Effects of thyroid hormones on adipose tissue development in Sherman and Zucker rats

It has been reported that mild hyperthyroidism in the young Sherman rat induces adipose tissue hyperplasia, concomitant with cell size reduction, and that hypothyroidism induces opposite effects. The present experiments were designed to study the evolution of cellularity in retroperitoneal and epididymal adipose tissue during a long term thyroxine (T4) treatment or in T4-treated rats, after the treatment had been stopped. In both cases, hyperplasia was transient with the observation that the adipocyte number observed in 3-mo-old control rats was reached earlier in T4-treated rats. In hypothyroid rats, hypoplasia was also transient, because once the treatment was stopped, the cell number overtook that of controls. Zucker rats were also treated with T4, because hypoplasia has been observed in young obese (fa/fa) rats and these rats are reported to be hypothyroid. T4 treatment increased their adipocyte number up to the level of nonobese (Fa/fa) untreated rats, while hypertrophy, although reduced, was persistent. In Sherman and Zucker rats, adipose tissue lipoprotein lipase activity was decreased by T4 treatment in parallel with and perhaps because of adipocyte size reduction. We suggest that hyperplasia induced by thyroid hormones results from a precocious differentiation of preadipocytes and does not necessarily imply an increased preadipocytes multiplication.