Inhibitory Action of Thyrotropin-Releasing Hormne on Serum Amylase Activity and Its Mechanism*

The effect of im administration of 500 micrograms TRH on serum amylase activity was studied in 34 normal women, 6 women with primary hypothyroidism, 1 woman with anorexia nervosa, 6 women with hyperthyroidism due to Graves' disease, and 5 women with renal failure on chronic hemodialysis. Serum amylase activity decreased significantly in 31 of 34 normal subjects 60 to 120 min after administration of TRH. However, amylase isoenzymes were not significantly affected after administration of TRH, suggesting that TRH equally affects pancreatic and salivary amylase activity. TRH was also effective in patients on chronic hemodialysis, indicating that TRH does not reduce serum amylase activity by increasing urinary excretion of amylase. Since TRH reduced serum amylase activity in hyperthyroid patients in whom TRH failed to stimulate TSH secretion, TRH does not reduce serum amylase activity through increased secretion of TSH. The action of TRH was not mediated by serum thyroid hormone levels since TRH was similarly effective in patients with hypothyroidism or hyperthyroidism. TRH did not inhibit or interfere with amylase determination when added in vitro to heparinized blood. Perfusion of the dog pancreas with TRH reduced amylase activity in pancreatic juice and pancreatic venous blood. The magnitude of the decrease was related to the dose of TRH used. Since decrease in amylase activity of the pancreatic juice preceded that in pancreatic venous blood, TRH probably directly acts on the pancreas to reduce amylase secretion. As a result, serum amylase activity decreased after administration of TRH.