Oxygen consumption is maintained in fetal sheep during prolonged hypoxaemia.

Experiments were conducted in 12 chronically-catheterized pregnant sheep to examine the effect of prolonged hypoxaemia secondary to the restriction of uterine blood flow on fetal oxygen consumption. Surgery was performed at 115 days gestation to place a teflon vascular occluder around the maternal common internal iliac artery and for insertion of vascular catheters. Following a 5-day recovery period, uterine blood flow was reduced in 6 animals for 24 hours and in 6 animals, the occluder was not adjusted. Fetal arterial PO2 decreased from 19.9 +/- 2.0 mmHg to 12.8 +/- 2.0 mmHg and 11.0 +/- 2.0 mmHg at 1 and 24 hours respectively in the experimental group and did not change the control group. Fetal pH decreased from 7.34 +/- 0.01 to 7.25 +/- 0.03 and 7.29 +/- 0.02 at 1 and 24 hours of hypoxaemia respectively. Fetal arterial lactate concentrations remained elevated throughout the experimental period with maximum concentrations of 6.6 +/- 2.1 mmol/l being present at 4 hours compared to 1.3 +/- 0.2 mmol/l during the control period. Umbilical blood flow increased from 186 +/- 19 ml/min/kg to 251 +/- 39 ml/min/kg at 1 h of hypoxaemia and returned to 191 +/- 21 ml/min/kg at 24 h. In association with the progressive fall in oxygen delivery to the fetus, oxygen extraction increased from 0.33 +/- 0.04 to 0.43 +/- 0.04 and 0.54 +/- 0.05 at 1 and 24 hours, respectively. Overall oxygen consumption by the fetus remained unchanged from control values.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in lung liquid dynamics induced by prolonged fetal hypoxemia

Our aim was to determine the effect of prolonged fetal hypoxemia, induced by reduced maternal uterine blood flow (RUBF), on fetal lung liquid secretion, flow, and volume. In chronically catheterized fetal sheep, lung liquid volume (VL) and the secretion rate of lung liquid (Vs) were measured before and after a 24-h period of either RUBF or normoxemia. Tracheal fluid flow and the incidence of fetal breathing movements (FBM) were measured before, during, and after the 24-h period. In normoxic control fetuses Vs was not significantly altered. After 24 h of RUBF, Vs was significantly (P less than 0.005) reduced compared with pre-RUBF values. During 24 h of RUBF the incidence of FBM declined initially but returned to control values after 12-16 h. In seven of eight fetuses, over the 12- to 24-h period of RUBF, large amounts of liquid (22.7-62.6 ml) were drawn into the lungs during FBM, resulting in a net movement of amniotic fluid into the lungs. During the 18- to 24-h period of RUBF, changes in the incidence of FBM were found to be significantly and positively correlated (r = 0.86, P less than 0.005) with the changes in VL that occurred over the 24-h period. Thus, prolonged RUBF can result in the inhalation of large volumes of amniotic fluid by the fetus, which could be a cause of in utero meconium aspiration.     

Renal sympathetic nerve activity during asphyxia in fetal sheep

The sympathetic nervous system (SNS) is an important mediator of fetal adaptation to life-threatening in utero challenges, such as asphyxia. Although the SNS is active well before term, SNS responses mature significantly over the last third of gestation, and its functional contribution to adaptation to asphyxia over this critical period of life remains unclear. Therefore, we examined the hypotheses that increased renal sympathetic nerve activity (RSNA) is the primary mediator of decreased renal vascular conductance (RVC) during complete umbilical cord occlusion in preterm fetal sheep (101 ± 1 days; term 147 days) and that near-term fetuses (119 ± 0 days) would have a more rapid initial vasomotor response, with a greater increase in RSNA. Causality of the relationship of RSNA and RVC was investigated using surgical (preterm) and chemical (near-term) denervation. All fetal sheep showed a significant increase in RSNA with occlusion, which was more sustained but not significantly greater near-term. The initial fall in RVC was more rapid in near-term than preterm fetal sheep and preceded the large increase in RSNA. These data suggest that although RSNA can increase as early as 0.7 gestation, it is not the primary determinant of RVC. This finding was supported by denervation studies. Interestingly, chemical denervation in near-term fetal sheep was associated with an initial fall in blood pressure, suggesting that by 0.8 gestation sympathetic innervation of nonrenal vascular beds is critical to maintain arterial blood pressure during the rapid initial adaptation to asphyxia.     

The effects of hypoxemia with progressive acidemia on fetal renal function in sheep

In order to determine the effects of fetal hypoxemia on renal blood flow, glomerular filtration rate (GFR) and urethral and urachal urine output, we examined the effects of 3 h maternally-induced (9% O2, 3% CO2, 88% N2) fetal hypoxaemia on 10 chronically-instrumented fetal sheep between 127-135 days of gestation. Fetal arterial pH fell significantly during the second and third hours of hypoxia and this coincided with a significant increase in fetal arterial blood pressure (P less than 0.05). During the second hour of hypoxia, with mild acidaemia, fetal GFR decreased significantly and then, during the third hour, fetal GFR, urethral and total urine output were significantly elevated. During the 2-h recovery period urachal and total, but not urethral urine output, were significantly elevated (P less than 0.05). The data suggested that the increase in GFR and urine output measured during the third hour of hypoxia and the recovery period may reflect a pressure diuresis.     

Acute anemic hypoxemia produces a transient depression in fetal respiratory activity

Isovolemic anemia was produced in 11 unanesthetized fetal sheep by withdrawal of blood and replacement with saline-dextran. Fetal hematocrit fell from 36 +/- 1 to 19 +/- 1% (SE). Fetal breathing movements, which were present during 34.4 +/- 5.5% of 3 h before the anemia, occurred 10.1 +/- 5.3, 14.8 +/- 4.4, and 27.1 +/- 6.7% in the 3 h following. The anemia caused a fall in arterial O2 concentration from 8.4 +/- 0.3 to 3.6 +/- 0.1 vol% and sagittal vein PO2 fell from 15.4 +/- 0.5 to 12.4 +/- 0.3 Torr. Cerebral metabolic rate during the period of anemia was 2.9 +/- 0.1 ml.100 g-1.min-1, which was unchanged from the control value of 3.0 +/- 0.2 ml.100 g-1.min-1. Sagittal vein PCO2 (54.2 +/- 1.4 Torr) remained constant after the fetus was made anemic. We conclude that respiratory activity in the sheep fetus is depressed by anemic hypoxemia but that the effect is transient.     

Maturation of spontaneous fetal diaphragmatic activity and fetal response to hypercapnia and hypoxemia

The electromyogram (EMG) of the diaphragm, lateral rectus, and nuchal and hindlimb muscles were studied during spontaneous activity and during hypercapnia or hypoxemia in eight fetal sheep from 0.5 to 0.8 gestation (73-128 days). At the earliest gestational age, diaphragmatic EMG activity was mainly tonic and associated with tonic activity of somatic muscles. The stimulus for the diaphragmatic activity originated centrally. Brief periods of a rapid-eye-movement (REM) state characterized by phasic lateral rectus and diaphragmatic activity and absence of nuchal activity were recognized. Furthermore, from 0.5 to 0.7 gestation onward, activity of all muscles increased. Thereafter increased specificity of activity in relation to the apparent REM and non-rapid-eye-movement (NREM) state occurred. With maturation, phasic diaphragmatic activity increased at the expense of tonic activity. The most striking effect of maturation on apnea was a greater proportion of apnea lasting greater than 1 min, but the total duration of apnea as a percent of a total recording remained unchanged. The quantitative response to hypercapnia during maturation was independent of the pattern of spontaneous diaphragmatic activity. Hypercapnia at 0.5 gestation changed the pattern of diaphragmatic EMG activity from mainly tonic to phasic. Thus the central chemoreceptors and appropriate neuronal pathways are present and functional as early as 0.5 gestation. Hypercapnia at 0.5 gestation caused a shift in diaphragmatic EMG power to lower frequencies similar to that found during control conditions in the older fetus. This might suggest that during maturation there is increased recruitment of phrenic motoneurons. Hypoxemia abolished tonic somatic activity at 0.5 gestation and decreased phasic diaphragmatic activity at more advanced gestational ages. Therefore the central inhibitory mechanisms of hypoxemia are developed by 0.5 gestation.     

Changes to metabolite concentration in fetal sheep subjected to prolonged hypobaric hypoxia

The effect of hypobaric hypoxaemia on the concentration of metabolic substrates in the ovine fetus and pregnant ewe with implanted vascular catheters, was investigated. At 120 to 141 days of gestation sheep were subjected to hypobaria (mean fetal carotid PO2 12.7 +/- 0.7 torr; n = 9) or normobaria (mean fetal carotid PO2 22.7 +/- 0.7 torr; n = 11; P less than 0.001). At 141 days gestation mean fetal weight was 3.46 +/- 0.72 kg in the hypobaric group compared to 4.15 +/- 0.51 in the normobaric group (P less than 0.05). Concentrations of glucose in maternal and fetal plasma and fructose in fetal plasma were similar in hypobaric and normobaric fetuses. The concentration of lactate in fetal plasma rose from 1.68 +/- 1.34 to 8.79 +/- 5.8 mmol/l (P less than 0.001) within 24 h of onset of hypoxia, but fell to 3.36 +/- 1.13 mmol/l by day 3 of treatment, though still significantly above the concentration of lactate in the control fetuses (1.47 +/- 0.47; P less than 0.001). There was no significant effect of hypoxia on the concentration of lactate or alanine in maternal plasma. Alanine concentration in the plasma of fetuses subjected to hypoxia significantly increased within 24 h of exposure (0.28 +/- 0.10 vs 0.58 +/- 0.39 mmol/l; P less than 0.01) and remained elevated for the duration of the study. There was no significant effect of gestational age on the concentration of metabolic substrates in either the control or experimental groups. Hypoxia is associated with a sustained rise in the concentration of plasma lactate and alanine in the fetus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of NO, phenylephrine, and hypoxemia on ductus venosus diameter in fetal sheep

To study the regulation of the ductus venosus (DV) inlet in vivo, we measured the effect of vasoactive substances and hypoxemia on its diameter in nine fetal sheep in utero at 0.9 gestation under ketamine-diazepam anesthesia. Catheters were inserted into an umbilical vein and a fetal common carotid artery, and a flowmeter was placed around the umbilical veins. Ultrasound measurements of the diameter of the fetal DV during normoxic baseline conditions [fetal arterial PO(2) (PaO(2)) 24 mmHg] were compared with measurements during infusion of sodium nitroprusside (SNP; 1.3, 2.6, and 6.5 microg. kg(-1). min(-1)) or the alpha(1)-adrenergic agonist phenylephrine (6.5 microg. kg(-1). min(-1)) into the umbilical vein or during hypoxemia (fetal Pa(O(2)) reduced to 10 mmHg). SNP increased the DV inlet diameter by 23%, but phenylephrine had no effect. Hypoxemia caused a 61% increase of the inlet diameter and a distension of the entire vessel. We conclude that the DV inlet is tonically constricted, because nitric oxide dilates it but an alpha(1)-adrenergic agonist does not potentiate constriction. Hypoxemia causes a marked distension of the entire DV.     

Genioglossus and alae nasi activity in fetal sheep

The functional development of two upper airway dilating muscles, the alae nasi and the genioglossus, has been studied in fetal sheep in utero from 112-140 days gestation. Before electrocortical differentiation phasic activity was present in both muscles for long periods, mostly when breathing movements were present. After 120 days gestation phasic genioglossal and alae nasi activity occurred only during periods of low voltage electrocortical activity. During high voltage episodes there was no phasic activity and tonic activity was not sustained. Although present during periods of breathing movements genioglossus activity was rarely synchronous with the diaphragm. The alae nasi showed both respiratory and non-respiratory related activity. Hypoxia abolished both alae nasi and genioglossus activity but whereas alae nasi rapidly developed an inspiratory rhythm during 5% CO2 administration this was not the case with the genioglossus and inspiratory activity was not always seen in the genioglossus even during 10% CO2 administration. It is concluded that there are fundamental differences between the control of genioglossus and alae nasi activity in the fetal sheep. The alae nasi behaves as an inspiratory muscle responding to hypoxia and hypercapnia as would be expected but the genioglossus shows no inspiratory activity during normal unstimulated fetal breathing. Thus the neural mechanisms for activation of inspiratory activity appear to be present late in gestation. However it is possible for the genioglossus to develop an inspiratory rhythm under conditions of much increased respiratory drive.     

Behavioral state transition and local cerebral blood flow in fetal sheep.

Local cerebral blood flow in four near term fetal sheep was evaluated continuously before and after natural alternations in fetal behavioral state. Measurements were made in fetuses several days following an aseptic surgery to place electrodes for behavioral state recordings as well as heated and reference thermojunctions in cortical and subcortical tissue. These thermojunctions were used to qualitatively assess local cerebral blood flow. The time of transition between rapid eye movement sleep (REMS) and non-rapid eye movement sleep (NREMS) was based on visual inspection of strip chart recordings of electrocortical, electroocular, and neck electromyographic activity and application of published criteria for their assessment. To confirm that transition had occurred, the amplitude of the spectrum of the electrocorticogram in one-third octave bands centered around 1 Hz and 20 Hz was measured before and after the transition point. Mean cerebral blood flow rose significantly by 24 s (P less than 0.05) after the transition from NREMS to REMS and fell by 12 s after the transition from REMS to NREMS (P less than 0.05).     

The effect of prolonged hypobaric hypoxia on growth of fetal sheep

The effect of prolonged hypobaric hypoxia on fetal sheep was studied. Pregnant ewes were subjected to an atmospheric pressure of 429 torr from 30 days to 135 days gestation (long-term study). Average fetal weight for the hypoxaemic group (3.35 +/- 0.53 kg; n = 4; mean +/- SD) was significantly lower than for the controls (4.23 +/- 0.29 kg; n = 7; P less than 0.05). A short-term study was undertaken with fetuses (n = 8) which were catheterized at 110 days gestation and whose dams were subjected to hypobaric hypoxia from 120 to 141 days gestation. The mean carotid PO2 of fetuses in the hypoxic group was 12.7 +/- 0.7 torr compared to 22.7 +/- 0.7 torr for the control group (n = 9; P less than 0.001) throughout the period of treatment. Fetal arterial oxygen content fell from 6.5 +/- 1.7 to 4.9 +/- 0.4 ml/dl (P less than 0.05), but rose to control values after 7 days due to an increase in fetal haemoglobin concentration (9.6 +/- 1.1 to 13.0 +/- 1.9 g/dl, P less than 0.001) and packed cell volume (33 +/- 3 to 45 +/- 4%, P less than 0.001). In the hypoxaemic fetuses, pH fell initially from 7.34 +/- 0.02 to 7.28 +/- 0.03 (P less than 0.05) and then recovered to 7.32 +/- 0.03 within 24 h. Mean fetal weight of the short-term hypoxic group was 3.46 +/- 0.72 kg compared to 4.15 +/- 0.51 for the control group (P less than 0.05). Both long- and short-term hypoxia produced a similar reduction in fetal body weight. The adrenal glands were significantly heavier in the hypoxic fetuses than in controls. Placental weight was not effected by hypoxia, but exposure from 30 days gestation reduced the average size of cotyledons (P less than 0.05). It is concluded that the fetal sheep increases its ability to acquire and transport oxygen in response to chronic hypoxia, but this compensation is not sufficient to prevent growth retardation or changes to the pattern of tissue growth.     

Purinergic contribution to circulatory, metabolic, and adrenergic responses to acute hypoxemia in fetal sheep

This study investigated the effects on femoral vascular resistance, blood glucose and lactate levels, and plasma catecholamine concentrations of fetal treatment with an adenosine receptor antagonist during acute hypoxemia in fetal sheep during late gestation. Under anesthesia, seven fetal sheep were instrumented between 117 and 118 days gestation (term is approximately 145 days) with vascular and amniotic catheters and an ultrasonic probe around a femoral artery. Six days after surgery, all fetuses were randomly subjected to a 3-h experiment consisting of 1 h of normoxia, 1 h of hypoxemia, and 1 h of recovery. This was done during either intravenous infusion of vehicle or the adenosine receptor antagonist [8-(p-sulfophenyl)-theophylline; 8-SPT] dissolved in vehicle. During vehicle infusion, all fetuses responded to hypoxemia with bradycardia, an increase in arterial blood pressure, and femoral vasoconstriction. Increases in blood glucose and lactate concentrations and in plasma epinephrine and norepinephrine concentrations also occurred in all fetuses during hypoxemia. Fetal treatment with 8-SPT markedly attenuated the bradycardic, hypertensive, vasoconstrictor, glycemic, and adrenergic responses to hypoxemia, but it did not affect the increase in blood lactate concentrations during hypoxemia. These data show that adenosine is involved in the mechanisms mediating fetal cardiovascular, metabolic, and adrenergic responses to hypoxemia in fetal sheep. Fetal treatment with 8-SPT mimics the effects of carotid sinus nerve section on fetal cardiovascular function during hypoxemia, suggesting a role for adenosine in mediating fetal cardiovascular chemoreflexes.     

Thymocyte subpopulations during early fetal development in sheep

Phenotypic analysis of thymocytes during fetal development may identify subpopulations which are either absent or difficult to detect in postnatal thymus. A panel of monoclonal antibodies specific for sheep lymphocyte antigens (SBU-T1, -T4, -T8, -T6) was used to identify thymocyte subpopulations in postnatal and fetal sheep. Thymuses were analyzed by two-color immunofluorescence and flow cytometry or by immunohistology. Two-color immunofluorescent staining of postnatal sheep thymus with anti-SBU-T4 and anti-SBU-T8 revealed four relatively distinct subpopulations with particular localizations: a) SBU-T4-T8-, predominantly outer cortex (12%); b) SBU-T4+T8+, inner cortex (74%); c) SBU-T4+T8-, medulla (10%), and d) SBU-T4-T8+, medulla (4%). One- and two-color immunofluorescent analysis of cells from early fetal thymuses demonstrated the appearance of SBU-T8+ cells well before SBU-T4+ cells. Immunohistologic staining of fetal sheep thymus at various stages of gestation (term = 150 days) revealed that lymphoid cells and MHC class II-positive dendritic cells first appeared at 35 days, at which stage the thymic epithelium was weakly positive for class I MHC antigens but negative for class II MHC antigens. The earliest lymphocyte antigens detectable on fetal sheep thymocytes were SBU-LCA and SBU-T1. By 40 days, the antigens SBU-T6, SBU-T4, and SBU-T8 were detectable on a small number of thymocytes; SBU-T8 preceded SBU-T4, and the number of SBU-T8+ thymocytes always exceeded the number of SBU-T4+ thymocytes throughout early gestation. At 50 days, a thymic medulla appeared and thereafter grew rapidly in size. Immunoperoxidase staining of serial sections of the fetal neck revealed cortical-type thymocytes outside the thymus from 40 days onward, before the appearance of a thymic medulla. However, by 60 days, only medullary-type thymocytes were observed either extrathymically or within the interlobular septa of the thymus, indicating that only thymocytes with a medullary phenotype leave the thymus from this stage of gestation.     

Cerebral oxygen consumption during asphyxia in fetal sheep

Cerebral blood flow and cerebral arteriovenous oxygen content difference were measured in 17 fetal sheep, and cerebral oxygen uptake was calculated. The measurements were made under control conditions and after profound fetal asphyxia induced of uterine blood flow for up to 90 min. In 14 of the fetal sheep, sequential measurements were made to examine hemodynamic changes and cerebral oxygen consumption at comparable intervals up to 36 min of asphyxia. These fetuses initially had elevated blood pressure and lowered heart rate became hypoxemic, hypercarbic, and acidotic. There was an initial decrease in cerebral oxygen consumption. Sequential measurements, however, showed a relative stability in this decreased oxygenation during 4 to 36 min of asphyxia despite a progressive metabolic acidosis. The cerebral fractional oxygen extraction remained unchanged despite a mean pH of 6.98 at 36 min. The calculated cerebral oxygen uptake during asphyxia in all 17 sheep was grouped according to whether the ascending aortic oxygen content was greater or less than 1.0 mmol/l. In the first group with mean ascending aortic oxygen content of 1.3 mmol/l, blood flow to the brain was increased and cerebral oxygen consumption was 85% of control. In the second group with mean arterial blood oxygen content of 0.8 mmol/l, there was a narrowing of the arteriovenous oxygen content difference, but no further increase in cerebral blood flow. Cerebral oxygen consumption was only 48% of control in this more asphyxiated group. We conclude that the degree of hypoxemia in the second group represents a point where physiologic mechanisms cannot compensate, and may be associated with neuronal damage.     

Hemodynamic responses to alpha-adrenergic blockade during hypoxemia in the fetal lamb.

The mechanisms responsible for the increase in umbilical venous and hepatic vascular resistance during hypoxemia are poorly understood. To assess the relative importance of alpha-adrenergic receptors, we produced an acute, severe hypoxemia in chronically instrumented fetal sheep. While fetal arterial oxygen saturation was maintained at the same level, we then injected phentolamine, a selective alpha-adrenoreceptor blocker. We found that the hypoxemia-induced vasoconstriction of the umbilical veins and hepatic vasculature was reversed by alpha blockade. Thus, alpha-adrenergic stimulation is necessary to maintain vasoconstriction of the umbilical veins and hepatic vasculature during acute fetal hypoxemia. Furthermore, alpha-adrenergic stimulation is responsible for the hypoxemia-induced vasoconstriction of the gut, spleen, and lower carcass. Thus, the alpha-adrenergic system mediates important fetal hemodynamic adaptations to acute hypoxemia. However, the alpha-adrenergic system is not responsible for the hypoxemia-induced constriction of the renal vasculature.     

Ovine surfactant protein cDNAs: use in studies on fetal lung growth and maturation after prolonged hypoxemia

cDNAs for ovine surfactant-associated protein (SP) A, SP-B, and SP-C have been cloned and shown to possess strong similarity to cDNAs for surfactant apoproteins in other species. These reagents were employed to examine the effect of fetal hypoxia on the induction of surfactant apoprotein expression in the fetal lamb. Postnatal lung function is dependent on adequate growth and maturation during fetal development. Insulin-like growth factor (IGF) I and IGF-II, which are present in all fetal tissues studied, possess potent mitogenic and proliferative actions, and their effects can be modulated by IGF-specific binding proteins (IGFBPs). Hypoxia can lead to increases in circulating cortisol and catecholamines that can influence lung maturation. Therefore, the effects of mild hypoxia in chronically catheterized fetal lambs at gestational days 126-130 and 134-136 (term 145 days) on the expression of pulmonary surfactant apoproteins and IGFBPs were examined. Mild hypoxia for 48 h resulted in an increase in plasma cortisol that was more pronounced at later gestation, and in these animals, there was a twofold increase in SP-A mRNA. SP-B mRNA levels also increased twofold, but this was not significant. SP-C mRNA was not altered. No significant changes in apoprotein mRNA were observed with the younger fetuses. However, these younger animals selectively exhibited reduced IGFBP-5 mRNA levels. IGF-I mRNA was also reduced at 126-130 days, although this conclusion is tentative due to low abundance. IGF-II levels were not affected at either gestational age. We conclude that these data suggest that mild prolonged fetal hypoxia produces alterations that could affect fetal cellular differentiation early in gestation and can induce changes consistent with lung maturation closer to term.     

Threshold of hormonal and biophysical responses to acute hypoxemia in fetal sheep at different gestational ages

We examined whether there was a threshold for change in fetal arterial PO2 to elicit alterations in plasma adrenocorticotropic hormone, arginine vasopressin, or cortisol, or to affect the incidence of fetal breathing movements or eye movements and we determined whether such a threshold changed with gestational age. Fetal sheep were exposed to two levels of hypoxemia, mild (4.6-5.3 mmHg PO2 drop) and moderate (8.3-8.8 mmHg PO2 drop) (1 mmHg = 133.322 Pa) for 1 h without pH change at 125-129 or 134-147 days of gestation within 7 days of spontaneous labor. Hypoxemia was induced by altering the inspired percent oxygen of the mother. No significant hormonal and biophysical changes were observed in mild hypoxemia at either age. In moderate hypoxemia at 125-129 days of gestation, there were significant increases of fetal adrenocorticotropic hormone, arginine vasopressin, and cortisol concentrations, and a decreased incidence of fetal breathing movements and eye movements. At 134-147 days of pregnancy, moderate hypoxemia induced a significant increase in adrenocorticotropic hormone, but the response was less than at 125-129 days of gestation. The arginine vasopressin response was similar to that at 125-129 days and there was no significant change in cortisol. There was a significant decrease in fetal breathing movements but not in eye movements. We conclude that a threshold of fetal arterial PO2 drop exists between 5 and 8 mmHg to elicit endocrine or biophysical responses to hypoxemia in fetal sheep at 125-129 days of gestation.(ABSTRACT TRUNCATED AT 250 WORDS)