis and vascular permeability during development, the cellular localization of KDR and Flt-1 mRNA and protein, and the distribution of(125)I-VEGF binding sites in placenta, chorion and amnion of ovine fetuses were examined at three different gestational ages. In placentae at 62, 103 and 142 days, the predominant site of KDR mRNA and protein, and VEGF binding was the maternal vascular endothelium. In addition, a specific, although weak, signal for KDR mRNA was found in the maternal epithelium. At 103 and 142 days but not 62 days gestation, KDR mRNA and protein as well as VEGF binding sites were abundantly present in the endothelium of villous blood vessels. In the fetal membranes at 62, 103 and 142 days gestation, KDR mRNA and protein were expressed in the amniotic epithelium and intramembranous blood vessel endothelium, where binding of(125)I-VEGF was strong. There was no KDR mRNA or VEGF binding in the chorionic cytotrophoblast. Flt-1 expression was not detectable in placentae or fetal membranes at the three ages studied.In summary, the results demonstrated that VEGF receptors are present in the maternal and fetal vasculatures of the ovine placenta. This expression is consistent with a capillary growth-promoting function of KDR and its ligand VEGF. Further, the presence of KDR and VEGF binding sites in ovine fetal membranes suggests a role for VEGF in promoting intramembranous vascularity and permeability throughout gestation.     

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.     

Rapid intramembranous absorption of water infused into the ovine allantoic cavity.

OBJECTIVE: Previously we found that water infused into the ovine amniotic cavity was rapidly absorbed into the fetal circulation through the vascularized fetal membranes and fetal surface of the placenta (i.e., the intramembranous pathway). The purposes of this study were to (1) estimate the conductance of the intramembranous pathway from the allantoic cavity and (2) determine if the conductance is adequate to offset the inflow of urine, which may be up to 500 ml/day in the near-term ovine fetus. STUDY DESIGN: Seven chronically catheterized fetal sheep averaging 132 +/- 2 (+/- SE) days' gestation underwent an infusion of warmed distilled water into the allantoic cavity at 6 ml/min. The infusions were continued until steady states were obtained in allantoic and amniotic fluid and in fetal and maternal blood osmolalities. During the steady state the conductance of the intramembranous pathway was estimated as the ratio of osmotic gradient to infusion rate. RESULTS: The allantoic and amniotic fluid and the fetal and maternal blood osmolalities decreased by 188 +/- 14, 36 +/- 8, 13 +/- 2, and 3 +/- 1 mOsm/kg, respectively, at steady state. From the fetal-allantoic osmolality gradients the conductance of the intramembranous pathway was 1.72 +/- 0.14 or 0.53 +/- 0.08 microliter/min/mm Hg/kg fetal weight. Assuming a similar conductance during the preinfusion period, the next volume movement would equal 0.67 ml/min (965 ml/day). CONCLUSIONS: The conductance of the intramembranous pathway in combination with the normal osmotic gradient is sufficient to remove the large volume of fetal urine that may enter the allantoic cavity each day.     

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.     

Review: Water channel proteins in the human placenta and fetal membranes

It has been established that the permeability of the human placenta increases with advancing gestation. Indirect evidence has also proposed that aquaporins (AQPs) may be involved in the regulation of placental water flow but the mechanisms are poorly understood. Five AQPs have been found in the human placenta and fetal membranes [AQP1, 3, 4, 8 and 9]. However, the physiological function(s) and the regulation of these proteins remain unknown. Emerging evidence has shown that human fetal membrane AQPs may have a role in intramembranous amniotic fluid water regulation and that alterations in their expression are related to polyhydramnios and oligohydramnios. In addition, we have observed a high expression of AQP3 and AQP9 in the apical membrane of the syncytiotrophoblast. Moreover, AQP9 was found to be increased in preeclamptic placentas, but it could not be related to its functionality for the transport of water and mannitol. However, a significant urea flux was seen. Since preeclampsia is not known to be associated with an altered water flux to the fetus we propose that AQP9 might not have a key role in water transport in human placenta, but a function in the energy metabolism or the urea uptake and elimination across the placenta. However, the role of AQP9 in human placenta is still speculative and needs further studies. Insulin, hCG, cAMP and CFTR have been found to be involved in the regulation of the molecular and functional expression of AQPs. Further insights into these mechanisms may clarify how water moves between the mother and the fetus.     

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.)