Inhibition of insulin secretion by L-thyroxine and D-thyroxine treatment in rats under the influence of drugs affecting the adrenergic nervous system.

Both L-thyroxine and D-thyroxine induced an inhibition of glucose-induced insulin secretion with comparable time- and dose-dependent characteristics. L-thyroxine was ten times more potent than D-thyroxine. While L-thyroxine and a ten times higher dose of D-thyroxine had a similar potency in inducing hyperthermia and hypocholesterolaemia, hyperglycaemia in response to D-thyroxine was less pronounced than in response to L-thyroxine. This difference may be explained by a greater depletion of liver glycogen stores and consequently more limited capacity for provision of glucose for the circulation. The results support the view that the differences between L-thyroxine and D-thyroxine are quantitative. Adrenergic contribution to L-thyroxine- and D-thyroxine-induced inhibition of insulin secretion by rat pancreas is apparently of minor importance. Treatment of the rats with propranolol as well as with reserpine or 6-hydroxydopamine did not alleviate L-thyroxine- or D-thyroxine-induced inhibition of insulin secretion by rat pancreas.