Effect of oral sucrose on ingestive behavior in the near-term ovine fetus

BACKGROUND: Dipsogen-mediated ingestion matures acutely in late gestation because the preterm fetus may demonstrate absent responses to putative dipsogens. Although central appetite-mediated ingestive behavior is functional near term, it is unknown whether peripheral mechanisms for stimulation of appetite also are functional. In the adult, sweet taste stimulates and potentiates ingestive behavior. We sought to determine whether oropharyngeal sucrose exposure stimulates ingestive behavior in the near-term ovine fetus. STUDY DESIGN: Time-dated pregnant ewes with near-term singleton fetuses (n = 6) were chronically prepared with fetal vascular and sublingual catheters and esophageal electromyogram electrodes and studied at 129 +/- 1 days of gestation. After an initial 2-hour baseline period, successive solutions of distilled water and 2.5%, 10%, and 40% sucrose were infused sublingually (0.25 mL/min), each for 2 hours. Maternal and fetal arterial blood samples were drawn at timed intervals. RESULTS: During the basal period, fetal swallowing averaged 0.9 +/- 0.1 swallows per minute. Swallowing did not change in response to distilled water (0.9 +/- 0.2 swallows per minute) but significantly increased after sublingual infusion of 2.5% sucrose (1.3 +/- 0.1 swallows per minute), 10% sucrose (1.8 +/- 0.1 swallows per minute), and 40% sucrose (1.3 +/- 0.1 swallows per minute, P =.001). There were no significant changes in other fetal or maternal parameters. CONCLUSIONS: The stimulation of fetal swallowing in response to sublingual sucrose infusion suggests that taste-mediated ingestive behavior is functional in the near-term fetus and that both central and systemic appetite mechanisms are intact near term. Fetal swallowing increased in response to an increase in sucrose concentration to peak at 10% and then decreased with further rises in concentration, possibly mediated by aversive fetal reaction to a high-intensity sucrose concentration.     

Unopposed appetite (orexigenic) mechanisms in the near-term ovine fetus: central leptin does not inhibit sucrose ingestion.

OBJECTIVE: Leptin is produced in adipocytes and is present in the term fetus. In the adult, leptin acts centrally to inhibit neuropeptide Y-induced carbohydrate intake. We sought to examine if central leptin alters fetal ingestion of oral sucrose in the near-term ovine fetus. METHODS: Five pregnant ewes and fetuses were prepared with fetal vascular, sublingual and intracerebroventricular (ICV) catheters and esophageal electromyogram electrodes, and studied at 132 +/- 1 days' gestation (term 145-150 days). Following a 2-h baseline period, 10% sucrose was infused sublingually (0.25 ml/min) for the duration of the study. At time 4 h, leptin (0.075 mg/kg) was administered ICV and fetal swallowing was monitored for an additional 6 h. RESULTS: During the basal period, fetal swallowing averaged 0.7 +/- 0.1 swallows/min. Fetal swallowing increased significantly in response to 10% sucrose (1.2 +/- 0.1 swallows/min; p < 0.05). In response to ICV leptin, fetal swallowing remained significantly elevated at 2, 4 and 6 h (1.3 +/- 0.4, 1.4 +/- 0.3 and 1.5 +/- 0.2 swallows/min, respectively; p < 0.05 vs. control). CONCLUSIONS: These results indicate that central leptin inhibition of sucrose ingestion is not functional in the near-term fetus. We speculate that a leptin-mediated anorexigenic response is not present at birth, such that unopposed appetite stimulatory mechanisms in the newborn may facilitate rapid newborn weight gain despite high body fat levels.     

Unopposed appetite (orexigenic) mechanisms in the near-term ovine fetus: central leptin does not inhibit sucrose ingestion

Objective: Leptin is produced in adipocytes and is present in the term fetus. In the adult, leptin acts centrally to inhibit neuropeptide Y-induced carbohydrate intake. We sought to examine if central leptin alters fetal ingestion of oral sucrose in the near-term ovine fetus.

Methods: Five pregnant ewes and fetuses were prepared with fetal vascular, sublingual and intracerebroventricular (ICV) catheters and esophageal electromyogram electrodes, and studied at 132?±?1 days' gestation (term 145–150 days). Following a 2-h baseline period, 10% sucrose was infused sublingually (0.25?ml/min) for the duration of the study. At time 4?h, leptin (0.075?mg/kg) was administered ICV and fetal swallowing was monitored for an additional 6?h.

Results: During the basal period, fetal swallowing averaged 0.7?±?0.1 swallows/min. Fetal swallowing increased significantly in response to 10% sucrose (1.2?±?0.1 swallows/min; p?<?0.05). In response to ICV leptin, fetal swallowing remained significantly elevated at 2, 4 and 6?h (1.3?±?0.4, 1.4?±?0.3 and 1.5 ±?0.2 swallows/min, respectively; p?<?0.05 vs. control).

Conclusions: These results indicate that central leptin inhibition of sucrose ingestion is not functional in the near-term fetus. We speculate that a leptin-mediated anorexigenic response is not present at birth, such that unopposed appetite stimulatory mechanisms in the newborn may facilitate rapid newborn weight gain despite high body fat levels.     

Neuropeptide Y administered into cerebral ventricles stimulates sucrose ingestion in the near-term ovine fetus

OBJECTIVE: In adults, nutrient intake is controlled by opposing actions of appetite stimulants (eg, neuropeptide Y [NPY]) and suppressors (eg, leptin). Because NPY may exert a preferential role in mediating adult carbohydrate intake, we sought to determine the effect of central NPY on near-term fetal carbohydrate ingestion. STUDY DESIGN: Five pregnant ewes and fetuses were prepared with fetal vascular, sublingual, and intracerebroventricular catheters, electrocorticogram, and esophageal electromyogram electrodes and studied at 131+/-2 days' gestation. After a 2-hour baseline period, 10% sucrose was infused sublingually for the duration of the study. At 4 hours' time, NPY was injected into the fetal cerebral ventricles and fetal swallowing monitored for an additional 6 hours. RESULTS: During the basal period, mean (+/-SEM) swallowing averaged 0.8+/-0.1 swallows per minute. Fetal swallowing increased significantly in response to sublingual sucrose (1.3+/-0.1 swallows/min, P=.001), and further significantly increased at 4 to 6 hours after NPY injection into the cerebral ventricles (1.8+/-0.3, P=.001). CONCLUSION: These results indicate central NPY stimulation of fetal ingestion beyond that resulting from sublingual 10% sucrose. The in utero development of NPY-induced ingestive behavior may be in preparation for high neonatal caloric intake.     

Effects of central angiotensin II receptor antagonism on fetal swallowing and cardiovascular activity

OBJECTIVE: Fetal plasma angiotensin II levels are 10 times the levels found in adults. Despite these high levels, central injection of angiotensin II may stimulate fetal swallowing and increase fetal arterial blood pressure. We postulated that the high rate of spontaneous fetal swallowing and normal fetal pressor regulation may be dependent, in part, on central angiotensin II. In view of the potential dipsogenic role of both type 1 and type 2 angiotensin II receptors, we examined the central effect of the nonselective angiotensin II receptor antagonist saralasin on fetal swallowing and cardiovascular responses. STUDY DESIGN: Six time-dated pregnant ewes and fetuses were chronically prepared with fetal vascular and intracerebroventricular catheters, electrocorticograms, and esophageal electromyogram electrodes and studied at 130 +/- 1 days' gestation. After an initial 2-hour baseline period (0 to 2 hours), saralasin (1 mL, 64 microg) was injected intracerebroventricularly (2 to 4 hours). After 4 hours the dose of saralasin was repeated together with angiotensin II (1 mL, 6.4 microg), and the fetuses were monitored for a final 2 hours. Four fetuses also underwent an identical control study (on an alternate day) in which saralasin was replaced with artificial cerebrospinal fluid. RESULTS: Blockade of central angiotensin II receptors by intracerebroventricular saralasin significantly reduced mean (+/- SEM) spontaneous fetal swallowing (1.3 +/- 0.1 to 0.4 +/- 0.1 swallows per minute; P <.001) but did not alter fetal mean blood pressure (50 +/- 5 versus 56 +/- 5 mm Hg). Intracerebroventricular angiotensin II, in the presence of saralasin, did not affect swallowing (0.6 +/- 0.1 swallows per minute) or fetal blood pressure. In the control study, intracerebroventricular artificial cerebrospinal fluid did not change fetal swallowing (0.9 +/- 0.1 versus 1.0 +/- 0.1 swallows per minute), whereas intracerebroventricular angiotensin II significantly increased swallowing activity (1.0 +/- 0.1 versus 2.0 +/- 0.1 swallows per minute; P <.001) and fetal blood pressure (51 +/- 2 to 59 +/- 3 mm Hg; P =.003). CONCLUSIONS: Tonic activity of central angiotensin II receptor stimulation contributed to the high rate of basal ovine fetal swallowing but not fetal basal blood pressure. Angiotensin II-mediated fetal dipsogenic and pressor responses are a result of specific angiotensin II receptor binding in central brain regions. These results indicate that fetal exposure to angiotensin II antagonists or angiotensin-converting enzyme inhibitors may have adverse effects on fetal and amniotic fluid homeostasis.     

Ovine fetal swallowing: expression of preterm neurobehavioral rhythms

OBJECTIVE: Fetal swallowing contributes importantly to amniotic fluid volume regulation and fetal gastrointestinal maturation. Near-term ovine fetal swallowing occurs in discrete bouts of activity (at approximately 30-min intervals) in association with fetal electrocortical voltage changes. Thus, swallowing rhythms have been hypothesized to be entrained to fetal neurobehavioral states. In the preterm ovine fetus, electrocortical activity does not demonstrate differentiation into high- and low-voltage periods until 120-130 days' gestation. We sought to quantify patterns of preterm (114 days, 0.75 gestation) ovine fetal swallowing activity and volume, and, in view of the lack of electrocortical pattern changes, to explore whether swallowing activity was regulated by an independent central pacemaker. METHODS: Six singleton ovine pregnancies were chronically prepared with fetal and maternal femoral artery and vein catheters. Biparietal electrocortical electrodes were placed on the fetal skull. Following a minimum 5-day recovery period, fetuses were studied at 114 +/- 1 days. Patterns of fetal swallowing behavior were quantified by computer analysis of laryngeal-esophageal electromyography (EMG) and thoracic esophageal fluid flow during a 12-h period. RESULTS: Esophageal fluid flow was bidirectional, although antegrade flow predominated, leading to an average fluid acquisition rate of 13 +/- 3 ml/h (7.3 +/- 1.8 ml/h per kg) during the 12-h study (302 +/- 87 ml/day). Propagated esophageal EMG activity, representing coordinated 'swallows', averaged 56 +/- 6 swallows/h and correlated well with net esophageal fluid flow. 'Bouts' of swallowing activity (> or = 3 swallows/min) averaged 9 +/- 1 swallows/bout, lasted 1.8 +/- 1.4 min and accounted for 31 +/- 4% of the swallowed volume. Despite the absence of fetal electrocortical high-voltage/low-voltage transitions, there was a 26.1 +/- 3.9-min interval between periods of swallowing bout activity. CONCLUSIONS: Preterm (0.75 gestation) ovine fetal volume swallowed (302 ml/day) and volume swallowed for body weight (175 ml/day per kg) was significantly less than that previously noted at 0.85 gestation (831 ml/day, 274 ml/day per kg, respectively; p < 0.05) although the rates of swallowing activity were similar. The presence of swallowing bout activity at periodic intervals, in the absence of electrocortical differentiation, suggests an intrinsic central pacemaker regulating preterm fetal neurobehavior.     

Ovine fetal breathing and swallowing activity in response to plasma glucose changes.

Fetal swallowing activity generally occurs simultaneously with fetal breathing movements (FBM) in sheep. The present study investigated the FBM and swallowing responses to altered fetal plasma glucose. Fetal lambs were chronically prepared with laryngeal, esophageal and diaphragm electromyogram (EMG) wires, an esophageal flow probe and vascular catheters. Beginning at 138 +/- 1 day, FBM and swallowing were monitored during control periods and in response to intravenous glucose infusions (14 mg/kg/min for 120 min) to fetuses of fed and fasted ewes. Glucose infusions to fetuses of fed ewes resulted in significant increases in fetal plasma glucose (21.2 +/- 0.7 to 40.5 +/- 1.9 mg/dl) and time breathing (46.2 +/- 6.3 to 60.0 +/- 9.5 min/2 h). In response to maternal fasting, fetal glucose levels (13.4 +/- 1.0 mg/dl) and time breathing (23.0 +/- 7.2 min/2 h) decreased significantly. Glucose infusion to fetuses of fasted ewes resulted in significant increases in time breathing (50.3 +/- 13.4 min/2 h) and diaphragmatic EMG activity (1,295 +/- 654 to 3,012 +/- 1,182 spikes/2 h). There was no change from basal levels of fetal EMG swallows (83.2 +/- 4.3 swallows/2 h) or esophageal flow (40.8 +/- 7.9 ml/2 h) in response to maternal fasting or fetal glucose infusions.     

Fetal vascular responses to prostacyclin

Prostacyclin is a potent vasodilator produced by both maternal and fetal tissues that dilates the umbilical placental vasculature in vitro. To test the hypothesis that prostacyclin dilates the fetal placental circulation in vivo, we measured blood flow by the radioactive microsphere technique in six unanesthetized near-term ovine fetuses before and during prostacyclin infusion. Fetal mean arterial pressure fell 15% from 35 +/- 3 to 31 +/- 3 mm Hg (p less than 0.05) during prostacyclin infusion, and heart rate increased from 182 +/- 6 to 208 +/- 19 beats/min (p less than 0.05). Placental blood flow changed from 240 +/- 58 to 191 +/- 46 ml.min-1.kg-1 fetal weight (p = 0.07), whereas vascular resistance was unchanged (0.16 +/- 0.04 to 0.18 +/- 0.06 mm Hg.ml-1.min.kg fetal weight). Fetal arterial pH decreased from 7.33 +/- 0.03 to 7.28 +/- 0.02 (p less than 0.05) during prostacyclin infusion, with a significant decrease in base excess from -1.2 +/- 1.4 to -3.1 +/- 1.6 (p less than 0.05) and a trend toward hypercarbia (p = 0.07). We conclude that in vivo administration of prostacyclin to the ovine fetus does not cause fetal placental vasodilation and does cause a significant fetal acidemia. The mechanism for these unexpected observations is likely shunting of blood away from the placenta to other organs in the face of systemic vasodilation.     

Nitric oxide modulates angiotensin II-induced drinking behavior in the near-term ovine fetus

OBJECTIVE: Human and ovine fetuses demonstrate an enhanced rate of spontaneous and angiotensin II-stimulated swallowing. Angiotensin II and nitric oxide synthase have been localized to thirst centers in the brain. This study was performed to determine whether central nitric oxide contributes to the regulation of angiotensin II-induced fetal swallowing. STUDY DESIGN: Six pregnant ewes with near-term singleton fetuses were chronically prepared with fetal vascular and lateral ventricle catheters and electrocorticogram and esophageal electromyogram electrodes. After a 2-hour control period, fetuses were administered serial lateral ventricle injections (1 mL) of angiotensin II (3.2 microg; time, 2 hours) and N omega-nitro-L -arginine methyl ester (3 mg; time, 3 hours) and a repeat angiotensin II injection (3.2 microg; time, 5 hours). All fetuses received an additional control study of lateral ventricle injections of artificial cerebrospinal fluid on a previous day. RESULTS: Angiotensin II injection significantly increased mean +/- SEM fetal swallowing (0.9 +/- 0.1 to 2.7 +/- 0.4 swallows/min). N omega-nitro-L -arginine methyl ester significantly decreased fetal swallowing to below the basal rate (0.4 +/- 0.1 swallows/min), and swallowing did not increase with the second angiotensin II dose (in the presence of nitric oxide blockade). CONCLUSIONS: These results demonstrate that inhibition of central nitric oxide suppresses fetal swallowing behavior in response to central angiotensin II. We speculate that tonic nitric oxide facilitates angiotensin II swallowing stimulation by maintenance of glutamate activation of hypothalamic N -methyl-D -aspartate receptors.     

Urinary and cardiovascular responses to indomethacin infusion in the ovine fetus.

OBJECTIVE: Our objective was to explore the urinary and cardiovascular responses of the near-term ovine fetus to plasma indomethacin levels similar to those in the human neonate undergoing indomethacin therapy. STUDY DESIGN: Chronically catheterized ovine fetuses between 125 and 139 days of gestation were studied. After a 1-hour control period we gave a bolus of 0.35 mg/kg estimated fetal weight of indomethacin into a fetal vein, followed by a 0.017 mg/kg/min continuous infusion for 5 hours (n = 9). Results were compared with a vehicle-infusion-only group (n = 10). RESULTS: During the first 3 hours of indomethacin infusion, fetal urinary output was increased by an average of 84.9% +/- 55.6% (analysis of variance, p less than 0.01). Urinary osmolality and sodium and chloride concentrations underwent sustained increases throughout the infusion period (p less than 0.001). Sodium excretion increased by 212% +/- 111% (p less than 0.05). Fetal arterial and venous pressures increased (p less than 0.001), and the change in urinary flow correlated positively with the change in arterial pressure (R = 0.55, p = 0.014). Fetal heart rate increased by 10% +/- 4% 1 hour after the bolus and remained elevated throughout the remainder of the infusion relative to vehicle-infused animals (p less than 0.001). Vehicle infusion had no effect on any fetal variable. CONCLUSIONS: This study does not support the hypothesis that indomethacin acutely reduces urinary flow rate in the late-gestation ovine fetus. Further, the observed urinary flow increases may be mediated in part by a pressure diuresis.     

Increased urinary flow without development of polyhydramnios in response to prolonged hypoxia in the ovine fetus

OBJECTIVE: In the ovine fetus subjected to 24 hours of hypoxia, urinary flow is normal within a few hours from the onset of hypoxia and there is a maintained inhibition of swallowing. We hypothesized that 4 days of fetal hypoxia would lead to polyhydramnios. STUDY DESIGN: Five late-gestation fetal sheep were subjected to hypoxia for 4 days and 7 other late-gestation fetal sheep served as time control animals. Fetal hypoxia was produced on postsurgical days 5 through 9 by continuous intratracheal nitrogen insufflation to the ewe. On days 3, 5, 7, and 9 after surgery, amniotic fluid volume, fetal urinary flow rate, and the compositions of maternal and fetal blood, amniotic fluid, and fetal urine were measured. A 3-factor analysis of variance was used for statistical analysis. RESULTS: During the period of experimental hypoxia the mean (+/-SE) fetal PaO(2) was 16.0 +/- 0.6 mm Hg, versus 21.2 +/- 0.7 mm Hg in control sheep (P <.001). Fetal hypoxia was associated with increased urinary flow on days 7 and 9, averaging 1410 +/- 310 and 2101 +/- 345 mL/d, respectively, versus 585 +/- 92 and 699 +/- 78 mL/d, respectively, in control animals (P <.001). Amniotic fluid volume was unchanged with time and averaged 960 +/- 159 mL in hypoxic fetuses on postsurgical days 7 through 9 and 851 +/- 130 mL in control animals (P =.60). Fetal blood lactate increased in the hypoxic animals, averaging 3.4 +/- 2.1 mmol/L versus 1.6 +/- 0.3 mmol/L in control animals (P =.02). Fetal urinary excretions of sodium, potassium, chloride, and lactate increased significantly during hypoxia, by 170% to 400%. CONCLUSION: Four days of nitrogen-induced hypoxia in the ovine fetus resulted in excess fetal urinary flow approximating 1000 mL/d greater than normal without the development of polyhydramnios. Because amniotic fluid volume did not change and hypoxia is a known inhibitor of fetal swallowing, we speculate that intramembranous absorption of amniotic water, electrolytes, and lactate increased.     

Ovine fetal lung fluid response to intravenous saline solution infusion: fetal atrial natriuretic factor effect

The fetal lung, a significant source of in utero fluid production, has been postulated to serve a regulatory role in maintenance of fetal body fluid homeostasis. Whereas the fetus responds to intravascular saline solution infusions with increased urine output, the fetal lung fluid response to this stimulus is unclear. Tracheal fluid output was measured in four chronically catheterized ovine fetuses (mean gestation, 129 +/- 1 days) subjected to successive 40-minute intravenous 0.9% saline solution infusions at rates of 0.5 and 1 ml/min/per kilogram of body weight. Tracheal fluid output decreased significantly (1.7 +/- 0.1 to 1.1 +/- 0.1 ml/10 min, p less than 0.01) during the infusion and returned to basal levels during the recovery period. Lung fluid osmolality and electrolyte concentration did not change. Fetal plasma atrial natriuretic factor increased significantly in response to the saline solution infusion (364 +/- 90 to 790 +/- 286 pg/ml, p less than 0.05) and returned to basal levels during the recovery period. There was a significant inverse correlation between plasma atrial natriuretic factor levels and tracheal fluid output. These results suggest that increased fetal plasma atrial natriuretic factor decreases lung fluid production. Lung fluid does not appear to compensate for fetal body water excess. Rather, lung fluid production appears to promote intrauterine pulmonary growth and to facilitate the transition to the extrauterine environment.     

Maternal and fetal metabolic effects of prolonged ritodrine infusion

To investigate metabolic and endocrine changes in the fetus during prolonged maternal tocolysis with beta sympathomimetic drugs, ritodrine hydrochloride (2.1 micrograms/kg per minute) was infused into pregnant sheep near term. Confirming earlier studies, maternal plasma metabolite and hormone levels changed greatly during the first six to eight hours of infusion. Changes in the fetus paralleled these closely: glucose, lactate, and insulin increased sharply, but glucagon and alpha-amino acid nitrogen decreased. After this, most maternal and fetal plasma metabolite and hormone levels returned to the normal range and were unchanged by infusion for 72 to 96 hours. Fetal lactate levels, however, remained elevated. Similar changes occurred during interrupted maternal infusions of ritodrine. Prolonged infusion of ritodrine leads to diminished responsiveness in beta-adrenergic mechanisms regulating maternal plasma metabolite and hormone levels. Comparable unresponsiveness of fetal beta-adrenergic mechanisms, though less certain, could increase hazards during delivery and adaptation to postnatal life.     

Transplacental, amniotic, urinary, and fetal fluid dynamics during very-large-volume fetal intravenous infusions

With rapid intravenous infusion of very large volumes of isotonic saline solutions into the fetus, the fluid could stay within the fetal body, thereby creating hydrops fetalis, be transferred into the amniotic fluid through the fetal kidneys, thereby creating polyhydramnios, or be transferred across the placenta into the maternal circulation. This study was designed to explore these possibilities. After a 1-hour control period, 10 near-term chronically catheterized ovine fetuses were infused intravenously with 4 L (greater than 100% of fetal weight) of either isotonic saline solution or lactated Ringer's solution over 4 hours. Fetal arterial pressure was significantly elevated by 7 mm Hg throughout the infusion (p less than 0.00001). Venous pressure underwent a transient rise (4.8 mm Hg) at 20 minutes of infusion and remained elevated (2.7 mm Hg) during the rest of the infusion (p less than 0.00001). Fetal urine flow increased by an average of 5.7 +/- 0.4 ml/min throughout the infusion (p less than 0.00001) and accounted for 34.1% +/- 2.6% of the infused volume. Estimated fetal extracellular fluid volume increased by 17.7% +/- 1.8% of the infused volume. Because fetal fluid retention, urine flow, and amniotic fluid volume changes accounted for only half of the infused fluid, the remainder of the infused volume must have crossed the placenta and entered the maternal circulation. Given the above changes in vascular pressures, this requires a filtration coefficient of the placenta 50 to 100 times the previously reported values. Thus we conclude that relatively small changes in fetal vascular pressures dramatically alter the filtration capacity of the ovine placenta and transplacental volume flow.     

Indomethacin-induced urinary flow rate reduction in the ovine fetus is associated with reduced free water clearance and elevated plasma arginine vasopressin levels.

OBJECTIVE: The purpose of our study was to explore the urinary responses of the ovine fetus to indomethacin levels comparable with those used therapeutically in the human fetus. STUDY DESIGN: After a 1-hour control period, chronically catheterized ovine fetuses between 125 and 139 days of gestation were given an intravenous bolus of indomethacin (0.05 mg/kg estimated fetal weight) followed by a 0.0025 mg/kg/min continuous infusion for 5 hours. The experimental group (n = 9) was compared with a vehicle-only infusion group (n = 10). RESULTS: There was a sustained 55.7% +/- 9.5% (mean +/- SEM) decrease in urinary output by 2 hours of indomethacin infusion (p < 0.00001, analysis of variance). Urinary osmolality, potassium, and chloride concentrations underwent sustained increases during the infusion period (p < 0.005). Free water clearance decreased by 67.5% +/- 12.0% (p < 0.001). Fetal arterial pressure increased only transiently (p < 0.05), and increases in venous pressure (p = 0.013) and heart rate (p < 0.0001) were sustained. Fetal plasma arginine vasopressin concentration increased during indomethacin infusion (p < 0.05) and was correlated with the fall in urinary flow rate and free water clearance (p = 0.002). During vehicle infusion no significant changes were observed in any of the variables. CONCLUSIONS: Our data indicate that the fetus undergoes antidiuresis when exposed to low levels of indomethacin and that the observed antidiuresis is mediated by a decrease in free water clearance. The reduction in free water clearance may be mediated by increases in plasma arginine vasopressin concentrations.     

Fetal blood volume, urine flow, swallowing, and amniotic fluid volume responses to long-term intravascular infusions of saline

The purpose of this study was to determine the effects of a long-term infusion into the fetal circulation on fetal and amniotic fluid dynamics. In 10 chronically catheterized fetal sheep averaging 130 +/- 1 (SE) days' gestation, a balanced, isotonic electrolyte solution (Isolyte S) was infused continuously for 5 days into a fetal vein at a rate of 0, 1, 2, 4, and 0 L/day, respectively. During the infusion, fetal blood volume increased by a maximum of 6.4 +/- 2.0% (p less than 0.001), and the daily swallowing of amniotic fluid doubled (p less than 0.001). Fetal urine flow increased (p less than 0.0001) above preinfusion rates by a volume equal to the infusion rate plus the increase in swallowing, whereas renal excretion of sodium and chloride increased by the amount infused. The increase in the plasma concentration of atrial natriuretic factor (p less than 0.0001) and the decrease in arginine vasopressin (p less than 0.05) were not linearly related to urine flow changes. Amniotic fluid volume increased (p less than 0.0001) by 20% of the infused volume. All values returned to normal on day 5 except amniotic fluid volume, which remained elevated. Estimated allantoic fluid volume at the end of day 5 was 800 ml above normal. Thus it appears that on a long-term basis, the ovine fetus eliminates infused water and electrolytes through its kidneys rather than across the placenta. Although all of the infused volume left the fetus through its kidneys, only 30% of the infused volume remained in the amniotic and allantoic fluid compartments, suggesting transfer to the mother by unknown mechanisms.     

Placental transfer of ritodrine hydrochloride in sheep

The purpose of this study was to examine the placental passage of ritodrine hydrochloride in relation to the drug's effects on the fetal circulation. Studies were carried out on nine nulliparous pregnant (120-140 days) ewes with chronically implanted cannulae for measurements of maternal and fetal arterial pressures and for blood sampling. One group of animals received sequential infusions of doses ranging from 0.1 to 30 micrograms/kg per min for 30 min (group 1). A second group was given a constant infusion of the drug at a dose of 3.0 micrograms/kg per min for 4 h (group 2). The peak concentrations of ritodrine in maternal and fetal blood were determined by radioimmunoassay. In group 1 they were 313.4 +/- 24.1 ng/ml (mean +/- S.E.) and 12.6 +/- 3.7 ng/ml at the finish of 30.0 micrograms/kg per min infusion for maternal and fetal blood, respectively. In group 2, maternal drug levels were 81.3 +/- 20.4 ng/ml after 30 min and 95.9 +/- 17.1 ng/ml after 4 h of the infusion. Fetal plasma concentrations increased slowly from trace levels at 30 min to 3.3 +/- 0.7 ng/ml at 4 h. Fetal blood pressure and heart rate did not show any significant changes during and after the infusion of ritodrine in both treatment groups. Our findings demonstrate the maternal administration of ritodrine produces no significant effects on the circulatory system of the fetal lamb because of the low transplacental passage of this drug.     

Antibody-induced anemia in fetal sheep: model for hemolytic disease of the fetus and newborn

OBJECTIVE: Our purpose was to produce a condition analogous to alloimmune hemolytic disease of the fetus and newborn by infusing antierythrocyte antibodies in fetal sheep. METHODS: Antierythrocyte antibodies were infused intravascularly into late-gestation ovine fetuses over a 10-day period. Fetal blood was sampled daily for complete blood cell counts, blood gases, iron, erythropoietin (EPO), and electrolyte concentrations. Red cell mass (RCM) and blood volume were determined every other day using indicator dilution techniques. Results were compared with eight similarly aged control animals. Statistical analysis included Student t test, three-factor analysis of variance, and least squares regression. RESULTS: The hematocrit in seven fetal sheep receiving antibody infusion declined significantly by 10.3 +/- 1.7%, whereas it increased in control animals 2.3 +/- 0.6% (P <.001). RCM was reduced by 18.9 +/- 3.2% over the 10-day protocol while increasing 34.1 +/- 4.2% in control animals, representing more than a 50% difference in RCM (P <.001). Fetal EPO was significantly increased with lower hematocrit and lower PO(2) (P <.001). As fetal hematocrit declined below 25%, lactate and reticulocytes also increased (P <.001). Plasma iron concentration was not significantly altered (P =.47). CONCLUSIONS: The chronically catheterized fetal sheep is a viable model for studying immunologically induced fetal anemia as hematocrit can be titrated and the fall in RCM and hematocrit are associated with fetal hypoxia and elevated EPO as occurs in the anemic human fetus. Furthermore, there appears to be a threshold degree of anemia required to elicit responses as the fetal EPO, PO(2), and lactate appeared unresponsive until hematocrit fell below 25%.     

Rapid intramembranous absorption of water infused into the ovine allantoic cavity.

OBJECTIVE: Previously we found that water infused into the ovine amniotic cavity was rapidly absorbed into the fetal circulation through the vascularized fetal membranes and fetal surface of the placenta (i.e., the intramembranous pathway). The purposes of this study were to (1) estimate the conductance of the intramembranous pathway from the allantoic cavity and (2) determine if the conductance is adequate to offset the inflow of urine, which may be up to 500 ml/day in the near-term ovine fetus. STUDY DESIGN: Seven chronically catheterized fetal sheep averaging 132 +/- 2 (+/- SE) days' gestation underwent an infusion of warmed distilled water into the allantoic cavity at 6 ml/min. The infusions were continued until steady states were obtained in allantoic and amniotic fluid and in fetal and maternal blood osmolalities. During the steady state the conductance of the intramembranous pathway was estimated as the ratio of osmotic gradient to infusion rate. RESULTS: The allantoic and amniotic fluid and the fetal and maternal blood osmolalities decreased by 188 +/- 14, 36 +/- 8, 13 +/- 2, and 3 +/- 1 mOsm/kg, respectively, at steady state. From the fetal-allantoic osmolality gradients the conductance of the intramembranous pathway was 1.72 +/- 0.14 or 0.53 +/- 0.08 microliter/min/mm Hg/kg fetal weight. Assuming a similar conductance during the preinfusion period, the next volume movement would equal 0.67 ml/min (965 ml/day). CONCLUSIONS: The conductance of the intramembranous pathway in combination with the normal osmotic gradient is sufficient to remove the large volume of fetal urine that may enter the allantoic cavity each day.     

The effect of insulin on ovine fetal oxygen extraction

Infusion of exogenous insulin (54 +/- 19 mU/kg/hr) to seven fetal lambs caused hyperinsulinism and arterial hypoxemia but not hypoglycemia. We measured the relationship between fetal oxygen delivery and oxygen use for a better understanding of the cause of the observed hypoxemia. Oxygen delivered to the fetus is the product of fetal umbilical venous oxygen content and umbilical blood flow. Both of these quantities decreased as fetal insulin concentration rose. The fall in umbilical blood flow was due to a change in the distribution of cardiac output. Cardiac output rose, but placental perfusion decreased while blood flow to the fetal carcass increased. Oxygen consumption by the ovine fetus increased as insulin concentration rose. Since the delivery of oxygen to the fetus did not increase when its use was rising, fetal extraction of available oxygen increased. Fetal arterial hypoxemia is the result of this increased extraction of available oxygen.