Studies on the Sensitivity to Catecholamines After Thyroidectomy

Abstract

The present experiments have explored some effects of thyroidectomy on the response of alpha and beta adrenergic receptor mechanisms to catecholamines. The increase in diastolic pressure and the relaxation of an isolated duodenal loop in vivo have been used to assess the alpha adrenergic receptors. The increase in pulse pressure, the depletion of cardiac glycogen and the release of the free fatty acids from adipose tissue incubated in vitro have been used as the beta adrenergic receptor systems. Phentolamine was the alpha adrenergic blocking drug and nethalide or propranolol the beta adrenergic blocking drugs. The resting diastolic blood pressure and pulse pressure were unaffected by thyroidectomy. The increment in diastolic pressure following epine-phrine was greater in the normal than in the hypothyroid rats, but there was no difference in the increments of pulse pressure or duodenal loop pressure after epinephrine. Isoproterenol did not increase diastolic pressure or decrease duodenal loop pressure. The increase in diastolic pressure and the decrease in duodenal loop pressure following epinephrine were completely eliminated by alpha adrenergic blockade but not by beta adrenergic blockade. The increment in pulse pressure due to epinephrine was diminished by nethalide in both normal and hypothyroid rats. In normal rats the cardiac glycogen was depleted 75% by isoproterenol; thyroidectomy abolished this response. The effect of isoproterenol on cardiac glycogen in normal rat hearts was inhibited by beta adrenergic blockade but not by alpha adrenergic blockade. Thyroidectomy shifted the dose-response curve for the release of free fatty acids isolated from fat cells incubated with epinephrine to the right of the curve for isolated fat cells from normal rats. The release of free fatty acids from adipose tissue in the presence of epinephrine was blocked by propranolol but not by phentolamine. Isoproterenol produced a significant hypertrophy of the salivary glands of thyroidectomized rats. Propranolol did not prevent the increase in heart rate or increased sensitivity to epinephrine of adipose tissue incubated in vitro. The present experiments have shown that thyroidectomy has little effect on the adrenergic responses mediated by alpha receptors. They suggest that certain metabolic effects mediated by beta adrenergic receptors (cardiac glycogenolysis and lipolysis) become less sensitive after thyroidectomy, while another beta adrenergic response (myocardial contraction) was uninfluenced by thyroidectomy. Hypertrophy of salivary glands after isoproterenol, which is neither an alpha or beta adrenergic response, was also uninfluenced by thyroidectomy.