Development of brown adipose tissue in the rat and thyroxine administration influence.

The role of the thyroid hormone in the activity of brown adipose tissue (BAT) is in most papers underestimated and it is the noradrenaline which is considered as the main regulator. The aim of the presented experiments was the study of the effect of 2 or 3 administrations of thyroxine (T4) applied to young rats of different age on the weight of BAT. T4 was injected in relatively large doses, i.e. 1 microgram/gram BW/day dissolved in 0.005 mmol NaOH intraperitoneally. Changes in T4 injected animals were compared with their litter-mate control, which were injected with solvents. In control animals the relative weight in mg/100 g BW was significantly higher during the entire first postnatal month in comparison with adult animals. Three administrations of T4 caused significant increase in relative as well as in absolute weight in all age groups. The difference was significant in dry tissue also. When the change was expressed in percentage the greatest effect of T4 was found in 15- and 19-day-old animals. Another experiment followed the effect of only two injections of T4 on BAT to animals of 7 different litters but of the same age, i.e. on day 5 and 6 and euthanized as 7-day-old. In all litters the increase after T4 treatment was significant; the increase in % ranged from 20 to 40% and was influenced by the level in controls. The relative weight in control animals ranged from 620 to 920 mg/100 g BW. This between-litter variance is greater than the effect of T4.(ABSTRACT TRUNCATED AT 250 WORDS)