Effects of 3,5,3'-triiodothyronine (T3) on rat liver peroxisomal compartment during cold exposure.

In this study, by enzymatic assays and morphometric-stereologic analyses, we show that the remodelling of the rat liver peroxisomal compartment, occurring during cold exposure, is largely influenced by the thyroid state of the animal. The research was performed both on the whole rat liver peroxisomal population and on 2 peroxisomal subpopulations: one having a diameter greater than 0.5 microns, sedimenting at 10,000 g and called Pe (Peroxisomes); and the other having a diameter less than 0.5 microns, sedimenting at 27,000 g and called sPe (small Peroxisomes). The increase in the peroxisome number and in the peroxisome volume densities, elicited by cold exposure, is greatly reduced in hypothyroid cold-exposed animals (-63% and -40% respectively, P less than 0.01), while triiodothyronine (T3) administration to these rats restores the values observed in normal cold-exposed animals. The "clusters" and the strict association between mitochondria and peroxisomes, more frequently found in cold-exposed rats, are rarely observed in hypothyroid cold-exposed animals. At peroxisomal enzymic level, the thyroid hormone inhibits, during cold exposure, the uricase activity (-29% in Pe and -40% in sPe), and stimulates the palmitoyl-coenzyme A oxidase activity (+44% in Pe and +88% in sPe). The results also demonstrate that T3 exerts all these effects by acting principally on sPe.