Adenosine A(1) receptor mediated suppression of adrenal activity in near-term fetal sheep.

Activation of the hypothalamic-pituitary-adrenal (HPA) axis is a critical response to perinatal hypoxia. Recent data show that adenosine appears to inhibit baseline levels of fetal cortisol and to restrict the increase in ACTH and cortisol during moderate hypoxia. Because adenosine increases substantially during profound asphyxia, it is possible, but untested, that counterintuitively it might restrict the HPA response to more severe insults. It is unclear which receptors mediate the effects of adenosine on the HPA axis; however, adenosine A(1) receptor activation is important for adaptation to hypoxia. We therefore investigated whether adenosine A(1) receptor blockade modulates ACTH and cortisol levels in fetal sheep at 118 to 126 days gestation, randomly allocated to receive an intravenous infusion of either vehicle (vehicle-occlusion, n = 7) or 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, an A(1) receptor antagonist, DPCPX-occlusion, n = 7) infused 60 min before and during 10 min of umbilical cord occlusion, or infusion of DPCPX for 70 min without occlusion (DPCPX-sham, n = 6). Experiments were terminated after 72 h. Fetal ACTH levels increased significantly (P < 0.01) during occlusion, but not sham occlusion, and returned to baseline values by 60 min after occlusion. In the vehicle-occlusion group, fetal cortisol and cortisone plasma levels increased significantly (P < 0.05) 60 min after the occlusion and returned to baseline values by 24 h. In contrast, there was a marked increase in both fetal cortisol and cortisone during DPCPX infusion before occlusion to a level greater even than the maximum rise seen after occlusion alone. This increase was sustained after occlusion, with increased cortisol levels compared with occlusion alone up to 72 h. In conclusion, fetal cortisol concentrations are suppressed by adenosine A(1) receptor activity, largely though a direct adrenal mechanism. This suppression can be partially overcome by supraphysiological stimuli such as asphyxia.

Respiratory function of the placenta as determined with carbon monoxide in sheep and dogs.

A technique is described for studying the respiratory function of the placenta using carbon monoxide, a gas whose exchange across the placenta between the maternal and fetal circulations is limited by diffusion rather than blood flow. During the steady state before the introduction of CO, the normal concentration of carboxyhemoglobin in the ewe, [COHb](M), is approximately 0.90%, and that in the fetus is 2.9%, the ratio [COHb](F)/[COHb](M) being 3.2. In dogs the corresponding values are 1.9%, 4.8%, and 2.4%. After the introduction of CO into the mother animal, CO diffused across the placenta slowly with an equilibration half-time of approximately 2 hours. The average carbon monoxide diffusing capacity (D(Pco)) of the placenta during maternal to fetal exchange was 0.54 ml per (minute x mm Hg x kg fetal weight) (SD +/- 0.13) in sheep and 0.57 ml per (minute x mm Hg x kg) (SD +/- 0.18) in dogs. The fetal to maternal placental diffusing capacity in two sheep was 0.54 ml per (minute x mm Hg x kg). Calculations considering the relative rates of reaction of O(2) and CO with red cell hemoglobin and the relative rates of diffusion of the two gases suggest that the true D(Po2) should be about 1.2 to 2 times greater than the D(Pco) or 0.65 to 1.1 per (minute x mm Hg x kg). This is about 5 times greater than the reported value of D(Po2) calculated from measurements of P(O2) in the mixed uterine and umbilical venous blood. With a diffusing capacity of this magnitude the maternal and fetal placental end capillary P(O2) would approach equilibrium, becoming too small to measure, and the calculation of D(Po2) would be unreliable. We suggest that the apparent end capillary P(o2) gradients of 15 to 20 mm Hg, obtained from sampling uterine and umbilical venous blood, result from a combination of uneven distribution of maternal and fetal placental blood flow and from placental oxygen consumption.

Determination of distribution of diffusing capacity in relation to blood flow in the human lung.

A method for appraising the distribution of diffusing capacity of the lungs (D(L)) in relationship to pulmonary capillary blood flow ([unk]Q(C)) in normal human subjects was derived from measurements of oxygen diffusing capacity (D(LO2)) and carbon monoxide diffusing capacity (D(LCO)) performed during breath holding. This method utilizes the fact that the observed D(LO2) is considerably reduced in value if uneven distribution of D(L) with respect to [unk]Q(C) (uneven D(L)/[unk]Q(C)) is present. In contrast, D(LCO) is barely affected by uneven D(L)/[unk]Q(C), and from its measured value one can calculate the value D(LO2) would have if no uneven D(L)/[unk]Q(C) were present (true D(LO2)). Once observed D(LO2) and true D(LO2) are known, the degree of uneven D(L)/[unk]Q(C) in the lung can be calculated. In five normal, resting, sitting subjects average values for true D(LO2) were 57 ml per (minute x mm Hg), and the directly measured D(LO2) was 33 ml per (minute x mm Hg). These values could be explained if one-half of total [unk]Q(C) were distributed to approximately 15% of total D(L). These measurements did not permit the determination of the alveolar to end capillary O(2) gradient, but calculations demonstrate that an important factor in determining its size may be the pattern of uneven D(L)/[unk]Q(C) present in the lungs. Estimations of the alveolar-end capillary O(2) gradient from measurements of D(LCO) or D(LO2) that do not take into account uneven D(L)/[unk]Q(C) may underestimate its size.

Uneven distribution of maternal and fetal placental blood flow, as demonstrated using macroaggregates, and its response to hypoxia.

A technique is described for studying the distribution of blood flow to the maternal and fetal placental vessels in sheep and dogs with radioactive labeled macroaggregates of albumin. When the maternal animal breathed room air the distribution of maternal placental blood flow was uneven among the cotyledons as well as within a given cotyledon. Fetal blood flow was also distributed nonuniformly among and within the cotyledons. The relation of maternal to fetal placental blood flow was also markedly uneven (coefficient of correlation, tau = 0.066). After the animal was made hypoxic by breathing 10-12% O(2) the distribution of maternal, fetal, and maternal/fetal placental flows became more uniform. The coefficient of correlation of maternal to fetal flow was high (tau = 0.53, P < 0.01). While the maternal animal breathed room air, after ligation of a major branch of the umbilical artery the distribution of maternal, fetal, and maternal/fetal flows in the remaining two-thirds to three-fourths of the placenta became more uniform. The correlation coefficient for maternal to fetal flow was high (tau = 0.35, P < 0.01).It appears that under normal circumstances with uneven distribution of blood flows there is a considerable portion of the placenta that does not receive blood flow in optimum quantities to promote efficient O(2) exchange. Failure to consider the influence of nonuniform maternal flow/fetal flow will result in overestimation of mean maternal-fetal oxygen tension gradients, and thus underestimation of the placental diffusing capacity for oxygen. In response to maternal hypoxia or compromise of the fetal placental circulation the distribution of maternal, fetal, and maternal/fetal flows becomes more uniform, thereby increasing the efficiency of placental O(2) exchange.

Modulation of growth hormone secretion by thermogenically derived free fatty acids in the perinatal lamb.

To evaluate the hypothesis that the rapid fall in circulating GH concentrations at birth is secondary to the initiation of nonshivering thermogenesis and the consequent rise in FFA levels, a series of experiments was performed in late-gestation fetal sheep. By sequentially cooling the fetus by means of a coil placed around the fetal thorax, ventilating with oxygen via an exteriorized tracheostomy tube, and separating the fetus from the placenta by occluding the umbilical cord, nonshivering thermogenesis could be induced in utero. In the first protocol (n = 6) cooling alone had no effect on fetal plasma FFA levels, oxygenation elevated FFAs slightly from 64 +/- 7 mu Eq/liter to 183 +/- 29 mu Eq/liter, and cord occlusion caused a further marked rise (P less than 0.005) to 635 +/- 69 mu Eq/liter. Neither cooling nor ventilation affected fetal plasma GH concentrations which fell (P less than 0.001) from 160 +/- 17 ng/ml to 65 +/- 13 ng/ml upon cord occlusion. When the cord occluder was removed FFA levels fell (P less than 0.001) and GH concentrations rose (P less than 0.001) once more, and when the cord was again occluded FFA levels rose (P less than 0.001) and GH concentrations fell (P less than 0.001). In a second protocol nine fetuses were cooled, ventilated, and the umbilical cord occluded. Once more, plasma FFA levels rose (P less than 0.001) and GH concentrations fell (P less than 0.001); when thermogenesis was inhibited by the infusion of the adenosine agonist N6-(L-2-phenyl isopropyl)-adenosine, FFA levels fell from 725 +/- 88 mu Eq/liter to 265 +/- 56 mu Eq/liter and GH concentrations rose from 54 +/- 13 ng/ml to 323 +/- 73 ng/ml. In two further protocols the possibility that PIA was acting directly on GH secretion was excluded in six fetuses with low plasma FFA levels and in three fetuses with elevated plasma FFA levels secondary to a fatty acid emulsion infusion. These studies provide direct evidence that the pattern of change in plasma GH concentrations at birth in the sheep is determined in part by the rise in plasma FFAs of thermogenic origin.

Fetal plasma hypoxanthine level in growth-retarded fetuses before labor.

Hypoxanthine is one of the purine nucleotides and is presumed to accumulate during hypoxia and acidemia. It remains uncertain, however, whether plasma hypoxanthine concentration is a useful indicator of fetal asphyxia; and its relationship to other markers of fetal physiologic state is not clearly defined. The aim of this study was to evaluate whether the level of fetal plasma hypoxanthine is correlated with fetal hypoxia and acidosis in growth-retarded fetuses before the onset of labor. Cordocentesis was performed in 34 growth-retarded fetuses at 31-35 weeks' gestation for the measurement of umbilical venous plasma concentrations of hypoxanthine, hemoglobin and lactate concentrations, blood gases, and base deficit. Umbilical venous plasma hypoxanthine concentration was found to be increased significantly, in parallel with the degree of acidosis (r = -0.74, P < 0.05) and base deficit (r = -0.41, P < 0.05), but not to bear a significant relationship to the degree of hypoxemia or other measured variables. We conclude that increases in the plasma concentration of hypoxanthine may reflect an impaired physiological state in growth-retarded fetuses before labor.

Application of laser Doppler flowmetry to measure cerebral microvascular perfusion in the fetal sheep.

Laser Doppler flowmetry (LDF) has been used to measure flow in various organs of the adult, but has not been applied to the mammalian fetus. The purpose of this study was to apply LDF to measure cerebral blood flow of the fetal sheep and to assess the possible errors and artifacts of the method caused by myometrial, fetal, and maternal movements. By three days after probe placement, the flow signal had decreased 55% from initial post surgical readings and thereafter it became stable. During fetal hypoxia, the signal increased 48% and during hypercarbia it increased 59%. After fetal death, the signal decreased to 48% of control level. After maternal death, it decreased to 9% and electrical zero could not be reached. LDF is useful to measure changes of fetal cerebral microvascular perfusion because it can provide continuous signals but care is required in data handling and probe fixation when used for the mammalian fetus.

Fetal and uteroplacental heat production in sheep.

To separate heat production of the fetus from that of the placenta, endometrium, and uterine muscle, we measured total uterine heat production first with the fetus intact and then after the umbilical cord was snared and the fetus killed. Heat production was measured with the Fick principle using thermistors chronically implanted in a maternal artery and major uterine vein and a flowmeter placed on the common internal iliac artery. In nine ewes, carrying lambs weighing 4.46 +/- 0.42 (SE) kg, total uterine heat production fell from 10.6 to 2.9 W after fetal death. Uterine blood flow fell progressively to 90% of control levels during the first hour after death. The caloric equivalent for O2 averaged 4.1 cal/ml O2 for the uterus, 2.2 for the uteroplacenta, and 4.6 for the fetus per se. It was not possible to explain these results using a simple model of maternal-fetal heat transfer. Rather, it was necessary to assume an additional pathway for heat transfer between small uterine veins on the surface of the uterus and cooler structures in the maternal abdomen, presumably the ventral abdominal wall.

Changes in plasma adenosine during simulated birth of fetal sheep.

Adenosine is known to inhibit nonshivering thermogenesis in adult brown fat. These experiments were undertaken to test whether fetal adenosine, normally present in high concentrations, suppresses lipolysis in utero and then falls after birth, permitting thermogenesis to begin. To test this hypothesis, we measured fetal plasma adenosine concentration [ADO] using high-performance liquid chromatography in 11 fetal sheep at 135-140 days gestation during simulated birth. During an initial control period, fetal [ADO] averaged 1.9 +/- 0.3 microM, about four times maternal [ADO] (0.4 +/- 0.1 microM, P less than 0.001). The fetus was then cooled by circulating cold water through a plastic coil encircled about the fetal torso. One hour later, when fetal core temperature had decreased 2.3 degrees C, fetal [ADO] averaged 2.8 +/- 0.5 microM, a 50% increase (P less than 0.05), while thermogenesis remained inactive. Next the fetal lungs were ventilated with O2 to raise arterial Po2 to greater than or equal to 150 Torr. Fetal [ADO] decreased only slightly, and thermogenic responses were modest. Finally, the umbilical cord was occluded. Fetal [ADO] decreased rapidly and 60 min later averaged 1.1 +/- 0.2 microM, 40% below initial control (P less than 0.05) and 57% below the previous period (P less than 0.001). As [ADO] fell, strong thermogenic responses became apparent, as indicated by seven- to eightfold increases in plasma glycerol (P less than 0.001) and a doubling in fetal O2 consumption (P less than 0.001). These results are consistent with the hypothesis that high fetal [ADO] inhibits thermogenesis before birth but then decreases after cord occlusion, allowing thermogenesis to begin.(ABSTRACT TRUNCATED AT 250 WORDS)

Fetal breathing, sleep state, and cardiovascular responses to graded hypoxia in sheep.

Graded isocapnic hypoxemia was produced in unanesthetized fetal sheep by varying the inspired O2 concentration (21, 12, 10.5, and 9%) of the ewe. This produced corresponding mean preductal arterial O2 tension (PaO2) values of 25.2 +/- 1.1 (control), 20.1 +/- 1.0 (mild hypoxia), 17.8 +/- 0.9 (moderate hypoxia), and 16.8 +/- 1.4 Torr (severe hypoxia). These were associated with mean arterial O2 contents (CaO2) of 7.18 +/- 0.44, 5.19 +/- 0.34, 4.24 +/- 0.33, and 3.27 +/- 0.20 ml/dl, respectively. The most severe hypoxia was associated with metabolic acidosis and fetal bradycardia. Hypoxia did not reduce significantly the incidence of low-voltage electrocortical activity. The incidence of breathing and rapid eye movements was not affected by mild hypoxia; however, the incidence of both was significantly reduced during moderate and severe hypoxia. It is concluded that 1) acute reductions in the mean PaO2 of 5.9 +/- 0.6 Torr and CaO2 of 2.00 +/- 0.23 ml/dl are critical in that greater reductions inhibit fetal eye and breathing activity and 2) hypoxia probably inhibits eye and breathing movements by altering sleep state.

Fetal breathing, sleep state, and cardiovascular responses to graded anemia in sheep.

Graded anemia was produced for 2 h in 10 unanesthetized fetal sheep by infusing plasma in exchange for fetal blood. This reduced the mean fetal hematocrits during the 1st h of anemia to 19.7 +/- 0.5% [control (C) = 28.2 +/- 1.1%] for mild anemia, 17.4 +/- 0.9% (C = 30.0 +/- 1.1%) for moderate anemia, and 15.1 +/- 1.0% (C = 29.2 +/- 1.3%) for severe anemia. The respective mean arterial O2 contents (CaO2) were 4.46 +/- 0.20, 3.89 +/- 0.24, and 3.22 +/- 0.19 ml/dl. Mean arterial PO2 was reduced significantly (by 2 Torr) only during moderate anemia, and mean arterial pH was decreased only during severe anemia. No significant changes occurred in arterial PCO2. Fetal tachycardia occurred during anemia. Mean arterial pressure was reduced by 2-3 mmHg during mild anemia; however, no significant blood pressure changes were observed for moderate or severe anemia. The incidence of rapid-eye movements and breathing activity was not affected by mild anemia, but the incidence of both was reduced significantly during moderate and severe anemia. It is concluded that 1) a reduction in CaO2 of greater than 2.48 +/- 0.22 ml/dl by hemodilution inhibits rapid-eye movements and breathing activity, and 2) the PO2 signal for inhibition does not come from arterial blood but from lower PO2 in tissue.

Fetal breathing and sleep state responses to graded carboxyhemoglobinemia in sheep.

To investigate CO effects on brain oxygenation, graded carboxyhemoglobinemia (HbCO) was produced in nine unanesthetized fetal sheep by infusing CO-laden erythrocytes in exchange for fetal blood. For the 1st h after this procedure, the mean fetal carboxyhemoglobin levels were 16.5 +/- 0.4% [control (C) = 1.4 +/- 0.4%] for mild HbCO, 22.7 +/- 0.6% (C = 1.8 +/- 0.4%) for moderate HbCO, and 27.8 +/- 0.5% (C = 2.1 +/- 0.7%) for severe HbCO. This induction of HbCO significantly reduced mean preductal arterial PO2 values to 4.3 Torr below control for mild HbCO, 4.6 Torr below control for moderate HbCO, and 5.5 Torr below control for severe HbCO. The respective arterial O2 contents were decreased by 17, 21, and 29%. Mean arterial pH was lowered only during severe HbCO, and arterial PCO2 values were unchanged. HbCO produced a fetal tachycardia. Mean arterial blood pressure was only increased during severe HbCO. The incidences of rapid eye movements and breathing activity were decreased by HbCO in a dose-dependent manner. When related to calculated brain tissue PO2, these decreases were similar to those measured during hypoxic hypoxia and anemia, suggesting that carboxyhemoglobin effects result solely from diminished oxygenation. It is concluded that 1) the peripheral arterial chemoreceptors in the fetus apparently have little effect on hypoxic inhibition of breathing and 2) the carboxyhemoglobin concentrations required to inhibit fetal breathing are greater than those likely to be encountered clinically.

Computer model of fetal-maternal heat exchange in sheep.

We constructed and used a mathematical model of maternal-fetal heat exchange in the sheep to explore the effects of changes in certain parameters on steady-state fetal temperatures and to determine whether the fetus in the model has any potential to control its own temperature. The model took into account both fetal and placental heat production and exchange of heat in the placenta, across the fetal skin, via amniotic fluid, and through the uterine wall. The maternal ewe was assumed to be a constant temperature heat sink. Changes in placental or fetal heat production were calculated to change the ratio of heat exiting across the placenta or fetal skin significantly but to have little effect on fetal core temperature, e.g., a rise of only 0.8 degrees C was predicted after a twofold increase in fetal heat production. Fetal placental blood flow was calculated to affect fetal temperature the most of any flow, a reduction to zero causing fetal temperature to rise 5.0 degrees C. Changes in heat conductances between fetal skin and amniotic fluid, or between amniotic fluid and uterine wall, had minimal effect on fetal temperature. From the model calculations here and because heat exchange within the sheep placenta has previously been calculated to be extremely efficient, we conclude that the fetal sheep has little ability to control its temperature by changes in heat dissipated through extraplacental pathways. Thus the model predicts an effective heat clamp that closely links fetal to maternal temperature.

Exogenous infusion of adenosine depresses whole body O2 use in fetal/neonatal sheep.

To examine a possible metabolic regulatory role for adenosine, infusions of adenosine and adenosine deaminase were given to 11 near-term fetal sheep during the simulation of birth in utero. Fetal arterial blood gases, the concentration of a number of metabolites, insulin, and whole body O2 consumption (VO2) were measured. After intrauterine ventilation and cord occlusion, fetal/neonatal VO2, measured by closed-circuit respirometry, averaged 11.0 +/- 1.1 (SE) ml (STPD).min-1.kg fetal wt-1 and plasma adenosine concentration ([Ado]) was 1.29 +/- 0.21 microM. Infusion of adenosine (1.5 mumol.min-1.kg-1) during the next 30-min interval increased [Ado] to 1.57 +/- 0.28 microM (not significant) and decreased VO2 to 7.7 +/- 0.5 ml.min-1.kg-1 (P < 0.05). The infusion reduced systolic blood pressure by 19% (P < 0.01) and diastolic blood pressure by 25% (P < 0.01) and increased heart rate by 19% (P < 0.01). At the highest rate of adenosine infusion studied (6 mumol.min-1.kg-1), [Ado] increased to 4.27 +/- 0.46 microM (P < 0.001) and VO2 did not measurably decline further, although there were further decreases in blood pressure and increases in heart rate. After administration of adenosine deaminase, [Ado] decreased to 0.58 +/- 0.13 microM (P < 0.05), whereas VO2 increased to 11.2 +/- 0.8 ml.min-1.kg-1 (P < 0.05); blood pressure and heart rate returned to basal levels. The dependence of VO2 on [Ado] is described by the relationship VO2 = 6.14 + 4.89 exp(-0.45[Ado]) (n = 144; r = 0.34; P < 0.001). Throughout the experiment, arterial O2 content and plasma glucose, lactate, glycerol, and fatty acid concentrations were normal or elevated, and, therefore, O2 lack and substrate deficiency were unlikely to have caused the reduction in VO2. We conclude that plasma adenosine may act as a messenger of energy status for the ovine fetus/neonate and may contribute thereby to a maintenance of a balance between O2 supply and O2 demand.

Fetal breathing and cardiovascular responses to graded methemoglobinemia in sheep.

Graded methemoglobinemia (MetHb) was produced in unanesthetized fetal sheep to determine the effects on brain oxygenation. MetHb was induced by infusing methemoglobin-containing erythrocytes in exchange for fetal blood. During the hour after MetHb was established, fetal methemoglobin concentrations averaged 1.23 +/- 0.12 (mild MetHb), 1.71 +/- 0.13 (moderate MetHb), and 2.27 +/- 0.17 g/dl (severe MetHb). MetHb reduced mean arterial O2 content by approximately 19 (mild MetHb), 29 (moderate MetHb), and 39% (severe MetHb). The average preductal arterial PO2 fell by 1.6 (-7%), 2.8 (-11%), and 4.0 Torr (-16%) for mild, moderate, and severe MetHb, respectively. Fetal heart rate increased significantly during mild and moderate MetHb, and mean arterial pressure fell slightly during moderate and severe MetHb. The incidences of fetal breathing and eye movements were reduced in a dose-dependent manner when the calculated brain end-capillary PO2 was less than 14 Torr. We conclude that: 1) the effective capillary PO2 in the fetal brain can be significantly reduced by increasing the distance between non-methemoglobin-laden erythrocytes in capillaries and 2) hypoxic inhibition of fetal breathing probably arises from discrete areas of the brain having a PO2 less than 3 Torr.

Disappearance of palmitic acid from plasma of fetal and newborn sheep.

These studies were undertaken to measure the kinetic constants that characterize the disappearance of a representative free fatty acid (FFA) from the plasma of fetal and newborn sheep. A bolus of albumin-complexed [14C]palmitic acid was infused intravenously, and during the next 8 min, 24 arterial samples were collected to characterize the disappearance curve. Palmitic acid disappearance from plasma was well described by a double-exponential model. When birth was simulated in utero, kinetic values were not changed by cooling. However, after intrauterine ventilation with O2, the volume of distribution of the FFA increased 29%, its plasma clearance rate decreased 26%, and its apparent half-life in the plasma lengthened from 0.8 to 1.2 min (all P < 0.01, n = 8). After umbilical cord occlusion, plasma clearance rate decreased a further 19% and half-life lengthened to 1.4 min. About 60% of the increase in FFA concentration during simulated birth is explained by increased release from adipose stores, and 40% is explained by decreased clearance. Further experiments tested the influence of FFA concentrations themselves. After infusion of unlabeled FFA, clearance of the tracer decreased 23% (P < 0.05, n = 5), a result consistent with a saturable membrane transporter of FFAs.

Maternal theophylline administration and breathing movements in late-gestation human fetuses.

OBJECTIVE: To investigate the relation between maternal administration of theophylline and breathing movements during late gestation in human fetuses. METHODS: After a 1-hour control period, 17 women with normally grown fetuses at 33-38 weeks' gestation were given 400 mg of sustained-release theophylline orally. Maternal plasma theophylline and glucose concentrations were measured every hour, and the incidence of fetal breathing movements and breathing rates were measured continuously during the next 8 hours. Results were compared with those of a similar control group that did not receive theophylline. RESULTS: The maternal plasma theophylline concentration increased to detectable levels after 1 hour, reached therapeutic levels of 6.9 +/- 0.4 micrograms/mL (standard error of the mean) after 6 hours, and thereafter averaged 8.3 +/- 0.3 micrograms/mL, significantly higher than initial control values (P < .05). The mean glucose concentration was unchanged during the first 6 hours (79.2 +/- 1.1 mg/dL) and then decreased somewhat, averaging 75.8 +/- 1.1 mg/dL in the 7-8 hour period. The incidence of fetal breathing increased 26.4 +/- 1.9% after 5 hours and remained elevated during the next 3 hours at levels significantly higher than in the initial control period (P < .05) and higher than in the control group that did not receive theophylline. The mean hourly breathing rate averaged 40.9 +/- 1.3 breaths per minute after ingestion of theophylline, a nonsignificant change. CONCLUSIONS: Ingestion of theophylline by pregnant women in late gestation is associated with an increase in fetal breathing movements. Theophylline has been used widely in the management of asthma in pregnancy, and the assessment of fetal breathing movements is used routinely for evaluation of fetal status. Therefore, account must be taken of changes in breathing movements caused by maternal theophylline administration.

Adaptation of laser-Doppler flowmetry to measure cerebral blood flow in the fetal sheep.

The purpose of this study was to devise a means to use laser-Doppler flowmetry to measure cerebral perfusion before birth. The method has not been used previously, largely because of intrauterine movement artifacts. To minimize movement artifacts, a probe holder was molded from epoxy putty to the contour of the fetal skull. A curved 18-gauge needle was embedded in the holder. At surgery, the holder, probe, and skull were fixed together with tissue glue. Residual signals were recorded after fetal death and after maternal death 1 h later. These averaged <5% of baseline flow signals, indicating minimal movement artifact. To test the usefulness of the method, cerebral flow responses were measured during moderate fetal hypoxia induced by giving the ewes approximately 10% oxygen in nitrogen to breathe. As fetal arterial PO(2) decreased from 21.1 +/- 0.5 to 10.7 +/- 0.4 Torr during a 30-min period, cerebral perfusion increased progressively to 56 +/- 8% above baseline. Perfusion then returned to baseline levels during a 30-min recovery period. These responses are quantitatively similar to those spot observations that have been recorded earlier using labeled microspheres. We conclude that cerebral perfusion can be successfully measured by using laser-Doppler flowmetry with the unanesthetized, chronically prepared fetal sheep as an experimental model. With this method, relative changes of perfusion from a small volume of the ovine fetal brain can be measured on a continuous basis, and movement artifacts can be reduced to 5% of measured flow values.

Rate of disappearance of glycerol from plasma of fetal and newborn sheep.

The disappearance of glycerol from plasma was studied after a single intravenous injection to estimate its volume of distribution (Vdist), plasma clearance rate, and rate constant for irreversible loss (kd). Studies were repeated before and after birth of the lamb to test whether loss of the placenta could account for rapidly increasing plasma concentrations in the newborn. The disappearance of glycerol was closely described by a double-exponential model in each instance. In fetal sheep Vdist averaged 0.41 +/- 0.15 (SD) 1/kg fetal wt (n = 15). This volume decreased to 0.33 +/- 0.11 l/kg (n = 8) soon after functionally removing the placenta (by snaring the umbilical cord and maintaining the fetus with intrauterine ventilation), but the change was not significant. In newborn lambs 1-3 days of age, Vdist averaged 0.45 +/- 0.11 l/kg (n = 5, NS). Plasma clearance rate also did not change significantly, averaging 7.9 +/- 2.9, 7.9 +/- 3.8, and 9.0 +/- 5.9 ml.min-1.kg-1 in the fetus, after simulated birth, and in the newborn lamb, respectively, kd also was not altered measurably and averaged 0.020 +/- 0.006, 0.024 +/- 0.007, and 0.019 +/- 0.007 min-1 during the same time periods. Similar results were obtained by using three widely different amounts of infused glycerol. The results indicate that removal of glycerol does not depend on placental function to an appreciable extent. It is concluded that plasma glycerol concentration reflects principally glycerol turnover and, hence, lipolysis before and after birth.

Oxygen supply and the placenta limit thermogenic responses in fetal sheep.

The aim of this study was to assess the individual effects of cooling, increased oxygenation, and umbilical cord occlusion on nonshivering thermogenesis in utero. A cooling coil was placed around eight fetal sheep of 132-145 days gestation; thermistors were placed in the fetal esophagus and maternal iliac artery, vascular catheters and a tracheal catheter were inserted, and a snare was placed loosely around the umbilical cord. The next day cold water was circulated through the coil for 5 h. During the 1st h of cooling alone, fetal core temperature fell 2.79 degrees C, but indexes of brown fat activity increased only slightly. After ventilation with O2, plasma free fatty acid concentration (FFA) rose 7.4-fold to 244 +/- 42 mu eq/l, glycerol concentration rose fourfold to 376 +/- 85 microM, and the difference between brown fat and core temperature widened to 0.60 +/- 0.10 degrees C. Ventilation with N2-enriched air did not evoke similar responses. After snaring the umbilical cord while ventilation was continued, FFA rose to 554 +/- 95 mu eq/l, glycerol rose to 684 +/- 76 microM, and the temperature difference widened to 0.77 +/- 0.13 degrees C. Whole-body O2 consumption peaked at 19.6 ml.min-1.kg-1 of fetal tissue. We conclude that fetal thermogenic responses are limited in part by O2 delivery to brown fat and are augmented by occlusion of the umbilical cord.