Lipolysis in isolated epididymal adipocytes from genetically obese Zucker rat treated with 3,5,3'-L-triiodothyronine

Glycerol released by isolated epididymal adipocytes was measured in 6-8 weeks old genetically obese (fa/fa) and lean (C.c.a/-) male Zucker rats, ip treated or not treated with T3 (0.46, 2.3 and 4.6 nmol/100 g in a single injection or 4.6 nmol.(100 g)-1.day-1 for 5 days). In the non-treated rats, lipolysis was less stimulated by adrenalin or isoproterenol in obese than in lean rats, whereas it was stimulated by theophylline up to the same level in both kinds of rats. A single injection of the T3 (highest dose) increased isoproterenol-induced lipolysis in the lean but not in the obese rats. The 5-day T3 treatment caused a stimulation of the adrenalin- or isoproterenol-induced lipolysis in both kinds of rats. However, the stimulation was lower in the obese than in the lean rats. This treatment increased theophylline-induced lipolysis in the lean but not in the obese rats. It did not affect significantly the theophylline-induced lipolysis stimulated by adrenalin or isoproterenol in either obese or lean rats. These findings show that mild hypothyroidism of obese rat is partly responsible for the impaired sensitivity of its adipose tissue to lipolytic drugs. The apparently low T3 sensitivity of obese rat adipose tissue is discussed.