The Effects in Rabbits of Thyroidectomy and Treatment with Triiodothyronine on the Sensitivity to Noradrenaline and the Content of Noradrenaline in Aorta and Spleen

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Abstract
The sensitivity of spiral strips of aorta and strips of isolated spleen to sympathomimetic amines and nicotine has been studied in the thyroidectomised rabbit and in rabbits treated with triiodothyronine. Aortic strips from hyperthyroid rabbits were less sensitive to noradrenaline and less able to contract than those from normal rabbits; they also usually failed to respond initially to nicotine or tyramine although the nicotine response could be restored by exposure of the tissues to noradrenaline. Strips from hypothyroid rabbits were slightly less sensitive to low doses of noradrenaline than the normal although their maximal responses to high doses was greater. The dose-response curves for noradrenaline of spleen capsules from hyperthyroid and hypothryoid rabbits were more shallow than the normal. While spleen capsules from both experimental groups responded to nicotine with tachyphylaxis, they failed to respond to tyramine. Exposure of the capsules to noradrenaline temporarily restored the nicotine response. The noradrenaline content of aorta and spleen from hyperthyroid and hypothyroid rabbits was determined and in each case was found to be reduced to about 50 per cent of normal. These observations indicate that not all sympathetically innervated tissues of the hyperthyroid rabbit develop an increased sensitivity to sympathomimetic amines. Changes in tissue noradrenaline levels during periods of altered thyroid activity do not explain the altered sensitivity of the aorta and spleen to noradrenaline.