Suppressive action of endogenous adenosine on ovine fetal nonshivering thermogenesis

Nonshivering thermogenesis is not initiated when the fetal sheep is cooled in utero but appears to require the removal of an inhibitor of placental origin at birth. To test whether adenosine is such an inhibitor, we examined the effect of the adenosine antagonist theophylline on the initiation of nonshivering thermogenesis during sequential cooling, ventilation, and umbilical cord occlusion in utero. Theophylline (18 mg/kg bolus and 0.6 mg.kg-1.min-1 thereafter) was infused for 90 min before and 90 min after cord occlusion. Theophylline enhanced the nonshivering thermogenic free fatty acid (FFA) and glycerol responses before cord occlusion, raising FFA concentrations 99% to 415 +/- 60 mueq/l (P < 0.01) and glycerol levels 87% to 526 +/- 135 mumol/l (P < 0.05). These FFA (P < 0.001) and glycerol (P < 0.05) concentrations were significantly greater than the corresponding period during the birth-simulation control. Umbilical cord occlusion did not alter FFA levels but induced a 41% rise in glycerol concentrations to 774 +/- 203 mumol/l (P < 0.05). The increases in nonshivering thermogenic indexes after the administration of the adenosine-receptor antagonist suggest that the quiescent state of ovine fetal brown adipose tissue may result, in part, from the tonic inhibitory actions of adenosine and that a decrease in adenosine concentrations enhances nonshivering thermogenesis. However, the further rise after umbilical cord occlusion suggests that at least one other inhibitor of placental origin inhibits nonshivering thermogenesis before birth.

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.

A potential role for adenosine in the inhibition of nonshivering thermogenesis in the fetal sheep

Adenosine is released by the placenta into the fetal circulation and has potent antilipolytic properties in vitro. Nonshivering thermogenesis cannot be demonstrated by cooling fetal sheep in utero but can be induced by supplemental oxygenation and umbilical cord occlusion; this suggests the presence of inhibitor(s) of placental origin. To test whether circulating adenosine could be such an inhibitor, a series of experiments was carried out in nine fetal sheep at 136-145 d gestation. Birth was simulated in utero by sequentially cooling the fetus 2.49 +/- 0.23 degrees C with no change in the low levels of plasma FFA or glycerol; ventilating with O2 via an exteriorized tracheostomy tube and umbilical cord occlusion. Thermogenic indices rose markedly, and plasma FFA and glycerol concentrations peaked at 725 +/- 88 microEq/L (p < 0.01) and 771 +/- 154 mumol/L, (p < 0.001), respectively, O2 consumption rose to 20 +/- 2 mL/min/kg, and temperature increased 1.99 +/- 0.35 degrees C. The long-acting adenosine analog N6-(L-2-phenylisopropyl)-adenosine (PIA) was then infused (90 micrograms/kg bolus, then 300 micrograms/kg/h for 30 min); plasma FFA and glycerol decreased to 265 +/- 56 microEq/L (p < 0.003) and 477 +/- 102 mumol/L (p < 0.04), respectively; O2 consumption fell rapidly to 4.5 +/- 0.3 mL/min/kg (p < 0.01); temperature decreased 1.89 +/- 0.39 degrees C (p < 0.001); and fetal arterial BP decreased to 38 +/- 5 mm Hg (p < 0.004) in 30 min. A stepped dose-response study was performed in three fetal sheep.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between the fall in growth hormone secretion at birth and the onset of nonshivering thermogenesis is independent of beta adrenergic stimulation

At the time of birth, many rapid metabolic changes occur including the initiation of nonshivering thermogenesis and a rapid fall in growth hormone concentrations. To evaluate the interaction between these events 5 fetal sheep were studied at 135-144 days' gestation. The fetuses were first cooled 2.22 +/- 0.19 degrees C by circulating cold water through a coil placed around the fetal thorax and then ventilated with oxygen through an exteriorized tracheostomy tube to raise fetal arterial PO2 above 67.5 +/- 14.1 Torr. An hour later the beta adrenergic agonist isoproterenol was infused intravenously for 90 min. The fetuses were then separated from the placenta by occluding the umbilical cord. After 60 min the cooling and then the isoproterenol infusion were stopped and the responses monitored. Basal plasma free fatty acid (FFA 35 +/- 5 microEq L-1) and growth hormone (GH 141 +/- 12 ng mL-1) concentrations were not significantly altered by cooling alone, but oxygenation modestly increased plasma FFA to 237 +/- 55 microEq L-1 (P < 0.01) while GH concentrations fell to 58 +/- 27 ng mL-1 (P < 0.05). Isoproterenol administration did not significantly affect either FFA or GH concentrations. Occlusion of the umbilical cord caused a rapid nearly threefold increase in plasma FFA concentrations to 903 +/- 71 microEq L-1 (P < 0.01) and a fall in the same proportions in GH concentrations to 16 +/- 2 ng mL-1 (P < 0.005). Maximal fetal oxygen consumption was 24.2 +/- 4.4 mL kg-1 min-1. Cessation of cooling induced a significant fall in plasma FFA to 480 +/- 58 microEq L-1 (P < 0.01) and rise in GH concentrations to 46 +/- 5 ng mL-1 (P < 0.01). Following the withdrawal of isoproterenol, the fall in plasma FFA and rise in GH concentrations continued while the fetal oxygen uptake fell to 6.4 +/- 1.7 mliter kg-1 mL-1 (P < 0.01). During the study the variation in plasma GH was inversely correlated with changes in FFA concentrations (R = 0.77, P < 0.001). This study confirms that the major factors initiating nonshivering thermogenesis at birth are: sympathetic stimulation from cutaneous cooling, which was not significantly enhanced by isoproterenol; adequate oxygenation; and removal of placental inhibitor(s). The findings are in agreement with a causal relationship between the initiation of nonshivering thermogenesis and consequent rise in FFA concentrations and the rapid fall in circulating GH concentrations after birth in the lamb, independent of beta adrenergic stimulation.

Role of prostaglandin I2 and prostaglandin E2 in the initiation of nonshivering thermogenesis during the simulation of birth in utero

Because maximal nonshivering thermogenesis can commence only after occlusion of the umbilical cord, circulating stimulators and inhibitors were hypothesized to alter brown fat activity in the perinatal period. The roles of prostaglandin I2 (PGI2) and PGE2 in the initiation of nonshivering thermogenesis at birth were investigated. Indomethacin (45 mg bolus, 3 mg h-1 thereafter) was infused into 10 near-term fetal sheep to decrease prostanoid synthesis; 6 age-matched fetuses were infused with saline as controls. Sixteen hours later, birth was simulated in utero by sequentially cooling the fetus, ventilating its lungs with oxygen and occluding the umbilical cord. In the control fetuses, the plasma concentrations of PGI2 and PGE2 and free fatty acids, an index of nonshivering thermogenesis, were unaffected by cooling. Ventilation caused the concentration of PGI2 to increase 108% (P < 0.001) and that of PGE2 to decrease 26% (P < 0.05), while fatty acid concentrations increased 100% (P < 0.05). After cord occlusion, PGI2 concentrations remained elevated whereas PGE2 concentrations decreased a further 46% (P < 0.01), and fatty acid concentrations increased a further 100% (P < 0.05). In the indomethacin-treated fetuses, PGI2 and PGE2 concentrations decreased to 20% of the preinfusion values (P < 0.001) and did not change during the experiment. Cooling initiated a 300% increase in fatty acid concentrations (P < 0.05) and ventilation and cord occlusion induced no further significant changes. Thus, prostanoid concentrations follow changes in nonshivering thermogenic activity and support a regulatory role for PGI2 and PGE2 in the initiation of thermogenesis. Before birth, high concentrations of PGE2 favour suppression of thermogenesis, and after birth this inhibition is removed and there is stimulation by PGI2.

Withdrawal of placental prostaglandins permits thermogenic responses in fetal sheep brown adipose tissue

Significant nonshivering thermogenesis cannot be demonstrated in fetal sheep cooled in utero but can be induced by supplemental oxygenation and umbilical cord occlusion, which suggests the presence of inhibitor(s) of placental origin. To test the hypothesis that an ecosanoid could be such an inhibitor, we studied eight fetal sheep at 136-141 days gestation. Thermistors were placed in the fetal esophagus, a cooling tube was placed around the trunk, a tracheal cannula and carotid catheters were inserted, and a snare was placed loosely around the umbilical cord. After indomethacin infusion for 18 h, the fetuses were cooled by 2.13 +/- 0.13 degrees C by circulating cold water through the coil. Within 60 min plasma free fatty acid levels rose threefold to 245 +/- 82 mu eq/l (P < 0.01) and glycerol levels rose to 197 +/- 17 mumol/l (P < 0.01). Ventilation caused a further rise in thermogenic indexes, and fetal oxygen consumption rose to 19.9 +/- 1.2 ml.kg-1.min-1. In four fetuses we ceased cooling, which caused thermogenic indexes to fall and oxygen consumption to fall to 6.9 +/- 1.1 ml.kg-1.min-1. We continued to cool three fetuses and infused prostaglandin E2 into the fetuses for 60 min; thermogenic indexes and oxygen consumption fell rapidly on infusion and rose rapidly when infusion ceased. We suggest that placental prostaglandins inhibit brown adipose tissue thermogenesis before birth and that withdrawal after placental separation is one factor in the initiation of nonshivering thermogenesis at birth.

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.

Reversible umbilical cord occlusion: effects on thermogenesis in utero

The initiation of thermogenesis at birth is an important adaptation for survival. We examined the sequential effects of cooling, increased oxygenation, and repeated episodes of umbilical cord occlusion on nonshivering thermogenesis in six fetal sheep at 139 to 145 d of gestation. The fetal sheep were cooled by circulating cold water through a coil placed around the trunk for 4 h. The fetal core temperature fell 2.47 +/- 0.24 degrees C in the first 60 min of cooling with minimal changes in plasma FFA and glycerol levels. After fetal arterial O2 tension was increased above 6.65 kPa by ventilation, fetal temperature and thermogenic indices rose significantly in 60 min. After occlusion of the umbilical cord by a reversible occluder cuff, plasma FFA levels rapidly increased to 635 +/- 69 muEq/L (p less than 0.005) by 30 min, fetal temperature increased a further 0.96 +/- 0.20 degrees C (p less than 0.001) and fetal O2 consumption peaked at 25.3 +/- 4.9 mL.min-1.kg-1. Release of cord occlusion caused a rapid fall in FFA to 149 +/- 23 muEq/L (p less than 0.005) and a fall in fetal core temperature of 0.90 +/- 0.13 degrees C (p less than 0.001) in 30 min. After irreversibly snaring the umbilical cord, the plasma FFA rose to 611 +/- 83 muEq/L (p less than 0.005) and the fetal temperature rose 0.78 +/- 0.09 degrees C (p less than 0.02). The effects on thermogenesis of interrupting and reestablishing placental flow are rapid and reversible and suggest the presence of placental inhibitors of brown adipose tissue thermogenesis.

Interdependence of arterial PO2 and O2 consumption in the fetal sheep

These experiments were undertaken to measure the effects of changing arterial oxygen tension (PaO2) on oxygen use by the fetal body (VO2). Six fetal sheep at 130-140 days gestation were prepared with an endotracheal tube, carotid artery catheter, body-core thermistor, cooling coil and loosely-applied umbilical cord snare. The next day the cord was occluded and the fetal lungs were ventilated with gas mixtures containing different concentrations of oxygen. While fetal core temperature was kept constant, fetal arterial PO2 was cycled between high and low values (span = 7 to 359 mmHg, n = 103) and O2 consumption was measured by the rate of O2 uptake from a closed-rebreathing circuit. VO2 changed directly with changes in PO2 from 10 to 40 mmHg but became insensitive to changes in PO2 above about 50 mmHg. The results were well described over the entire range by the equation: VO2 (ml/min per kg fetal wt) = -9.62 + 6.99 ln PO2(mmHg)-0.66 ln2 PO2. Thus the oxygen consumption of the near-term fetal sheep varies with changes in arterial PO2 in the physiologic range. This finding is distinctly different than the adult at rest but resembles adult tissues such as exercising muscle at VO2max. This finding is consistent with differences in fetal metabolic controls, limited cardiac reserve, and limited tissue diffusion rates in actively metabolizing tissues.

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.

Presence of a diurnal rhythm in fetal prolactin secretion and influence of maternal nutrition

The possible effect of altered maternal nutrition on fetal prolactin (PRL) secretion and the possible presence of a diurnal rhythm in fetal PRL secretion has been investigated in 11 fetuses aged between 132 and 140 days of gestation. Fetal PRL levels in 4 of 5 ewes fed once a day were high (greater than 40 ng/ml) and exhibited a distinct diurnal rhythm in PRL secretion. Fetal PRL levels in those fetuses where ewes fed 4-hourly (n = 6) were all low (less than 10 ng/ml) and did not exhibit a diurnal rhythm. This study suggests that frequent maternal feeding results in low fetal PRL concentrations and that maternal undernutrition results in high fetal PRL levels that exhibit a distinct diurnal rhythm.

Disappearance of growth hormone from plasma of fetal and newborn sheep

The disappearance of growth hormone (GH) from plasma was measured after a single intravenous injection in fetal and newborn sheep and fetal sheep after simulated birth in utero. The process was adequately described when separated into two exponential components, consistent with an inner (plasma) and outer (composite tissue) pool. Plasma clearance rate increased from 3.4 +/- 0.2 (SE, n = 6) in fetuses to 3.9 +/- 0.1 (n = 5) ml.min-1.kg-1 in newborns (P less than 0.05), but was not altered significantly after simulated delivery in utero. The volume of distribution decreased from 74 +/- 4 ml/kg before birth to 47 +/- 2 ml after natural birth (P less than 0.001). The basal secretory rate decreased from 2.4 +/- 0.2 before birth to 0.27 +/- 0.02 microgram/min after birth (P less than 0.001) and to a lesser extent after simulated delivery. The rate constant for irreversible loss, Kd, increased from 0.052 +/- 0.004 min-1 before birth to 0.093 +/- 0.002 min-1 after birth (P less than 0.001). Because plasma GH concentration in steady state equals secretory rate/(volume of distribution X Kd), one may calculate that 83% of the total decrease in GH, which occurs after birth, can be explained by diminished secretory rate, whereas 17% can be explained by more rapid loss from the plasma.