Thyrotropin-releasing hormone in rat pituitary stalk blood and hypothalamus: studies with high performance liquid chromatography.

Immunoreactive TRH (TRH-IR) in rat pituitary stalk blood and hypothalamus was investigated with high performance liquid chromatography in conjunction with a sensitive RIA. The TRH-IR of ethanol extracts of stalk blood resolved into three peaks, whether the blood was collected with the pituitary gland in situ or after the gland had been removed. The first peak corresponded to authentic TRH. By contrast, all the immunoreactive TRH in pituitary and hypothalamic extracts and in peripheral blood to which hypothalamic or synthetic TRH had been added eluted as a single peak with the same retention time as authentic TRH. The additional IR peaks in stalk blood did not correspond to known metabolites of TRH, and, since absent from the hypothalamus, they are unlikely to represent stored TRH precursors. Although authentic TRH constituted only 37% of total TRH-IR in pituitary stalk blood, the amount released during the first hour into stalk blood (1.7 ng) in relation to hypothalamic content (approximately 5 ng) was still high (approximately 34%) compared with that of LHRH (approximately 0.6%) and somatostatin (approximately 0.3%).