Influence of hypo- and hyperthyroidism on plasma catecholamines in pithed rats

Abstract

Altered thyroid states are known to produce profound changes in sympathetic mechanisms. The influence of thyroid hormones on plasma catecholamines, adrenal catecholamines and on circulatory parameters were studied in pithed rats. Animals were made hypothyroid by feeding 6-propyl-2-thiouracil for 6 weeks. Hyperthyroidism was induced by triiodothyronine injections (0.5 mg/kg body weight) for 7 days. 1. The basal heart rate was decreased in hypothyroidism and accelerated in hyperthyroidism. Basal diastolic blood pressure was reduced in both dysthyroid states. 2. Electrical stimulation of autonomic outflow from the spinal cord induced tachycardia and increased diastolic blood pressure. The increase in heart rate due to electrical stimulation was reduced in hypo- and hyperthyroidism. In hyperthyroid animals this may be due to the already accelerated basal heart rate. The initial rise in diastolic blood pressure was decreased in the hypothyroid and increased in the hyperthyroid state. 3. Basal adrenaline plasma levels were higher than control values in hypothyroidism and unaltered in hyperthyroidism. Basal noradrenaline levels were not significantly influenced by either thyroid state. 4. The increase in plasma noradrenaline during electrical stimulation was enhanced in hyperthyroid animals and remained unaltered by hypothyroidism. 5. The increase in plasma adrenaline during electrical stimulation was significantly changed in both dysthyroid states. Hyperthyroidism reduced the initial peak of adrenaline release. Hypothyroidism raised plasma adrenaline in the steady state after sustained stimulation. 6. Elevated basal and stimulated adrenaline plasma levels corresponded to an increased adrenaline content and enhanced phenylethanolamine-N-methyltransferase activity in adrenal glands of hypothyroid rats.