Characterization of changes in volume, osmolarity and electrolyte composition of porcine fetal fluids during gestation.

Allantoic fluid volume of the porcine conceptus undergoes dynamic changes throughout gestation. Volume (@ Â±SEM) increases from Day 20 (3.7 Â±0.3 ml) to an initial peak at Day 30 (189.0 Â±5.6 ml) and then decreases to Day 40 (69.5 Â±4.6 ml). It increases to a second mid gestation peak on Day 58 (451.3 Â±53.5 ml) and then steadily decreases to Day 112 (23.8 Â±5.5 ml). The sodium [Nal and potassium [KI concentrations in allantoic fluid undergo equally dynam ic changes. During periods of increasing allantoic fluid volume the (Na] IlK] ratio is greater than 1, while during periods of decreasing volume the (Na] /(KI ratio is less than 1. Calcium (Ca] , chlor ide (Cl I , and bicarbonate [HCO3 I concentrations and the osmolarity of allantoic fluid also change throughout gestation. Changes in osmolarity indicate that changes in ion concentration cannot be explained on the basis of concentration and dilution effects brought about by changes in allantoic fluid volume. Changes in amniotic fluid volume and ion composition during pregnancy are markedly different from those of allantoic fluid and indicate that the two fluid pools may be of different origin or subject to different regulatory mechanisms. Data on volume and composition of fetal urine, stomach fluid and yolk sac fluid indicate that these fluids are not major sources of allantoic or amniotic fluid, especially early in gestation. We conclude that the chorioallantois of the porcine placenta separates two fluid pools, plasma and allantoic fluid, which differ markedly in ionic composition. The chorioallantois may be intimately involved in regulating the water and electrolyte milieu in the porcine conceptus. PORCINE FETAL FLUIDS 1169 The reproductive organs were exposed following midventral laparotomy and the ovaries, oviducts, uterus and a portion of the cervix removed. The uterus was first dissected free of the ovaries, oviducts and supporting connective tissues. Dissection of the uterus was then begun along the mesometrial border so that each conceptus (fetus, fetal membranes and fetal fluids) could be exposed and removed intact into a clean pan. A 10 ml sample of allantoic fluid was obtained by puncturing the chorioailantoic membrane with a sterile 18 gauge needle attached to a sterile 10 ml syringe. The chorioallantoic membrane was then cut, the free allantoic fluid collected in the pan and the volume measured in a graduated cylinder. A 10 ml sample of amniotic fluid was obtained and its total volume determined as described for allantoic fluid. Yolk sac fluid samples, when available, were obtained in the same manner. Samples of fetal urine and stomach fluid were obtained from the fetal bladder and stomach, respec tively, after the umbilical cord was clamped and the abdominal organs exposed by midventral incision. Samples of urine and stomach fluid of up to 10 ml were withdrawn into a sterile syringe fitted with a 20 gauge needle. All samples were stored in sterile plastic storage tubes at 3 â€”? 4Â° C until analyzed for electrolyte composition. Electrolyte and Osmolarity Analyses Sodium and potassium ions were analyzed by flame photometry using a Beckman Klinaflame. Calcium and chloride ion concentrations were deter mined with an atomic absorption spectrophotometer (Perkin Elmer 306). Bicarbonate ion was a measure of carbonate, bicarbonate and free CO, using a Tech nicon Autoanalyzer (see N-8B Methodology for Technicon Instruments). Osmolarity was determined by means of a Wescor Vapor Pressure Osmometer. Statistical Analysis Data were analyzed by the procedure of least squares analysis of variance (Harvey, 1960). The model accounted for effects due to day of gestation, gilts within days of gestation, position of the conceptus within the uterine horn, right or left uterine horn, conceptus position by uterine horn interaction and number of embryos within the uterine horn. The residual mean square was used to test all main effects except day effects for which the gilts within days mean square was used. Least squares regression analyses were also conducted, and the highest order regression curve that was statistically significant and best de scribed the data was plotted to demonstrate the pattern of change in fetal fluid volume and electro lytes during the period of gestation studied.