The Influence of Prolonged Hyper-and Hypothyroid States on the Noradrenaline Content of Rat Tissues and on the Accumulation and Efflux Rates of Tritiated Noradrenaline

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The influence of chronic hyper- and hypothyroidism on the uptake and retention of tritiated noradrenaline ([3-H]NA) and on the endogenous noradrenaline (NA) content of various adrenergically innervated tissues was studied in thyroidectomized and sham-operated euthyroid rats. Half of the thyroidectomized rats were treated daily with thyroxine (25 mug/kg) for 3 or 12 weeks to simulate a condition of chronic hyperthyroidism, while the other half was left untreated to form a hypothyroid group. The body weight and the heart rate of each rat were measured at the end of each experiment, and in addition, at the end of the 3 week experiment, the oxygen consumption and the plasma thyroxine levels were measured to confirm the thyroid state of the animals. At the end of both experiments, each animal was given an intravenous injection of [3-H]NA and the [3-H]NA and the total endogenous NA content of the heart and various other adrenergically innervated tissues were measured on a timed schedule, to compare the initial accumulations and the rates of efflux of [3-H]NA under different thyroid states. Although the hyperthyroid rats had higher heart rates and heart weights, they were not significantly different from the euthyroid controls with respect to their body weights, tissue NA content, or accumulation and efflux rates of [3-H]NA. In contrast, the hypothyroid rats showed significantly lower heart and other tissue weights, but higher tissue concentrations of NA and rates of efflux of [3-H]NA than the euthyroid group. In the hypothyroid state, the NA turnover appeared to be increased as the [3-H]NA efflux rate was increased from the hearts and adrenal glands. There were no significant differences between the results of the 3 week and the 12 week experiments and no evidence that prolongation of the hyperthyroid state gave different results from those found by other workers who used much shorter treatment periods and larger doses of thyroxine to develop hyperthyroidism.