Ultrastructural and Biochemical Changes in Brown Fat in Cold-Exposed Rats

Abstract

During the first 3 days of exposure of rats to 5°C, the nitrogen concentration of interscapular brown fat increased by 50% and remained at this elevated level for the duration of the 8-wk observation period, while the mass of tissue increased fourfold. The concentration of both DNA and RNA per unit nitrogen reached a maximum after 3 days, then declined; however, the total quantity of each continued to rise. The concentration of various respiratory enzymes decreased during the first few days and then increased, but at different rates. The morphological changes in mature brown fat cells during cold acclimation were observed to be: a reduction in fat droplet size during the first 3 days, followed by a gradual increase in size through 6 wk in the cold; a continual increase in the amount of intermitochondrial ground substance during the first 3 wk, with increased granularity and glycogen content after 1 wk; initial disappearance of glycogen between mitochondria, followed by the reappearance of a few isolated particles in the intermitochondrial ground substance after 1 wk in the cold; initial increase in the density of intramitochondrial matrix for the first 3–4 days, followed by a gradual return to the control density; loss in integrity of mitochondrial outer membranes during the first 4 days, followed by gradual but incomplete restoration; temporary loss of the dense material in lipid droplets during the first 24 hr, with return after 1 wk in the cold; and a 40% increase in mitochondrial diameter within 1 day, followed by a decrease in diameter within 1 wk to a constant value about 15% larger than the controls.

Comparison of Uptake of Multiple Clinical Radiotracers into Brown Adipose Tissue Under Cold-Stimulated and Nonstimulated Conditions

“…1B and 1C). Thomson et al reported that a reduction in fat droplet size in BAT was observed during the first 3 d of stimulation by cold and was followed by a gradual increase in size over a 6-wk period (15). In our study, BAT changed to a reddish and dark color after exposure to cold (Fig.…”

Morphological studies on brown adipose tissue in the bat and in humans of various ages.

“…The organized elements of the granular endoplasmic reticulum shown by the electron microscope have been overlooked by many authors. THOMSON et al (1969) reported, however, that, in the immature brown fat cells, both the granular and agranular endoplasmic reticulum were found in considerable amounts, but none were seen in large mature cells. To the contrary, almost all investigators noticed numerous free ribosomes widely distributed singly or grouped into rosette-like clusters.…”

“…25). He further showed that cytolysomes occurred far more frequently in the brown fat cells of the April bat; especially those containing a degenerating mitochondrion were found THOMSON et al (1969), closely packed mitochondria October (fat repletion stage) (Fig. 19).…”

“…BARNARD (1969) and SUTER (1969a) demonstrated that, in immature brown fat cells of rat fetuses, glycogen particles accumulated prenatally, but were depleted within several hours after birth. This glycogen depletion and repletion in brown fat cells have been examined in cold-exposed and coldacclimated rats (SUTER, 1969b;THOMSON et al, 1969). UMAHARA (1968) could not demonstrate glycogen accumulation in bat brown fat cells.…”

Regulation of fructose 2,6‐bisphosphate levels in cold‐acclimated brown adipose tissue of rat

“…Cold adaptation is closely related to an increase in the amount of brown adipose tissue (BAT) and mitochondria [1][2][3]. In addition to the role of fatty acid as the major fuel during cold exposure, recent studies have indicated that BAT plays an important role in regulating blood glucose concentration after a carbohydrate load, especially in coldacclimated rats [4,5].…”