Document Detail

Regulation of amniotic fluid volume: intramembranous solute and volume fluxes in late gestation fetal sheep.
MedLine Citation:
PMID:  15467551     Owner:  NLM     Status:  MEDLINE    
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.
Robert A Brace; Maria L Vermin; Evelien Huijssoon
Publication Detail:
Type:  Journal Article; Research Support, U.S. Gov't, P.H.S.    
Journal Detail:
Title:  American journal of obstetrics and gynecology     Volume:  191     ISSN:  0002-9378     ISO Abbreviation:  Am. J. Obstet. Gynecol.     Publication Date:  2004 Sep 
Date Detail:
Created Date:  2004-10-06     Completed Date:  2004-11-06     Revised Date:  2007-11-14    
Medline Journal Info:
Nlm Unique ID:  0370476     Medline TA:  Am J Obstet Gynecol     Country:  United States    
Other Details:
Languages:  eng     Pagination:  837-46     Citation Subset:  AIM; IM    
Department of Reproductive Medicine, University of California San Diego, La Jolla, Calif 92093-0802, USA.
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MeSH Terms
Amniotic Fluid / chemistry,  physiology*
Bicarbonates / analysis
Biological Transport
Calcium / analysis
Chlorides / analysis
Esophagus / embryology,  surgery
Fetal Blood / chemistry
Fetus / physiology*
Gestational Age*
Sodium / analysis
Trachea / embryology,  surgery
Grant Support
Reg. No./Substance:
0/Bicarbonates; 0/Chlorides; 7440-23-5/Sodium; 7440-70-2/Calcium

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