Differential Effects of Hypothyroidism on the Specific Uptake of Growth Hormone and Prolactin by the Rat Liver*

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Abstract
The possible effects of severe hypothyroidism on the specific uptake of ovine PRL (oPRL) and bovine GH (bGH) by the liver have been investigated in the rat. Female rats were thyroidectomized (T). These animals and age-paired controls (C) were injected with less than 20 ng [125I]iodo-oPRL or [125I]iodo-bGH alone or in combination with 100–200 µg native oPRL, bGH, or human GH per 100 g BW. The animals were killed at different intervals, and the total 125I was determined in plasma and liver. From these data, the liver to plasma 125I ratio was calculated. In some experiments, the proportion of 125I which was soluble in 10% trichloroacetic acid was also determined. The T rats used were all severely hypothyroid. A specific uptake of [125I]iodo-oPRL by the liver could be shown for C rats. It was, however, markedly decreased in T rats (in agreement with previous findings showing a decrease of liver lactogenic binding sites) studied in vitro. On the contrary, the specific liver uptake of [125I]iodo-bGH was not decreased in T rats. Some differences were, however, observed between T and C rats with respect to saturation of the liver uptake of bGH and the metabolism of the [125I]iodo-bGH. The amount of native bGH required to reduce the liver to plasma 125I ratio after injection of [125I]-iodo-bGH by 50% was smaller (1.5 µg/100 g BW) in T rats than in C animals (5 µg/100 g BW). The rate of degradation of the labeled hormone, as far as could be assessed from the increasing solubility of 125I in 10% trichloroacetic acid, was slower in T than in C animals. Present findings indicate that the specific liver uptakes of bGH and oPRL are not affected in the same manner by severe hypothyroidism.

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