Inhibitor of intramembranous absorption in ovine amniotic fluid

Abstract

Intramembranous absorption increases during intra-amniotic infusion of physiological saline solutions. The increase may be due partly to the concomitant elevation in fetal urine production as fetal urine contains a stimulator of intramembranous absorption. In this study, we hypothesized that the increase in intramembranous absorption during intra-amniotic infusion is due, in part, to dilution of a nonrenal inhibitor of intramembranous absorption that is present in amniotic fluid. In late-gestation fetal sheep, amniotic fluid volume and the four primary amniotic inflows and outflows were determined over 2-day intervals under three conditions: 1) control conditions when fetal urine entered the amniotic sac, 2) during intra-amniotic infusion of 2 l/day of lactated Ringer solution when urine entered the amniotic sac, and 3) during the same intra-amniotic infusion when fetal urine was continuously replaced with lactated Ringer solution. Amniotic fluid volume, fetal urine production, swallowed volume, and intramembranous absorption rate increased during the infusions independent of fetal urine entry into the amniotic sac or its replacement. Lung liquid secretion rate was unchanged during infusion. Because fetal membrane stretch has been shown not to be involved and because urine replacement did not alter the response, we conclude that the increase in intramembranous absorption that occurs during intra-amniotic infusions is due primarily to dilution of a nonrenal inhibitor of intramembranous absorption that is normally present in amniotic fluid. This result combined with our previous study suggests that a nonrenal inhibitor(s) together with a renal stimulator(s) interact to regulate intramembranous absorption rate and, hence, amniotic fluid volume. amniotic fluid volume regulation; fetal swallowing; fetal urine production; lung liquid secretion; intramembranous absorption OUR UNDERSTANDING OF THE REGULATION of amniotic fluid volume has been continually evolving. Current concepts are that, in late gestation, amniotic fluid volume is dependent on a balance between two primary amniotic inflows (fetal urine and lung liquid) and two primary outflows (swallowing and intramembranous absorption). Although fetal urine production, lung liquid secretion, and swallowing have been shown to be regulated by the fetal nervous and endocrine systems, the regulation of intramembranous absorption rate is less well understood.Conceptually, intramembranous absorption may be regulated by chemical substances present in amniotic fluid and/or stretch of the fetal membranes. However, membrane stretch has been shown to have no effect on amniotic fluid volume in late-gestation sheep (7). The presence of stimulators and/or inhibitors of intramembranous absorption in amniotic fluid was demonstrated in amniotic fluid washout studies (1, 18). In those experiments, lactated Ringer solution was continuously infused into the amniotic compartment at 3 l/day for 3 days, while amniotic fluid was withdrawn at the same rate and discarded. Amniotic fluid volume...

Regulation of amniotic fluid volume: mathematical model based on intramembranous transport mechanisms

“…These flows are typically measured over a period of days. In our recent studies (5,19,34,57), the fetal trachea was catheterized so that LuFlo could be accurately determined. The catheterization allows all lung secretions to continuously enter and mix with the AF.…”

“…If not available, then urine flow rate and composition were calculated on the basis of the observation that fetal urine flow increases in response to intra-amniotic fluid infusions (15,19,36,57). For this urine flow rate calculation, excess fluid within the fetal body (ExFl in milliliters) was calculated from the volume mass balance equation (Eq.…”

“…In the model, lung liquid secretion rate was held constant. This was based on our experimental findings that lung liquid production rate did not undergo significant changes over a range of experimental conditions (5,19,57).…”

Prostaglandin E2 regulation of amnion cell vascular endothelial growth factor expression: relationship with intramembranous absorption rate in fetal sheep

“…Multiple catheters were attached to the fetal skin for AF sampling and returning urine and lung liquid to the AF. In vivo data from some of these animals have been presented in part (1,6). The present study reports new data on PGE2 concentrations and relationships with AF volume and IM absorption rates.…”

“…Fetal urine production, lung liquid secretion, and swallowing have been well described and are regulated by fetal endocrine and nervous systems. The remaining major pathway, IM absorption, has been suggested as the key regulator of AF volume under a variety of experimental conditions (1,6,19,34,35), but the regulation of IM absorption is currently poorly understood.…”