The influence of partial food deprivation on the quantity and source of triiodothyronine in several tissues of athyreotic thyroxine-maintained rats

In the present study the influence of partial food deprivation (PFD) on the quantity and source of T₃ [ i.e. T₃ derived from local T₄ to T₃ conversion (Lc T₃ (T₄)) vs. plasmaderived T₃] in several rat tissues was investigated. Two groups of athyroid rats on a synthetic diet received a continuous iv infusion consisting of T₄ (1.0 µg/100 g BWday), [¹²⁵I]T₄, and [¹³¹I]T₃ over a prolonged period. For one group of rats the daily food intake was restricted by one third to maintain constant BW during the infusion period. At isotopic equilibrium the mean plasma T₃, T₄, and TSH levels for control-fed rats were: 38 µg/dl, 5.1 µg/dl, and 470 µg/ml, respectively. The values for rats on PFD were T₃: 22 µg/dl, T₄: 4.8 µg/dl, TSH: less than 70 µg/ml. The [¹²⁵I]T₃ and [¹³¹I]T₃ contents of whole homogenates from liver, kidney, thigh muscle, cerebral cortex, cerebellum, and anterior pituitary gland as well as the subcellular fractions from liver, kidney (nuclei, mitochondria, microsomes, and cytosol), and anterior pituitary gland (nuclei) were determined (after extraction in ethanol) by thin layer chromatography. The contribution of Lc T₃(T₄) and the total T₃ levels in these tissue preparations could then be calculated. In the cerebral cortex, cerebellum, and anterior pituitary gland of PFD rats plasmaderived T₃ as well as Lc T₃(T₄) was decreased. The total T₃ level in the liver did not change under PFD, owing to an increase in Lc T₃(T₄). It is possible that the release of hepatic Lc T₃(T₄) into the blood stream was reduced. In neither group was there appreciable Lc T₃(T₄) in muscle. In contrast to the other tissues investigated, the [¹³¹I]T₃ tissue-plasma ratio for muscle had increased under PFD, suggesting a higher uptake of T₃. As a consequence, the T₃ levels in the muscles of PFD rats did not differ from those in normally fed animals. For both groups of rats the contribution of Lc T₃(T₄) in hepatic nuclei was far lower than that found for the other hepatic cellular fractions. This would suggest that the hepatic nucleus preferentially takes up plasma-derived T₃. In both control-fed and PFD rats the bulk of renal T₃ appeared to be exchangeable with plasma T₃. Hence the T₃ levels in renal nuclei were reduced under PFD. The nuclear T₃ levels in the anterior pituitaries from PFD rats were markedly decreased. Therefore it is likely that other factors determine TSH secretion under PFD.