Thyroid Hormones Modulate Thyrotropin-Releasing Hormone Biosynthesis in Tissues Outside the Hypothalamic-Pituitary Axis of Male Rats*

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Abstract
In the present study we have examined the in vivo effects of thyroid hormone and TRH on secretory tissue concentrations of TRH and TRH-Gly (pGlu-His-Pro-Gly), a TRH precursor. Within secretory granules, TRH-Gly is converted to TRH through .alpha.-amidation of the C-terminal proline residue, using Gly as the NH2 donor. Using specific RIA, we measured the TRH-Gly immunoreactivity (TRH-Gly-IR) and TRH-IR concentrations in tissues from the reproductive and gastrointestinal systems, adrenals, and other internal organs in euthyroid, hypothyroid, and T4-treated 250-g Sprague-Dawley male rats. TRH-Gly-IR concentrations were more than 2-fold higher than TRH-IR concentrations within the adrenal, pancreas, bowel, and stomach at the time of death. Untreated hypothyroidism and exogenous TRH significantly increased adrenal TRH-Gly-IR levels. Pancreatic TRH-Gly levels increased about 2-fold in hypothyroid rats. Incubation at 60 C significantly increased TRH-Gly-IR levels in the pancreas, adrenal, bowel, stomach, and epididymis by 14-, 3-, 6-, 6-, and 6-fold, respectively. Also after 60 C incubation increases in the TRH-Gly-IR/TRH-IR ratio of 2.7-, 4-, and 1.7-fold were observed in the pancreas, epididymis, and bowel, respectively. Pooled tissue extracts were fractionated by cation exchange and reverse phase HPLC for characterization of TRH-Gly-IR. Both chromatographic methods revealed a major peak of TRH-Gly-IR coeluting with synthetic TRH-Gly. Incubation at 60 C caused 13.5-, 4.1-, 1.5-, and 5-fold increments in the TRH-Gly-IR for adrenal, pancreas, prostate, and thyroid, respectively, compared to the immediately extracted control aliquots. Cation exchange and reverse phase HPLC also revealed production of higher mol wt TRH precursor peptides after incubation at 60 C for 4 or 20 h. Only the TRH-Gly-IR peak coeluting with pGlu-His-Pro-Gly was converted into TRH by rat brain .alpha.-amidating enzyme. The data suggest that biosynthesis of TRH occurs in rat extrahypothalamic tissues and may be modulated by thyroid status, iv TRH, and selective thermal inactivation of enzymes that convert prepro-TRH to TRH.