Development of Brown Adipose Tissue Thermogenesis in the Ovine Fetus and Newborn*

Abstract

Despite the importance of brown adipose tissue (BAT) in the regulation of thermogenesis and energy expenditure in both newborn and adult mammals, the functional ontogenesis of this tissue is largely unknown. In the present study, we describe the maturation of several aspects of BAT thermogenesis in fetal and newborn sheep. Cell respiration of brown adipocytes isolated from perirenal BAT was measured using a Gilson differential respirometer. Cells were isolated from four fetal animals at 121-124 days gestation (group 1), five fetal animals at 137-140 days gestation (group 2), and five newborns between birth and 4 days of age (group 3). In addition to basal oxygen consumption, in vitro cell respiration also was measured after the addition of norepinephrine (NE), (Bu)2cAMP, alpha-glycerophosphate (alpha GP), and butyric acid. Mean (+/- SEM) basal respiration (in microliters of O2 per 10(6) cells/h) increased from 11 +/- 1 in group 1 to 31 +/- 2 in group 2 and 45 +/- 7 in group 3. Cell volume increased from 9 +/- 1 pl in group 1 to 13 +/- 2 pl in group 2 and 18 +/- 2 pl in group 3. After adjustment for variations in basal respiration due to differences in cell volume, basal respiration in group 2 was greater than that in group 1 and equal to that in group 3. Maximal NE (10(-6) M)-stimulated respiration increased from 74 +/- 16 in group 1 to 294 +/- 47 in group 2. Maximal NE-stimulated respiration in group 3 (133 +/- 30) was less than that in group 2, but equal to that in group 1. (Bu)2cAMP-stimulated respiration increased from 51 +/- 12 in group 1 to 175 +/- 22 in group II, with no further increase in group III. Neither NE- nor (Bu)2cAMP-stimulated respiration varied significantly with cell volume. alpha GP substrate respiration demonstrated significant increases from group 1 to group 2, with another significant increase in Group 3. Butyric acid substrate respiration in group 1 was less than those measured in groups 2 and 3, while respiration values in groups 2 and 3 were equal. After adjustments for variations due to differences in cell volume, the patterns of development of both alpha GP and butyric acid substrate respiration were unaltered. The following conclusions were reached 1) Full maturation of BAT catecholamine-stimulated cellular respiration occurs before delivery near term in the ovine fetus. 2) In the neonatal lamb, a decrease in catecholamine-stimulated respiration occurs without a decrease in (Bu)2cAMP-stimulated respiration. This suggests that a decrease in BAT sensitivity to NE occurs after delivery at the receptor adenyl cyclase level. 3) The perinatal increase in alpha GP substrate respiration without an increase in butyric acid substrate respiration suggests that mitochondrial alpha-glycerophosphate dehydrogenase activity is increased. This was confirmed by measuring increased alpha-glycerophosphate dehydrogenase activity in crude BAT mitochondrial fractions in group 3 animals.

A Potential Role for Adenosine in the Inhibition of Nonshivering Thermogenesis in the Fetal Sheep

“…In this study cooling the fetal sheep failed to cause plasma FFA and glycerol concentrations to increase, confirming the results of our previous studies that showed that when the fetal sheep is cooled in urero it produces little heat from nonshivering thermogenesis in brown fat (1). This failure to respond is observed despite the capability of ovine fetal brown fat cells to respond to thermogenic stimuli in virro (2) and the ability of prematurely delivered and term-delivered lambs to produce significant amounts of additional heat from nonshivering thermogenesis in brown fat (3). Ventilation with 0, in our study caused modest thermogenic responses, and umbilical cord occlusion was followed by further increases in fetal core temperature, plasma FFA and glycerol concentrations, and 0, consumption.…”

“…0.35"C. The long-acting adenosine analog N6-(L-2-phenylisopropy1)-adenosine (PIA) was then infused (90 p@g bolus, then 300 p@gh for 30 min); plasma FFA and glycerol decreased to 265 + 56 pEq/L (p < When a fetal sheep is cooled in utero, nonshivering thermogenesis is not initiated, as evidenced by the failure of plasma FFA and glycerol concentrations to increase and the lack of additional heat production by brown adipose tissue (1). The failure of this thermogenic response is observed despite the capability of ovine brown fat cells to respond to thermogenic stimuli in vitro (2) and despite the ability of prematurely delivered and term-delivered lambs to thermoregulate (3). These findings indicate that brown adipose tissue is competent but unresponsive before birth.…”

Nutrition, temperature and homeostasis during perinatal development

“…This coincides with the period in which in vitro isoprenaline-stimulated adenylate cyclase activity and noradrenaline-stimulated respiration are maximal (Klein et al 1983). However, in contrast to the peak in mRNA for uncoupling protein (Casteilla et al 1989) and thermogenic activity (Symonds, 1995), in vitro indices of BAT function decrease between late gestation and after birth (Klein et al 1983;Casteilla et al 1994).…”

Effect of Fetal Thyroidectomy on Newborn Thermogenesis in Lambs

“…Mitochondria1 aGPD plays a pivotal role in this system, and aGPD activity doubles during the perinatal period in sheep (8,20). The present observations that aGP substrate failed to in- crease BAT respiration in the hypothyroid animals and that measured BAT aGPD activity was not reduced suggests that BAT aGPD is not affected by hypothyroidism.…”

“…BAT was isolated and oxygen consumption measured using our previously described methods (7,8). Briefly, BAT was excised from the perirenal deposits of one of each twin animal ( n = 4 thyroidectomized and 4 sham-operated fetuses) and placed in modified Krebs Ringer bicarbonate buffer at room temperature for transport to the laboratory.…”