Intramembranous solute and water fluxes during high intramembranous absorption rates in fetal sheep with and without lung liquid diversion

Abstract

Objective: To examine mechanisms that mediate increased intramembranous solute and water absorption.

Study design: Intramembranous solute and water fluxes were measured in fetal sheep under basal conditions and after intraamniotic infusion of lactated Ringer's solution of 4 L/d for 3 days with and without lung liquid diversion.

Results: Intramembranous sodium, potassium, chloride, calcium, glucose, and lactate fluxes increased 2.5- to 7.9-fold, were linearly related to volume fluxes (r = 0.83-0.99), and were unaffected by lung liquid. All clearance rates, except that of lactate, increased to equal the intramembranous volume absorption rate during infusion.

Conclusion: Under basal conditions, passive diffusion makes a minor and bulk flow a major contribution to intramembranous solute absorption. During high absorption rates, the increase in solute absorption above basal levels appears to be due entirely to bulk flow and is unaffected by lung liquid. The increased bulk flow is consistent with vesicular transcytosis.

FIGURE. Sodium absorption

Open circles, lung liquid entered the amniotic fluid; closed circles, lung liquid diverted and replaced with an equal volume of lactated Ringer’s solution. There was no difference in sodium absorption with and without lung liquid diversion (ANCOVA P = .093), so the data were combined. Solid line, regression line; dotted lines, 95% confidence interval about regression line.