Thyrotropin response to thyrotropin-releasing hormone in the euthyroid rat: dose-response, time course, and demonstration of partial refractoriness to a second dose of thyrotropin-releasing hormone.

Abstract

The concentration of TSH was measured in plasma and anterior pituitary after iv injection of TRH into groups of euthyroid rats. Integration of plasma TSH curves for 120 min after administration of graded doses of TRH (0.05–5.0 μg/100 g BW) showed that a maximal TSH response was achieved with a TRH dose of 1.0 μg/100 g BW or greater. Mean plasma TSH increased to near maximal values at 5 min and to its highest concentration (400–800% increase) 20 min after injection of 5 μg TRH/100 g BW. After 20 min, plasma TSH decreased at a constant fractional rate to pre-TRH injection values, suggesting cessation of TRH-induced TSH secretion after 20 min. Pituitary TSH decreased to a nadir at about 20-30 min after TRH. Moreover, the mean maximal decrement in pituitary TSH (40–45% decrease from control) was equivalent to the amount of TSH metabolized from 0–120 min. The latter was calculated from the mean plasma TSH concentration during this interval and the metabolic clearance rate of TSH. The cessation of TRH effect af...