Transamniotic fetal feeding. III. The effect of nutrient infusion on fetal growth retardation

A Rabbit Model for Optimization of Amniotic Fluid Components in the EXTrauterine Environment for Newborn Development (EXTEND) System

In this model article, we present a protocol for continuous amniotic fluid exchange in rabbits using a novel system to test the effects of growth factor-deficient, artificial amniotic fluid on bowel development.

BACKGROUND

Ideally, the EXTrauterine Environment for Neonatal Development (EXTEND) will provide physiologic support to the extreme premature infant. An important component of that environment is the amniotic fluid. Thus, we developed an animal model to study the growth factors found within amniotic fluid and inform design of a synthetic fluid to optimize fetal development.

METHODS

We designed a model of amniotic fluid exchange within the pregnant rabbit, continuously removing the natural fluid from around 2 fetuses per doe and replacing it with a physiologic electrolyte solution during the final 100 h of gestation. Two fetuses from the contralateral uterine horn were used as sham-operated controls. Thirty-eight fetuses were analyzed, 19 in each group. We analyzed the fetal growth and bowel development.

RESULTS

Ultrasound after 100 h of exchange showed equivalent fluid volumes, p = 0.63. Cultures were negative for bacterial colonization. Final fluid protein concentrations were 11.6% that of control fluid (mean 1,451 ± 224.2 vs. 12,491 ± 849.2 μg/mL). There was no significant difference in fetal growth, with experimental weights 91.4% of control weights, p = 0.07. Fetal bowel weights (90.1%, p = 0.16) and lengths (94.2%, p = 0.49) were also not significantly less compared to controls. There was no significant difference in villous height or crypt depth measurements between the groups, and absorptive capacity of the bowel was not different between groups, p = 0.44.

CONCLUSION

This animal model allows for manipulation of the components of amniotic fluid. Marked reduction of natural amniotic fluid proteins during gestation does not appear to significantly impair fetal growth or bowel development. Further work with this model will assess the importance of amniotic fluid components for normal development to inform design of a synthetic fluid for use during EXTEND.

Models of Intrauterine growth restriction and fetal programming in rabbits

Intrauterine growth restriction (IUGR) affects approximately 10% of human pregnancies globally and has immediate and life-long consequences for offspring health. However, the mechanisms underlying the pathogenesis of IUGR and its association with later health and disease outcomes are poorly understood. To address these knowledge gaps, the use of experimental animals is critically important. Since the 50's different environmental, pharmacological, and surgical manipulations have been performed in the rabbit to improve our knowledge of the control of fetal growth, fetal responses to IUGR, and mechanisms by which offspring may be programmed by an adverse gestational environment. The purpose of this review is therefore to summarize the utility of the rabbit as a model for IUGR research. It first summarizes the knowledge of prenatal and postnatal development in the rabbit and how these events relate to developmental milestones in humans. It then describes the methods used to induce IUGR in rabbits and the knowledge gained about the mechanisms determining prenatal and postnatal outcomes of the offspring. Finally, it discusses the application of state of the art approaches in the rabbit, including high-resolution ultrasound, magnetic resonance imaging, and gene targeting, to gain a deeper integrative understanding of the physiological and molecular events governing the development of IUGR. Overall, we hope to engage and inspire investigators to employ the rabbit as a model organism when studying pregnancy physiology so that we may advance our understanding of mechanisms underlying IUGR and its consequences in humans and other mammalian species.

Nutritional intra-amniotic therapy increases survival in a rabbit model of fetal growth restriction

OBJECTIVE

To evaluate the perinatal effects of a prenatal therapy based on intra-amniotic nutritional supplementation in a rabbit model of intrauterine growth restriction (IUGR).

METHODS

IUGR was surgically induced in pregnant rabbits at gestational day 25 by ligating 40-50% of uteroplacental vessels of each gestational sac. At the same time, modified-parenteral nutrition solution (containing glucose, amino acids and electrolytes) was injected into the amniotic sac of nearly half of the IUGR fetuses (IUGR-T group n = 106), whereas sham injections were performed in the rest of fetuses (IUGR group n = 118). A control group without IUGR induction but sham injection was also included (n = 115). Five days after the ligation procedure, a cesarean section was performed to evaluate fetal cardiac function, survival and birth weight.

RESULTS

Survival was significantly improved in the IUGR fetuses that were treated with intra-amniotic nutritional supplementation as compared to non-treated IUGR animals (survival rate: controls 71% vs. IUGR 44% p = 0.003 and IUGR-T 63% vs. IUGR 44% p = 0.02), whereas, birth weight (controls mean 43g ± SD 9 vs. IUGR 36g ± SD 9 vs. IUGR-T 35g ± SD 8, p = 0.001) and fetal cardiac function were similar among the IUGR groups.

CONCLUSION

Intra-amniotic injection of a modified-parenteral nutrient solution appears to be a promising therapy for reducing mortality among IUGR. These results provide an opportunity to develop new intra-amniotic nutritional strategies to reach the fetus by bypassing the placental insufficiency.

Localized fetomaternal hyperglycemia: spatial and kinetic definition by positron emission tomography

BACKGROUND

Complex but common maternal diseases such as diabetes and obesity contribute to adverse fetal outcomes. Understanding of the mechanisms involved is hampered by difficulty in isolating individual elements of complex maternal states in vivo. We approached this problem in the context of maternal diabetes and sought an approach to expose the developing fetus in vivo to isolated hyperglycemia in the pregnant rat.

METHODOLOGY AND PRINCIPAL FINDINGS

We hypothesized that glucose infused into the arterial supply of one uterine horn would more highly expose fetuses in the ipsilateral versus contralateral uterine horn. To test this, the glucose tracer [18F]fluorodeoxyglucose (FDG) was infused via the left uterine artery. Regional glucose uptake into maternal tissues and fetuses was quantified using positron emission tomography (PET). Upon infusion, FDG accumulation began in the left-sided placentae, subsequently spreading to the fetuses. Over two hours after completion of the infusion, FDG accumulation was significantly greater in left compared to right uterine horn fetuses, favoring the left by 1.9+/-0.1 and 2.8+/-0.3 fold under fasted and hyperinsulinemic conditions (p<10(-11) n=32-35 and p<10(-12) n=27-45) respectively. By contrast, centrally administered [3H]-2-deoxyglucose accumulated equally between the fetuses of the two uterine horns. Induction of significant hyperglycemia (10(3) mg/dL) localized to the left uterine artery was sustained for at least 48 hours while maternal euglycemia was maintained.

CONCLUSIONS AND SIGNIFICANCE

This approach exposes selected fetuses to localized hyperglycemia in vivo, minimizing exposure of the mother and thus secondary effects. Additionally, a set of less exposed internal control fetuses are maintained for comparison, allowing direct study of the in vivo fetal effects of isolated hyperglycemia. Broadly, this approach can be extended to study a variety of maternal-sided perturbations suspected to directly affect fetal health.

Evidence of fetal pulmonary aspiration of intra-amniotic administered surfactant in animal experiment

INTRODUCTION

The instillation of surfactant into the airways of patients with respiratory distress syndrome (RDS), especially in the neonatal period, is a proven therapy. The preventive therapy of RDS through intra-amniotic injection of surfactant has been reported recently. It has not been conclusively shown, however, that the surfactant administered in this way actually reaches the fetal pulmonary airways.

OBJECTIVE

To study the distribution in fetal organs of a natural surfactant labeled with technetium-99m and injected through amniocentesis into the amniotic sac of guinea pigs in the last third of pregnancy.

METHODS

After stimulating fetal respiratory movements with aminophylline 0.3 ml of an aqueous suspension containing 0.75 mg of phospholipids of a natural bovine surfactant labeled with technetium-99m, together with 0.1 ml of the biological dye carmine indigo, were injected into the amniotic sac. One hour later fetuses were delivered by cesarean section. In those that were dye-stained, dosimetric and gammagraphic tests were applied to trachea, lungs, esophagus, stomach, heart, liver, kidneys and placenta.

RESULTS

Significant radio isotopic activity was found in both lungs of six treated fetuses, with a dose capture of between 1.0% and 5.3% of total dose. The level of activity in the stomachs was similar to that in the lungs (0.9% to 3.0% dose capture), whereas activity in other organs was negligible except in two placentae. No radio isotopic activity was found in non-injected control fetuses.

CONCLUSIONS

In the present animal model natural surfactant injected intra-amniotically is aspirated into the lungs within one hour.

The effects of prenatal intraamniotic surfactant or dexamethasone administration on lung development are comparable to changes induced by tracheal ligation in an animal model of congenital diaphragmatic hernia

BACKGROUND/PURPOSE

Lung surfactant deficiency contributes to the pathophysiology of congenital diaphragmatic hernia (CDH) and the high neonatal mortality rate. Acceleration of lung surfactant system maturation by prenatal administration of hormones has been described in animal models of CDH. However, in utero tracheal ligation (TL) is the best method to accelerate lung growth and reverse the pulmonary hypoplasia associated with CDH. Although this method offers promise, its application in humans is limited. The aim of this study was to investigate a new noninvasive therapeutic strategy, that is, the prenatal intraamniotic administration of exogenous porcine surfactant or dexamethasone, and compare it with the effects of TL in an animal model of CDH.

METHODS

Twenty-four pregnant New Zealand rabbits underwent surgery on gestational day 24 or 25 to create CDH in 26 fetuses. Five groups of animals were studied: (1) Control, nonoperated fetuses (n=14), (2) CDH (n=6), (3) CDH plus TL (n 6), (4) CDH plus intraamniotic administration of Curosurf (40 mg; n=6), and (5) CDH plus intraamniotic infusion of dexamethasone (0.4 mg; n=8). On gestational day 30, the fetuses were delivered by cesarean section. Functional studies (lung hysteresis curves and lung distensibility), weight and volume of lungs, histopathologic and histomorphometric analysis of lungs were performed.

RESULTS

The authors demonstrated that the hysteresis curve of CDH animals was shifted downward in comparison with controls. The analyses of curves standardized for lung weight indicated that intraamniotic administration of surfactant or dexamethasone improved lung compliance in comparison with controls and CDH fetuses, but TL had no effect on this parameter. Lung distensibility (maximum lung volume at 32 cm of water pressure per gram of lung) was reduced by CDH, but this parameter was increased by intraamniotic administration of drugs and not by TL (P< .05). CDH decreased the weight and volume of lungs (P< .05), and these changes were reversed only by TL, which prevented the herniation of the liver from the abdomen to the thorax. Histologically, CDH lungs treated with TL or intraamniotic administration of drugs demonstrated structural patterns similar to those of controls. Histomorphometric studies proved that CDH promoted significant thickening of septa walls (P< .05), and all the therapeutic methods could reverse this alteration to control values. The alveolar number per area in control lungs, CDH, and CDH plus TL lungs were similar, but in CDH plus surfactant and CDH plus dexamethasone lungs, the decreased number per area (P< .05) demonstrated that the alveolar airspace was increased.

CONCLUSION

From these data the authors conclude that intraamniotic surfactant or dexamethasone administration is capable of preventing pulmonary hypoplasia in fetuses with CDH, and thus, this method may be a substitute for TL.

Development of ingestive behavior

Swallowing represents a primary physiological function that provides for the ingestion of food and fluid. In precocial species, swallowing activity likely develops in utero to provide for a functional system during the neonatal period. The chronically instrumented ovine fetal preparation has provided the opportunity for recent advances in understanding the regulation of in utero swallowing activity. The near-term ovine fetus swallows fluid volumes (100-300 ml/kg) that are markedly greater, per body weight, than that of the adult (40-60 ml/kg). Spontaneous in utero swallowing and ingestive behavior contribute importantly to the regulation of amniotic fluid volume and composition, the acquisition and potential recirculation of solutes from the fetal environment, and the maturation of the fetal gastrointestinal tract. Fetal swallowing activity is influenced by fetal maturation, neurobehavioral state alterations, and the volume of amniotic fluid. Furthermore, intact dipsogenic mechanisms (osmolality, angiotensin II) have been demonstrated in the near-term ovine fetus. It remains unknown to what degree, if any, fetal swallowing may be influenced by nutrient appetite, salt appetite, or taste. Nevertheless, the development of dipsogenic and additional regulatory mechanisms for ingestive behavior occurs during fetal life and may be susceptible to changes in the pregnancy environment. This review describes what is currently known regarding the in utero development of ingestive behavior and the importance of this activity for fetal and perhaps ultimately adult fluid homeostasis.

Paralysis of the preterm rabbit fetus inhibits the pulmonary uptake of intraamniotic iron dextran

OBJECTIVE

Whether fetal breathing movements or gasping result in the movement of amniotic fluid substances into the distal airways remains controversial. We evaluated the effect of paralysis of the preterm rabbit fetus on the pulmonary distribution of iron dextran.

STUDY DESIGN

Laparotomy was performed on 10 New Zealand White rabbits of 25 days' gestation (term 31 days) under general anesthesia. Fetuses in one uterine horn were given an intramuscular injection of pancuronium (1.5 mg/kg) and fetuses in the other horn were given an equal volume of normal saline solution as controls. A 1 ml volume of iron dextran (100 mg/ml) was injected into the amniotic sac of all fetuses. The laparotomy was closed, and 20 to 24 hours later the fetuses were removed by hysterotomy and assessed for paralysis. Necropsy was performed. Lungs were stained with prussian blue and evaluated histologically for the presence of iron.

RESULTS

A total of 92 pups were delivered (49 given pancuronium, 43 given normal saline solution), of which 64 were born alive. There were no differences between groups for live births (31 pancuronium, 33 normal saline solution), pup body weight, or lung weight. Pups given normal saline solution demonstrated more breathing motions, spontaneous movement, and brown (color of iron dextran) stomach contents than did the pups given pancuronium (p < 0.001). At necropsy a greater number of control pups (31/33) had brown lungs grossly compared with pups given pancuronium (2/31, p < 0.001). Lung histologic examination showed that more control pups (29/29) had iron in the trachea and main bronchi compared with pancuronium pups (0/27, p < 0.001), and more control pups (29/29) had iron in the distal lung airways compared with pancuronium pups (0/27, p < 0.001). With use of the Optimas Image Analysis System, iron in the lungs of control pups was found to be equally distributed between right versus left lungs, upper half versus lower half lungs, and anterior versus posterior lung sections. More iron was identified in the central airways than in the periphery (p < 0.001).

CONCLUSION

We conclude that paralysis prevents the uptake of iron dextran into the main and distal airways of the rabbit fetus. Although lung fluid production results in a net efflux of fluid, we speculate that fetal breathing movements can result in the movement of fluid into distal airways and potentially provide fetal therapy.

Decrease in severity of intrauterine growth retardation in subsequent pregnancies

OBJECTIVE

Intrauterine growth retardation (IUGR) is likely to recur in a subsequent pregnancy. We investigated the obstetric features of recurrent cases and the severity of IUGR by comparing initial and subsequent deliveries.

METHODS

From a total of 12,567 deliveries, 95 women who were delivered of small-for-gestational-age (SGA) infants and who became pregnant again within 5 years, were enrolled. A retrospective, comparative study of recurrent and non-recurrent groups was performed.

RESULTS

Twenty-two of ninety-five women gave birth to SGA infants again, and a relatively high risk of recurrence was confirmed, but no single recurrence-associated features were revealed. Within the recurrent group, the degree of IUGR was more severe in only five cases in the subsequent pregnancy.

CONCLUSIONS

IUGR tends to recur, but does not increase in severity in most cases. We conclude that there is no need for excessive concern about the recurrence of IUGR.

Surfactant replacement therapy in utero for prevention of hyaline membrane disease in the preterm baboon

OBJECTIVE

A previous study of intraamniotic administration of surfactant in preterm rabbit fetuses demonstrated that exogenous surfactant can be taken up into the lungs from amniotic fluid in quantities sufficient to alter pulmonary mechanical properties. On the basis of these findings we chose to test the hypothesis that intraamniotic administration of surfactant to the preterm baboon 24 hours before delivery will prevent the development of clinical and pathological aspects of hyaline membrane disease.

STUDY DESIGN

Characteristics of hyaline membrane disease in the preterm baboon model include atelectasis, the formation of hyaline membranes in the airways and distal air saccules, overexpansion of distal airways, and disruption of airways by barotrauma associated with neonatal intensive care practices. Nine preterm baboons were treated with either saline solution (n = 4) or surfactant (n = 5) by intraamniotic injection on 136 to 137 days' gestation. One day later fetuses were delivered by cesarean section and maintained for 24 hours with standard neonatal intensive care techniques.

RESULTS

All neonates completed the protocol, and surfactant-treated animals had better clinical courses as documented by Pao2/PAO2 (p < 0.05), FIO2 (p < 0.005), and Paco2 (p < 0.05). Significant radiographic differences (p < 0.02) were noted to be the result of surfactant treatment. No differences in ventilator pressures or neonatal cardiovascular parameters were found. Significant differences in pulmonary pathologic conditions (p < 0.01) were also found, but all animals had some degree of pathologic pulmonary changes associated with hyaline membrane disease.

CONCLUSION

Therefore a single treatment with surfactant in utero significantly improved the clinical course but did not completely prevent hyaline membrane disease in an established animal model.

The nutrition of the fetus with intestinal atresia: studies in the chick embryo model

This article examines the effects of experimental prenatal intestinal obstruction on the growth and blood composition of chick embryos. Intestinal atresia (IA) was produced by bipolar bowel electrocoagulation in fertile eggs on the 14th day of incubation. The chicks killed on the 19th day were measured, weighed, and blood-sampled. Twenty-three control, 10 sham-operated, and 11 IA chicks were studied. Animals with IA were severely undernourished by weight (43.4 +/- 4.7 v 70.3 +/- 7.6% of egg weight, P < .001) and length (15.3 +/- 1.1 v 18.1 +/- 0.9 mm tibial length, P < .001) in comparison with sham-operated ones. Their hematocrit was slightly lower, and total protein increased. Prealbumin was absent in their sera and albumin, alpha and beta globulins were significantly decreased, whereas gamma-globulin was greatly increased. Sodium, potassium chloride, urea, and glucose remained within normal limits. The lack of placenta in the avian embryo precludes any supply of nutrients by this route and the ingestion of amniotic fluid, which is protein-rich after the 13th day of incubation, when the opening of the seroamniotic connection allows albumen to be mixed with it, becomes the main source of nutrients until hatching. Obstruction of the main incoming avenue by IA induces severe malnutrition in this model which relies on this route to a greater extent than the human fetus. In spite of the obvious biological differences between the avian embryo and the human fetus, the present evidence supports the hypothesis that prenatal interruption of the amniotic fluid transit contributes to fetal undergrowth in IA.

Fetal rabbit intestinal absorption: implications for transamniotic fetal feeding

Delivery of nutrients to the fetus via the developing gastrointestinal tract has been advocated as a potential prenatal treatment for intrauterine growth retardation. Previous studies have demonstrated significant increases in fetal length and weight following continuous intraamniotic nutrient infusions in animals. The ideal composition of intraamniotic feedings has yet to be determined. Thirteen time-mated rabbit does underwent ceserean section between gestational days 27 and 33 to deliver 54 fetuses. The everted intestinal sleeve technique was used to measure radiolabeled carbohydrate and amino acid uptake rates from 1-cm segments of midjejunum. Uptake of galactose was more than double that of glucose and fructose. Active uptake of glucose, fructose, and galactose increased by 3.3-fold, 6.2-fold, and 2.1-fold, respectively, during the final 7 days of gestation when expressed as uptake per mg intestine. In contrast, uptake of the amino acids leucine, lysine, and proline decreased by 41%, 34%, and 17%, respectively, during the final 5 days of gestation. The demonstrated changes in intestinal absorption and nutrient delivery in the rabbit model, during the last days of gestation, suggest that certain substrates are absorbed more rapidly than others. Galactose may be the preferred intraamniotic carbohydrate for fetal growth.