Evidence of active sodium transport in the visceral yolk sac of the rat in vitro

Anionic amino acid transporter expression in late gestation rodent yolk sac

The yolk sac plays an important role in fetal nutrition. Transport of amino acids by the rodent visceral yolk sac has been shown previously. We have demonstrated the presence of several amino acid transport proteins capable of the Na(+)-dependent transport of anionic amino acids within late gestation mouse visceral yolk sac and uterine epithelium. We speculate that these proteins may be involved in the efflux of glutamate from the fetal to the maternal circulations.

Electrical potential difference between exocelomic fluid and maternal blood in early pregnancy

The forces that drive transfer of solutes between maternal blood and embryo in early human pregnancy are poorly understood. The aim of this study was to determine whether there is an electrical potential difference (PD) between maternal blood and the exocelomic cavity and between maternal blood and the amniotic cavity in the normal human conceptus at or before 10 wk of pregnancy. We measured PD between a saline-filled catheter in a forearm vein of women undergoing termination of pregnancy for psychological reasons in the first trimester and a second saline-filled catheter in the exocelomic cavity or amniotic cavity of their conceptus. The mean (+/- SE) maternal blood/exocelomic cavity PD in eight women was 8.7 +/- 1.0 mV and the mean maternal blood/amniotic cavity PD in four of the women was 6.7 +/- 1.3 mV, embryo side negative for both sets of measurement. These data show that there is a PD between maternal and embryonic extracellular fluid in the first trimester that will directly influence exchange of ions between the two compartments.

Diamide-induced alterations of intracellular thiol status and the regulation of glucose metabolism in the developing rat conceptus in vitro

Direct oxidation of embryonic reduced glutathione (GSH) by a thiol oxidant, diamide, has been demonstrated to result in increased glutathione disulfide (GSSG) and protein-glutathione mixed disulfide (protein-S-SG) formation, which is accompanied by embryotoxicity and reductions in amniotic fluid volume. The altered functions of critical proteins or enzymes caused by the formation of protein-S-SG perturb cellular metabolism and may be involved in the embryotoxicity produced by GSH oxidation. The present study investigates changes in the metabolism of glucose through glycolysis and the pentose phosphate shunt pathways (PPP) and their related enzymes under the oxidative conditions produced by diamide exposure in organogenesis-stage rat conceptus (gestational day 10) in vitro. The metabolism of glucose via the PPP, measured as amounts of CO2 production from D-[1-14C]-glucose, was significantly increased in the conceptus exposed to 100-500 microM diamide to levels 2.5-3-fold those of controls. It was found that these substantial increases in the PPP activity did not correlate well with a moderate activation of glucose 6-phosphate dehydrogenase (G6PD) activity, the key enzyme in the PPP pathway. Changes in glycolysis due to diamide treatment were also determined by measurements of lactate production from D-[U-14C]-glucose. Production of lactate by the conceptus exposed to 250-500 microM diamide for 60 min was reduced (to approximately 54% of control values) concomitantly with a significant inhibition of the glycolytic enzymes, glyceraldehyde 3-phosphate dehydrogenase (GPD) and phosphofructokinase (PFK), indicating an overall decrease in glycolysis. Diamide was found to produce a differential effect on the enzymatic activities determined in this study, with greater degrees of inhibition seen in the tissue supernatants from the visceral yolk sac (VYS) compared to those from the embryo. Activities of GPD and PFK were decreased to approximately 22% and 43% control values, respectively, when determined in the supernatants from the VYS of the conceptus exposed to 500 microM diamide for 60 min. In addition, more than 90% of the GPD activity in the VYS, but not the embryo, was rapidly inhibited by the thiol alkylating agent N-ethylmaleimide (NEM, 100 microM) within 15 min of the exposure. In contrast to diamide and NEM, no alterations in lactate production were seen in the conceptus treated with the GSH depletor L-buthionine-S,R-sulfoximine (1 mM) for 5 hr in the culture media. Further experiments demonstrated that the activity of the GPD, inhibited by a 30-min incubation with 500 microM diamide, can be reversed after removal of diamide and that this effect was potentiated by subsequent treatment with dithiothreitol (30 mM), a thiol reducing agent. These results indicated the involvement of thiol/disulfide status in regulation of the metabolism of glucose in the developing conceptus and support the hypothesis that GSH oxidation and protein-S-SG formation could be a critical event associated with mechanisms of embryotoxicity elicited by oxidative stress. It was suggested in this study that, under these experimental conditions, embryotoxicity induced by diamide is primarily mediated via altered VYS functions, including disrupted energy production (glycolysis).

Ion transport across the early chick embryo: I. Electrical measurements, ionic fluxes and regional heterogeneity

The chick blastoderm at the stage of late gastrula is a flat disc formed by three cell layers and exhibiting epithelial properties. Blastoderms were cultured in miniature chambers and their electrophysiological characteristics were determined under Ussing conditions. Under open-circuit condition and identical physiological solutions on both sides, spontaneous transblastodermal potential difference (Voc) of -7.5 +/- 3.3 mV (ventral side positive) was measured. Under short-circuit condition (transblastodermal delta V = 0 mV), the blastoderm generated short-circuit current (Isc) of 21 +/- 8 microA/cm2, which was entirely dependent on extracellular sodium, sensitive to ouabain applied ventrally and independent of extracellular chloride. The net transblastodermal Na+ flux fully accounted for the measured Isc, both under control conditions and with ouabain. The total transblastodermal resistance (Rtot) was 390 +/- 125 omega cm2. Frequently, the Voc, Isc and Rtot showed spontaneous oscillations with a period of 4-5 min. Removal of endoderm and mesoderm did not significantly affect the electrical properties, indicating that the electrogenic sodium transport is generated by the ectoderm. The Voc and Isc measured in the area pellucida (-1.3 +/- 0.8 mV, 9.3 +/- 4.4 microA/cm2) and extraembryonic area opaca (-7.8 +/- 1.1 mV, 31.2 +/- 12.7 microA/cm2) were significantly different. Such a heterogeneous distribution of electrical properties can explain the presence in the blastoderm of extracellular electrical currents found by using a vibrating probe.

Effects of cytochalasin B on calcium transport by 1,25(OH)2D3- or PTH-treated chick embryonic yolk sac in vitro and in vivo

The present study was done to elucidate the mechanism(s) by which calcium is taken up or transported across the yolk sac membrane of the embryonic chick. We examined the effect of various inhibitors or experimental conditions on the uptake of calcium in vitro. Treatment with ouabain, verapamil, antimycin A and calcium ionophore A23187; substitution of choline for sodium or potassium in the buffer; or incubation of the tissue at 0 degree C had no significant effect on calcium uptake by the yolk sac membrane. Dinitrophenol (DNP) and lanthanum chloride (LaCl2) reduced 45Ca uptake from day 6 and 9 embryos by 15% and 30%, respectively. Cytochalasin B decreased the uptake of 45Ca in yolk sac membrane disks of day 6 embryos, but not in older embryos. The effects of cytochalasin B were explored further in embryos pretreated with either 1,25(OH)2D3 or PTH, both of which enhance calcium uptake. Cytochalasin B decreased calcium uptake in 9-day and 12-day vitamin D-treated embryos to about 60% of their hormone-enhanced level and also decreased PTH-stimulated 45Ca uptake into yolk sac disks by about 50% in embryos of all age groups tested. We also examined the effect of cytochalasin B on 45Ca transport across the yolk sac membrane in vivo. Cytochalasin B did not affect this transport in control (vehicle-treated) embryos. However, it significantly decreased the enhanced in vivo 45Ca transport in day-9 and -12 vitamin D-treated embryos approximately 30% and 45%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium-dependent short-circuit current across the yolk sac membrane during embryonic development in normal and shell-less cultured chicks

The transepithelial electrical characteristics of the isolated yolk sac membrane of normal in ovo or shell-less cultured chick embryos were investigated. In normal chicks the potential difference (blood side positive relative to yolk side) and short-circuit current of the membrane increased during development. Ouabain (10(-4) M) on the blood side (basolateral side, serosal side) significantly decreased potential difference and short-circuit current but was without effect on the yolk side (brush border side, mucosal side). Substitution of choline for Na+ in the bathing solutions abolished the potential difference and the short-circuit current; when Na+ replaced choline this effect was reversed. Amiloride added to both sides of the yolk sac membrane had no effect on potential difference or short-circuit current. Injection of aldosterone (50 micrograms) and T3 (10 microM) into yolk did not induce amiloride sensitivity. The short-circuit current was not altered by addition of either glucose or alanine to the bath. The short-circuit current of the yolk sac membrane of shell-less cultured embryos was significantly lower than that of normal controls. Addition of Ca2+ to the serosal bathing medium did not reverse the foregoing condition, but decreased the short-circuit current. It is concluded that the yolk sac short-circuit current is Na+ dependent and increases with developmental age in the chick embryo.

Experimental manipulation of the rodent visceral yolk sac

The visceral yolk sac (VYS) is an especially important placental organ in the rodent because it is the primary source of exchange between the embryo and mother during early organogenesis before the chorioallantoic placenta circulation is established. The VYS is involved with nutritional, endocrine, metabolic, immunologic, secretory, excretory, and hematopoietic functions. The VYS also plays a role in steroid metabolism and interacts with a variety of blood-borne factors: parathyroid hormone, glucocorticoids, insulin, and vitamin D metabolites. The importance of the VYS during development is emphasized by the embryotoxicity resulting from exposure to agents which cause VYS dysfunction when administered to the pregnant animal during organogenesis. Several experimental procedures have provided useful information concerning a variety of VYS functions from early organogenesis to term: Culture of the Embryo, Fetal Incubation, Culture of the Fetus, Giant Yolk Sac, Short- and Long-Term Culture of the Yolk Sac, Modified Ussing's Chamber, Single or Double Diffusion Chamber, and the use of Heterologous Rodent Visceral Yolk Sac Antibodies. Since human yolk sac pathology has been associated with developmental toxicity and spontaneous abortion, it is important to discover whether there are some common functional roles among different mammalian species and to determine if other experimental animal models can be used to study the possible contribution of human yolk sac dysfunction to some human reproductive problems.

Permeability of the near-term rat placenta to hydrophilic solutes

The permeability of the rat placenta at 21 days' gestation (term is 23 days) has been measured with fetuses intact in situ and by perfusion of the fetal circulation. Intact measurements were made by removal of fetuses at known time intervals after injection of radioisotopes to the anaesthetized mother and measurement of their radioactivity content. Unidirectional clearance (Kmf) was calculated and found to be in proportion to the diffusion coefficient in water (Dw) for [14C]mannitol, [14C]sucrose, [51Cr]EDTA and [3H]inulin but was relatively high for 22Na and low for [125I]albumin. The high value for 22Na is partially explained by transfer across the yolk sac placenta; cautery of the vitelline vessels significantly reduced the Kmf for 22Na but not that for [51Cr]EDTA or [125I]albumin. The low [125I]albumin value is explained on the basis of restricted diffusion of this tracer. Perfused permeability measurements were made by injection of radioisotope into the anaesthetized mother and measurement of the radioactive content of fetal perfusate samples. Although the Kmf for Na+ was significantly (P less than 0.01) reduced by 25 per cent, there was no other significant difference between results with the perfused placentae compared with those from the intact placentae. Comparison of the intact data with similar measurements in guinea pig and human confirms the similar permeability properties of the haemochorial placentae as a whole; the extra trophoblast layers in the rat make no discernible difference.

The effects of triaminopyrimidine on the short circuit current and transmembrane potentials of the isolated rat visceral yolk sac in vitro

Intracellular potentials in the cells from 17.5-day old rat visceral yolk sacs were measured by a glass microelectrode. When penetrated from the maternal side, the cells have potentials of about 50.2 +/- 1.9 mV (inside negative) which were reduced by increasing the external K+ concentration and increased by removing Na+ ions from the bathing fluid. Triaminopyrimidine (TAP) which inhibited Na+ transport caused a dose-dependent depolarization of the cell membrane. The depolarization was dependent on the presence of extracellular Ca2+ ions. It is proposed that TAP may inhibit Na+ transport by increasing the intracellular concentration of calcium ions.

Catecholamines in the yolk sac epithelium of the rat

Catecholamines were found histochemically in the visceral yolk sac of the rat from embryonic day (ED) 10, i.e. before the amines become detectable in peripheral or central neurons of the fetus. Formaldehyde-induced fluorescence was confined to the apical part of the yolk sac epithelial cells. The specificity of histofluorescence has been confirmed by borohydride reduction, microspectrofluorimetry revealing an emission peak at 480 nm and administration of reserpine. The catecholamines present were identified by mass fragmentography using N,O-trifluoroacetyl derivatives. At ED 13 both dopamine and norepinephrine were present, while only dopamine was detected at ED 18 1/2. Maternal circulation or the epithelial cells themselves appear as possible sources of these catecholamines. The occurrence of amines in the yolk sac epithelium may reflect an intracellular role of these compounds, a barrier function of the epithelium or a step in a transport to the fetus where the amines might assume regulatory functions.