An In Vitro Effect of Triiodothyronine on Rat Adipose Tissue*

Abstract

Triiodo-L-thyronine (T₃) added in vitro to fat pads from normal, or propylthiouracil-treated rats enhanced the rate of release of glycerol and free fatty acids (FFA) in the presence of epinephrine. An effect of T₃ was also demonstrated in the presence of adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone, or glucagon in studies with tissue from normal rats. The minimal effective concentration of T₃ was approximately 2.5 × 10^-5 mole/liter for intact fat pads and 3 × 10^-6 mole/liter for fat cells. With fat pads from propylthiouracil-treated rats the effect of T₃ was not apparent until the 3rd hr of incubation. Enhancement of epinephrine-stimulated lipolysis by T₃ was evident during the 1st hr of incubation of fat pads from normal rats, and fat cells responded almost immediately to the presence of T₃. When added alone or in the presence of theophylline, 3′,5′-adenosine monophosphate or its dibutyryl derivative, T₃ had little or no effect on lipolysis. The effect of T₃ was observed with or without glucose in the medium, and was not inhibited by cycloheximide or actinomycin D. It did not persist when tissues, after incubation in the presence of T₃ were transferred to medium without T₃. No effect of T₃ on glucose uptake in the presence of epinephrine, ACTH, or insulin was demonstrated.