[The effect of age and thyroxine on lipogenesis during ontogeny. Activity of malic enzyme]

In young rats of different ages of postnatal development the effect of short-term administration of thyroxine (T4) on the activity of malic enzyme (ME) was studied in brown adipose tissue (BAT), liver, heart and kidney. Thyroxine was injected in a dose sufficient to saturate all receptor sites, i.e. 1 microgram per 1 gram of B.W. during 3 consecutive days. The highest activities of ME were found in BAT. An increase in enzyme activities with age was observed in all tissues studied in control animals. Injections of T4 increased the activity od ME in the liver; in BAT a significant increase was found only the younger age group (6 days). In older animals (40 days) with high values in BAT of controls no further increase induced by T4 was observed. In the kidney as well as in the heart no significant changes were found after T4 administration. It is concluded that in developing animals BAT may play a more important role in lipid synthesis than considered so far. (Fig. 1, Ref. 7.)