Effects of chronic reduction in uterine blood flow on fetal and placental growth in the sheep

Pregnancy is associated with a significant increase in uteroplacental blood flow (UBF), which is responsible for delivering adequate nutrients and oxygen for fetal and placental growth. The present study was designed to determine the effects of vascular insufficiency on fetal and placental growth. Thirty-nine late-term pregnant ewes were instrumented to investigate the effects of chronic UBF reduction. Animals were split into three groups based on uterine blood flow, and all animals were killed on gestational day 138. UBF, which began at 851 +/- 74 ml/min (n = 39), increased in controls (C) to 1,409 +/- 98 ml/min (day 138 of gestation) and in the moderately restricted (R(M)) group to 986 +/- 69 ml/min. In the severely restricted (R(S)) group, UBF was only 779 +/- 79 ml/min on gestational day 138. This reduction in UBF significantly affected fetal body weight with R(M) fetuses weighing 3,685 +/- 178 g and R(S) fetuses weighing 2,920 +/- 164 g compared with C fetal weights of 4,318 +/- 208 g. Fetal brain weight was not affected, whereas ponderal index was significantly reduced in R(M) (2.94 +/- 0.09) and R(S) fetuses (2.49 +/- 0.08) compared with the value of the C fetuses (3.31 +/- 0.08). Placental weight was also significantly reduced in the R(M) group, being 302 +/- 24 g, whereas the R(S) group placenta weighed 274 +/- 61 g compared with the C values of 414 +/- 57 g. Fetal heart, liver, lung, and thymus were all significantly smaller in the R(S) group. Thus the present study shows a clear relationship between the level of UBF and both fetal and placental size. Furthermore, the observation that fetal brain weight was not affected, whereas fetal body weight was significantly reduced suggests that this experimental preparation may provide a useful model in which to study asymmetric fetal growth restriction.     

Effect of reduced uterine blood flow on fetal and maternal cortisol

We have measured the changes in fetal and maternal plasma concentrations of cortisol in relation to blood gases and percent oxygen saturation during 2- and 4-h episodes of reversibly reduced uterine blood flow in sheep between 120 days gestation and term. During that period of reduced uterine blood flow there was a significant decrease in fetal arterial percent oxygen saturation (SaO2), PO2 and pH. Fetal SaO2 decreased from 59.5 +/- 3.2% to 31.8% +/- 2.8% by 15 min, 32.9 +/- 2.9% by 60 min, and 33.5 +/- 2.9% by 120 min. Fetal PO2 decreased from 3.2 +/- 0.1 KPa to 2.0 +/- 0.2 KPa by 15 min, 2.2 +/- 0.2 KPa by 60 min and 2.3 +/- 0.1 KPa by 120 min. Fetal pH decreased from 7.36 +/- 0.01 to 7.30 +/- 0.03 by 15 min, 7.27 +/- 0.02 by 60 min and 7.25 +/- 0.03 by 120 min. During the period of reduced uterine blood flow, fetal plasma concentrations of cortisol increased from 37.1 +/- 10.8 nmol/l to 53.3 +/- 9.2 nmol/l by 15 min, 49.2 +/- 11.4 nmol/l by 60 min and 43.3 +/- 9.0 nmol/l by 120 min. The greatest percentage increase in fetal plasma concentrations of cortisol occurred in fetuses of 126-139 days gestation. There was no significant change in maternal blood gases, SaO2 or plasma concentrations of cortisol. These experiments demonstrate that there is a significant increase in fetal plasma concentrations of cortisol in response to reductions in uterine blood flow from as early as 120 days gestation.     

The relationships between fetal and maternal placental blood flows.

Individual maternal and fetal flows to 706 placental cotyledons obtained from 9 chronically catheterized pregnant ewes and their fetuses (gestation age 123-141 days) were measured. The larger the cotyledon the greater the maternal and fetal blood flow to it. Both fetal and maternal flows to larger cotyledons, however, tended to be lower when corrected for the weight of the cotyledon perfused. Changes in fetal placental flow (dfgc, ml/min/g) occurring within 15 min of administration of 15 mg i.v. of captopril to the ewe were dependent on changes in fetal placental vascular resistance (dcotfr) and maternal flow (dmgc) according to the equation dfgc = 0.123 + 0.185 dmgc - 0.026 dcotfr. Changes in maternal placental flow occurring within 15 min of administration of 15 mg i.v. of captopril to the ewe were dependent on changes in maternal placental vascular resistance (dcotmr) and changes in fetal flow according to the equation dmgc = 0.483 + 0.496 dfgc - 0.0198 dcotmr. The changes in fetal flow over the next 1.5h of treatment with captopril at 6 mg/h were dependent on neither changes in fetal placental vascular resistance nor maternal placental flow. changes in maternal placental flows over the same time were no longer related to changes in fetal flow and depended only to a minimal extent on changes in maternal placental resistance. These analyses suggest that treatment of the pregnant ewe with captopril may have disturbed the normal relationships between maternal and fetal placental circulations.     

Embryo survival, and fetal and placental growth following elevation of maternal estradiol blood concentrations in the rat.

High doses of estrogens cause embryonic mortality, and fetal and placental growth retardation in rats. This study addresses the physiological relevance of such findings. Estradiol benzoate (EB), by s.c. injection, or estradiol-17beta (E2), delivered by a miniosmotic pump, raised maternal E2 concentrations from only slightly above control values to 5-fold. EB (1 microgram/day) over Days 6-13, 8-13, and 11-13, and continuous infusion of E2 (15 ng/h; Days 10-13) reduced fetal survival to 0%, 0%, 22%, and 75%, respectively. Single injections of EB showed that its lethal effect declined rapidly over Days 9 (44% survival) to 13 (90% survival). Embryos died within 48 h, but death was not due to luteal failure since progesterone levels were maintained and progesterone administered with EB did not reduce mortality. Administration of EB at 1 microgram/day (Days 14-21) or E2 at 40 ng/h (Days 13-16) retarded fetal and placental growth but did not affect survival. The rat embryo is highly sensitive to elevated maternal estradiol concentrations over much of gestation. The early lethal effect implies that endogenous E2 production is carefully regulated to maintain pregnancy; the latter growth-retarding effect suggests that E2 may have a role in the normal control of fetal growth.     

Effects of maternal nutritional status on fetal and placental growth and on fetal urea synthesis in sheep

Fetal and placental growth, and fetal and maternal urea synthesis in late gestation, were studied in 2-year-old Corriedale ewes on a maintenance ration (M) except when subjected to moderate dietary restriction from day 50 to day 100 (RM), day 100 to day 135 (MR) or day 50 to day 135 (RR). In comparison with fetuses of ewes maintained throughout the experiment (MM), RR fetuses were smaller and RM fetuses were larger whereas MR fetuses were unaffected; all restrictions were associated with increased placental size. Fetal urea synthesis at day 133 in the well-nourished ewes (MM) was 21.5 mg N h-1 kg-1 increasing to, respectively, 25.7, 27.3 and 38.8 mg N h-1 kg-1 in groups MR, RM and RR; these values were 1.6, 3.9, 2.2 and 3.8 times the maternal rates of synthesis. On the basis of the observed urea synthesis rates, amino acid oxidation could have accounted for up to, respectively, 32, 38, 40 and 57% of fetal oxygen consumption in groups MM, MR, RM and RR. Amino acids, in addition to their role in tissue accretion, may be key energy substrates for the fetus.     

Effects of severe reduction in maternal placental blood flow on blood flow distribution in the sheep fetus

To test the hypothesis that fetal lambs are able to maintain oxygen delivery to myocardial, brain and adrenal tissues during reduction in uterine blood flow to 25% of control, we performed experiments on five ewes and their fetuses. A snare occluder was placed around the maternal common hypogastric artery and catheters were placed for measurement of blood pressures, flows, blood gas tensions, pH and oxygen content. After a five day recovery period, control measurements were made. The snare occluder was then closed until the artery was fully occluded. The arterial occlusion caused uteroplacental blood flow to fall to 32 +/- 4% and maternal placental blood flow to fall to 25 +/- 3% of control values. This level of asphyxia was maintained for 19 +/- 3 minutes, when maternal and fetal blood flows were measured again. In response to occlusion, fetal ascending aortic PO2 fell from 21 +/- 2 (SEM) to 13 +/- 2 mmHg (P less than or equal to 0.01), oxygen content from 4.3 +/- 0.3 to 1.4 +/- 0.2 mM (P less than or equal to 0.01) and pH from 7.37 +/- 0.01 to 7.21 +/- 0.05 (P less than or equal to 0.01). PCO2 rose from 48 +/- 1 to 62 +/- 3 mmHg (P less than or equal to 0.01). Fetal arterial blood pressure increased from 51 +/- 3 to 61 +/- 3 mmHg (P less than or equal to 0.001) and heart rate decreased from 172 +/- 10 to 104 +/- 4 beats.min-1 (P less than or equal to 0.01). The heart, brain and adrenals showed vasodilation in response to the asphyxic stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of reducing uterine blood flow on fetal blood flow distribution and oxygen delivery.

We examined the effect of graded reduction in uterine blood flow on distribution of cardiac output and oxygen delivery to fetal organs and venous blood flow patterns in 9 fetal sheep using the radionuclide-labeled microsphere technique. We reduced uterine blood flow in two steps, decreasing fetal oxygen delivery to 70% and 50% of normal, and compared the results with those from a similar study from our laboratory on graded umbilical cord compression. With 50% reduction in fetal oxygen delivery, blood flow and the fraction of the cardiac output distributed to the brain, heart, and adrenal gland increased and that to the lungs, carcass, skin, and scalp decreased. Oxygen delivery to the brain and myocardium was maintained, while that to the adrenal doubled, and that to the brain stem increased transiently. The decrease in oxygen delivery to both carcass and lower body segment correlated linearly with oxygen consumption (P less than 0.001). The proportion of umbilical venous blood passing through the ductus venosus increased from 44.6% to 53% (P less than 0.05). The preferential distribution of ductus venosus blood flow through the foramen ovale to the heart and brain increased, but that to the upper carcass decreased so that ductus venosus-derived blood flow to the upper body did not change. Hence, the oxygen delivered to the brain from the ductus venosus was maintained, and that to the heart increased 54% even though ductus venosus-derived oxygen delivery to the upper body fell 34%. Abdominal inferior vena caval blood flow and its contribution to cardiac output decreased, but the proportion of the abdominal inferior vena caval blood distributed through the foramen ovale also increased from 23.0 to 30.9%. However, the actual amount of inferior vena caval blood passing through the foramen ovale did not change. There was a 70% fall in oxygen delivery to the upper body segment from the inferior vena cava. A greater portion of superior vena caval blood was also shunted through the foramen ovale to the upper body, but the actual amounts of blood and oxygen delivered to the upper body from this source were small. Thus, graded reduction of uterine blood flow causes a redistribution of fetal oxygen delivery and of venous flow patterns, which is clearly different from that observed previously during graded umbilical cord occlusion.     

Repetitive reduction of uterine blood flow and its influence on fetal transcutaneous PO2 and cardiovascular variables

The influence of repeated asphyxia on fetal transcutaneous PO2, relative local skin perfusion, heart rate, blood gases and pH was investigated in 15 experiments on 8 acutely instrumented sheep fetuses in utero between 125 and 145 days gestation (term is 147 days). Uterine blood flow was intermittently arrested (11 times within 33 min) by intra-vascular maternal aortic occlusion, exposing the fetuses to repeated episodes of asphyxia of 30 (n = 3), 60 (n = 9) and 90 (n = 3) s duration. The fetal transcutaneous PO2 fell as the duration of asphyxia (2 alpha less than 0.01), heart rate deceleration area (2 alpha less than 0.01) and acidaemia (2 alpha less than 0.01) increased. With decreasing skin perfusion, which was dependent on the duration of asphyxia (2 alpha less than 0.001) and acidaemia (2 alpha less than 0.001), a discrepancy developed between transcutaneous and arterial PO2. The increase (delta) in transcutaneous-arterial PO2 difference was related linearly to the duration of asphyxia (2 alpha less than 0.01), the mean haemoglobin oxygen saturation (2 alpha less than 0.001), acidaemia (2 alpha less than 0.001) and relative local skin flow (2 alpha less than 0.05). It was highest after severe episodes of asphyxia (90 s), when O2 saturation, skin blood flow and arterial blood pH values were low. Fetal heart rate deceleration area was only correlated with the cutaneous-arterial PO2 difference when the mean fetal haemoglobin oxygen saturation was below 35%. Thus, a discrimination of heart rate decelerations that are significant for the fetus seems to be possible, when associated with low transcutaneous PO2 values. We conclude that in the sheep fetus transcutaneous PO2 measurements during repeated asphyxial episodes yield information on fetal oxygenation and on the skin vasomotor response.     

Effect of maternal undernutrition in early gestation on ovine fetal blood pressure and cardiovascular reflexes

Human epidemiological and animal experimental studies suggest that maternal undernutrition during pregnancy may alter cardiovascular development of the offspring. The extent to which these effects involve changes in fetal cardiovascular function and whether they are necessarily linked to reduced fetal growth is unknown. In sheep, we investigated the effect of a 15% reduction in maternal global nutrition for the first 70 days of gestation (term = 147 days) on fetal blood pressure development, baroreflex control of fetal heart rate (FHR), and cardiovascular responses to acute hypoxemia in late gestation. Basal mean arterial pressure (P < 0.05), systolic blood pressure (P < 0.05), diastolic blood pressure (P < 0.05), and rate-pressure product (P < 0.001) were significantly lower in fetuses of nutritionally restricted ewes (R) compared with controls (C). FHR was not altered. The operating point for the fetal baroreflex was significantly lower in R fetuses compared with C (P < 0.01), but there was no difference between the groups in the cardiovascular response to hypoxemia. We conclude that mild maternal undernutrition alters fetal cardiovascular development, producing low blood pressure and resetting of baroreflex control mechanisms. This effect occurs without any changes in fetal growth or blood gas status.     

Intrauterine growth restriction is associated with alterations in placental lipoprotein receptors and maternal lipoprotein composition

Among other factors, fetal growth requires maternal supply of cholesterol. Cellular cholesterol uptake is mainly mediated by the LDL receptor (LDL-R) and the scavenger receptor family. We hypothesized that expression levels of key receptors of these families were regulated differently in placentas from IUGR pregnancies with varying degrees of severity. Third-trimester placentas from IUGR pregnancies with (IUGR-S) and without (IUGR-M) fetal hemodynamic changes and from control (AGA) pregnancies were studied. LDL-R, LDL-R-related protein (LRP-1), and scavenger receptor class B type I (SR-BI) mRNA and protein levels were measured. Cholesterol concentration and composition of lipoproteins were analyzed enzymatically and by lipid electrophoresis, respectively, in maternal and umbilical cord blood. LDL-R mRNA levels in IUGR-M were similar to AGA but lower (P < 0.05) in IUGR-S. In contrast, LDL-R protein was twofold (IUGR-M) and 1.8-fold (IUGR-S) higher (P < 0.05) than in the AGA group. LRP-1 mRNA and protein levels were not altered in the IUGR cases. SR-BI mRNA was unchanged in IUGR, but protein levels were lower (P < 0.05) in IUGR-S than in the other groups. Maternal plasma concentrations of LDL cholesterol were higher (P < 0.05) in the AGA group (188.5 +/- 23.6 mg/dl) than in the IUGR-S group (154.2 +/- 26.1). Electrophoretic mobility of the LDL fraction in maternal plasma demonstrated significant changes in migration toward higher values (AGA 0.95 +/- 0.06, IUGR-M 1.12 +/- 0.11, P < 0.001; IUGR-S 1.28 +/- 0.20, P = 0.002). We conclude that LDL-R and SR-BI levels are altered in IUGR pregnancies. These differences were associated with changes in LDL, but not HDL, mobility and cholesterol concentration in maternal circulation.     

Regional blood flow distribution in fetal sheep with intrauterine growth retardation produced by decreased umbilical placental perfusion

To determine the capacity of the fetus to adapt to chronic O2 deficiency produced by decreased placental perfusion in the early development of growth retardation, we embolized the umbilical placental vascular bed of fetal sheep for a period of 9 days. Fetal umbilical placental embolization decreased arterial O2 content by 39%, decreased total placental blood flow by 33%, and produced a 20% reduction in mean fetal body weight. Neither the combined ventricular output nor the regional blood flow distribution was significantly different between the 8 growth-retarded and 7 normally grown fetuses despite the 39% decrease in fetal arterial O2 content. Thus a 33% reduction in total placental blood flow restricts normal fetal growth, but does not exceed the placental circulatory reserve capacity necessary to maintain normal basal metabolic oxygenation. Because the proportion of combined ventricular output to the placenta at rest is decreased in late IUGR fetuses but not in early IUGR fetuses, despite chronic oxygen deficiency, we conclude that the growth retarded fetus maintains a normal regional blood flow distribution until the placental circulatory reserve capacity is depleted.     

Cotyledonary responses to maternal selenium and dietary restriction may influence alterations in fetal weight and fetal liver glycogen in sheep

To examine the effects of maternal supranutritional selenium (Se) and nutrient restriction during mid and late gestation on placental characteristics and fetal liver glycogen, ewes received either adequate Se (ASe) or high Se (HSe) prior to breeding. On d 64 of gestation, ASe and HSe ewes remained at 100% of requirements (controls; CON) or were restricted (RES; 60% of requirements). On d 135 of gestation, fetal weight (P ≤ 0.08) was greatest in both HSe and CON ewes. Placentome number, mass, and caruncular and cotyledonary weight were not different (P ≥ 0.17) among treatments. Fetal mass:placental mass ratio was less (P = 0.06) in RES compared to CON ewes. Compared to ASe, HSe exhibited increased (P ≤ 0.08) cellular proliferation and DNA concentration and decreased (P = 0.07) cellular size in cotyledonary tissue. Nutritional restriction decreased (P ≤ 0.08) cotyledonary protein concentration and cellular size. VEGF receptor 1 (Flt) mRNA in cotyledonary tissue was greater in HSe compared with ASe ewes (P = 0.06) and in RES compared with CON ewes (P = 0.08). There was no effect of diet on caruncular growth variables (P ≥ 0.13) or on placental vascularity (P ≥ 0.11). Progesterone was greater (P ≤ 0.08) in ASe–RES ewes compared to all groups at d 90 and ASe–CON and HSe–CON at d 104. Although fetal glucose and cortisol concentrations were not affected by diet, fetal liver glycogen was greater (P = 0.04) in ASe–RES compared to ASe–CON and HSe–RES ewes with HSe–CON being intermediate. Both Se and nutritional plane may impact placental function and fetal growth, as fetal weight and liver glycogen are altered despite similar placental vascularity measurements.