Elevation of Thyrotropin Release by Intrapituitary Infusion of Crude Hypothalamic Extracts

Abstract

In rats whose endogenous pituitary secretion had been depressed by phenobarbital and raised ambient temperature (34 C), increases of 131I levels in blood were measured following intrapituitary infusion (4.5-7 [mu]l/hr for 2.5-3 hr.) of extracts prepared from the hypothalami of rats and sheep. It was shown that the intrapituitary infusion technique was a sensitive method for testing thyrotropin- releasing factor (TRF) activity provided precautions were taken to prevent cytolysis due to faulty pH or tonicity of the solutions infused. Intrapituitary infusion of dialyzed extracts of rat hypothalamus and of stalk-median eminence (SME) and hypothalamic fragments from sheep at different dosages showed that the magnitude of the response increased with increasing dosage. Intrapituitary infusions of saline or intravenous infusions of sheep hypothalamic extract did not increase blood 131I levels, indicating that neither pituitary damage due to infusion nor TSH contamination of the extracts was responsible for the results. In the sheep, the concentration of TRF in the SME was approximately twice that in the hypothalamus but the latter contained 7 times as much TRF. When compared with 131I responses obtained from intravenous infusion of TSH at 3 dose levels, it was estimated that intrapituitary infusions of TRF in this experiment released only about 5% of the TSH present in the pituitary store.