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

Fever response to intravenous prostaglandin E2 is mediated by the brain but does not require afferent vagal signaling

PGE2 produced in the periphery triggers the early phase of the febrile response to infection and may contribute to later phases. It can be hypothesized that peripherally synthesized PGE2 transmits febrigenic signals to the brain via vagal afferent nerves. Before testing this hypothesis, we investigated whether the febrigenic effect of intravenously administered PGE2 is mediated by the brain and is not the result of a direct action of PGE2 on thermoeffectors. In anesthetized rats, intravenously injected PGE2 (100 μg/kg) caused an increase in sympathetic discharge to interscapular brown adipose tissue (iBAT), as well as increases in iBAT thermogenesis, end-expired CO2, and colonic temperature (Tc). All these effects were prevented by inhibition of neuronal function in the raphe region of the medulla oblongata using an intra-raphe microinjection of muscimol. We then asked whether the brain-mediated PGE2 fever requires vagal signaling and answered this question by conducting two independent studies in rats. In a study in anesthetized rats, acute bilateral cervical vagotomy did not affect the effects of intravenously injected PGE2 (100 μg/kg) on iBAT sympathetic discharge and Tc. In a study in conscious rats, administration of PGE2 (280 μg/kg) via an indwelling jugular catheter caused tail skin vasoconstriction, tended to increase oxygen consumption, and increased Tc; none of these responses was affected by total truncal subdiaphragmatic vagotomy performed 2 wk before the experiment. We conclude that the febrile response to circulating PGE2 is mediated by the brain, but that it does not require vagal afferent signaling.

Kupffer cell-generated PGE2triggers the febrile response of guinea pigs to intravenously injected LPS

Because the onset of fever induced by intravenously (iv) injected bacterial endotoxic lipopolysaccharides (LPS) precedes the appearance in the bloodstream of pyrogenic cytokines, the presumptive peripheral triggers of the febrile response, we have postulated previously that, in their stead, PGE2could be the peripheral fever trigger because it appears in blood coincidentally with the initial body core temperature (Tc) rise. To test this hypothesis, we injected Salmonella enteritidis LPS (2 μg/kg body wt iv) into conscious guinea pigs and measured their plasma levels of LPS, PGE2, TNF-α, IL-1β, and IL-6 before and 15, 30, 60, 90, and 120 min after LPS administration; Tcwas monitored continuously. The animals were untreated or Kupffer cell (KC) depleted; the essential involvement of KCs in LPS fever was shown previously. LPS very promptly (<10 min) induced a rise of Tcthat was temporally correlated with the elevation of plasma PGE2. KC depletion prevented the Tcand plasma PGE2rises and slowed the clearance of LPS from the blood. TNF-α was not detectable in plasma until 30 min and in IL-1β and IL-6 until 60 min after LPS injection. KC depletion did not alter the times of appearance or magnitudes of rises of these cytokines, except TNF-α, the maximal level of which was increased approximately twofold in the KC-depleted animals. In a follow-up experiment, PGE2antiserum administered iv 10 min before LPS significantly attenuated the febrile response to LPS. Together, these results support the view that, in guinea pigs, PGE2rather than pyrogenic cytokines is generated by KCs in immediate response to iv LPS and triggers the febrile response.

Effect of delivery temperature on endocrine stimulation of thermoregulation in lambs born by cesarean section

We examined the hypothesis that exogenous stimulation with physiological doses of 3,5,3'-triiodothyronine (T(3)) and/or norepinephrine at birth can improve thermoregulation in near-term lambs delivered by cesarean section. This was achieved by investigating the effect of delivery temperature [i.e., warm (30( degrees )C) vs. cool (15( degrees )C) ambient temperatures] on hormonal stimulation on uncoupling protein-1 (UCP1) abundance in brown adipose tissue. In vivo measurements of temperature control (i. e., colonic temperature, oxygen consumption, and incidence of shivering) were made over the first 2.5 h after birth. Each lamb was injected with saline with or without T(3), norepinephrine, or T(3) plus norepinephrine. Irrespective of delivery temperature, abundance of UCP1 increased and incidence of shivering decreased by all hormonal treatments, but this only reduced the rate of decline in colonic temperature of cool-delivered lambs. Oxygen consumption was higher in cool-delivered lambs that were able to fully restore body temperature, an adaptation not observed in controls or any warm-delivered groups. Exogenous administration of endocrine stimulatory factors can enhance the abundance of UCP1 in cesarean-section-delivered lambs with the magnitude of thermoregulatory response being greater at cool than warm delivery temperatures.

Beta2-agonist ritodrine, unlike natural catecholamines, activates thermogenesis prematurely in fetal sheep

Prolonged administration of the beta2-adrenergic agonist ritodrine to fetal sheep increases nonesterified fatty acid mobilization. To investigate whether changes in fetal growth or functional development of brown adipose tissue (BAT) also occur, ritodrine was infused at 5 microg/min iv into eight fetal sheep (6 twins and 2 singletons at 125-128 days of gestation) for 5 days and then at twice this rate for a further 7-11 days. Fetal growth was reduced significantly (P < 0.02) during ritodrine infusion relative to controls (5.8 +/- 17.5 vs. 79.7 +/- 10.3 g/day), with growth of skeletal muscles ceasing. Ritodrine reduced perirenal BAT weight by 50% from 18.6 +/- 1.89 to 9.3 +/- 0.60 g (P < 0.01) and its lipid content by >70% from 6.5 +/- 0.96 to 1.9 +/- 0.45 g (P < 0.01). Mitochondrial protein in BAT was also less (P < 0.002), but GDP binding to uncoupling protein increased (P < 0.05). In similar experiments, epinephrine and norepinephrine increased plasma nonesterified fatty acid initially, but neither altered perirenal BAT composition. The beta2-adrenergic agonist ritodrine appears able to promote lipid mobilization and thermogenesis in utero.

Metabolic and endocrine responses to cold exposure in chronically incubated extrauterine goat fetuses

To investigate developmental aspects of metabolic and endocrine responses to cold exposure in fetuses, we conducted experiments on six goat fetuses, three aged 95-116 d of gestation (dGA; group I), and three aged 122-134 dGA (group II), using an extrauterine fetal incubation system that provided arterio-venous extracorporeal membrane oxygenation (A-V ECMO). The fetuses were cannulated via the umbilical vessels, and their blood gas exchange was totally supported by A-V ECMO, while they were maintained in an isothermal incubator containing artificial amniotic fluid. After confirming that fetuses were in metabolically stable condition in the extrauterine incubation system, fetal core temperature was lowered by 2 degrees C over 2 h by decreasing the temperature of incubating fluid from 39.5 degrees C. During and after cold exposure, fetal heart rate and arterial blood pressure remained unchanged. We observed significant increases in oxygen consumption and plasma concentrations of norepinephrine, epinephrine, adrenocorticotropic hormone, and cortisol in group II but not in group I fetuses. In addition, based on regression analysis, maximal changes of these parameters during cold exposure were linearly correlated with gestational age significantly, and the regression lines were found to intersect the x (gestational age) axis at around 98-106 dGA. These results suggest that metabolic and endocrine responses to cold exposure develop with gestational age in the goat fetus, the responses being manifested around 100 dGA.

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.

Effect of beta 3-adrenergic agonist (Zeneca D7114) on thermoregulation in near-term lambs delivered by cesarean section

We investigated the effect of a beta 3-adrenergic agonist, Zeneca D7114, on thermoregulation in near-term lambs delivered by cesarean section. Lambs were delivered into a cool ambient temperature of 15 degrees C, and between 60 and 80 min of life were given an oral dose of Zeneca D7114 (10 mg.kg body weight-1) dissolved in 20 mL of milk, or milk alone. During the first 0.5 h of life colonic temperature decreased in all lambs, and then increased to plateau levels (39.6-40.4 degrees C) after 120-150 min of life, in 19 out of 23 lambs studied. In the remaining lambs, colonic temperature failed to return to normothermic values, plateauing at 34.3 degrees C. All control lambs were observed to shiver throughout the study, but after Zeneca D7114 treatment 7 out of 10 normothermic lambs stopped shivering, and plateau colonic temperature was 0.8 degree C higher. Hypothermic beta 3-agonist-treated lambs had significantly lower rates of heat production, breathing frequency, and plasma triiodothyronine and cortisol concentrations than normothermic lambs. the level of GDP binding and norepinephrine content of brown adipose tissue (BAT) sampled from hypothermic beta 3-agonist-treated lambs was significantly lower than in normothermic lambs. There was no difference in GDP binding in BAT between control and Zeneca D7114-treated groups, but the Hb content was higher in the latter group. It is concluded that administration of Zeneca D7114 to euthyroid lambs enhances their ability to thermoregulate and restore colonic temperature without altering the thermogenic activity of BAT. This response may be mediated by increasing blood flow to BAT and/or an improvement in the animal's thermal efficiency (i.e. decreased heat loss) due to a reduced reliance on shivering thermogenesis.

Perinatal thermogenesis

The rapid initiation of thermogenesis is crucial for the survival of newborn infants. At birth the fetus must adapt to cooling, increased oxygenation and separation from the placenta. An experimental approach in the chronically instrumental fetal sheep of 'simulated birth in utero' allowed the evaluation of each of these stimuli sequentially. Cooling stimulated shivering, cardiovascular and endocrine responses but not nonshivering thermogenesis (NST). Ventilation of the cooled fetus with oxygen caused only modest NST which was not altered by an infusion of triiodothyronine. Occluding the umbilical cord was followed by a rapid substantial rise in NST which was maintained until the placental circulation was re-established. Thus the placenta is secreting factors into the fetal circulation which inhibit the ability of the brown adipose tissue to respond to either hormonal or neural stimuli. Placental prostaglandin E2 and probably adenosine are tonic inhibitors of thermogenesis in utero. Effective thermogenesis after birth requires the combination of separation from the placental inhibitors of lipolysis, increased oxygenation from breathing and the stimulation of cutaneous cold receptors.

Role of prostaglandin E2 and prostacyclin in nonshivering thermogenesis during simulated birth in utero

Prostaglandin E2 (PGE2) inhibits and prostacyclin (PGI2), stimulates lipolysis in vitro. Their role in initiating nonshivering thermogenesis at birth was investigated in 16 fetal sheep at 129-143 days gestation. In 10 fetuses indomethacin, a prostaglandin synthesis inhibitor, was infused; in 6 fetuses saline was administered as a control. 16 h later birth was simulated in utero. The plasma levels of PGE2 and PGI2 were unaffected by cooling. In the control fetuses, ventilation with oxygen caused PGE2 to fall, PGI2 to rise, and initiated moderate thermogenesis, signaled by a twofold increase in plasma free fatty acids (FFA). After umbilical cord occlusion, PGE2 decreased further (PGI2 was unchanged) and thermogenesis accelerated. In indomethacin-treated fetuses, in which the prostanoids had decreased and remained at approximately 20% normal, cooling initiated moderate nonshivering thermogenesis, and ventilation and cord occlusion caused no further changes. Changes in plasma adenosine were similar in control and indomethacin-treated groups. We conclude that declining PGE2 and rising PGI2 contribute to the initiation of thermogenesis at birth, but that other agents possibly of placental origin may play a contributory role.