Ontogenesis of brown adipose tissue thermogenesis in the rabbit

Basal and norepinephrine (NE) stimulated oxygen consumption was measured in BAT cells isolated from fetal and newborn rabbits at 24, 28, and 31 days gestation, and 3 and 10 days postnatal age. Maximum catecholamine stimulated respiration was measured at a final NE concentration of 10(-6) M. Cell diameter, calculated cell volume and mitochondrial alpha glycerophosphate dehydrogenase (alpha GPD) were also determined at each age. Basal respiration increased continuously during fetal and neonatal life from a mean (+/- SEM) of 12.2 +/- 0.4 mul O2/10(6) cells . hr at 24 days gestation to a mean of 70.5 +/- 4.2 mul O2/10(6) cells . hr at 24 days gestation to 670.5 +/- 60.4 mul O2/10(6) cells . hr at 31 days. A further increase of 30% measured at 10 days of age was also observed. BAT mitochondrial alpha GPD activity was several fold greater than that measured in liver. Both cell size and enzyme activity increased in parallel to increasing respiratory response to NE. We conclude from this data that the maturation of catecholamine stimulated BAT thermogenesis occurs primarily in the fetus prior to delivery. This increase is associated with both an increase in mitochondrial enzyme activity and an increase in cell size.