In the present study we have examined the in vivo effects of
thyroid hormones and TRH on tissue and blood levels of TRH and
TRH-Gly (
pGlu-His-Pro-Gly), a TRH precursor. Using specific radioimmunoassays (RIAs), we measured TRH immunoreactivity (TRH-IR) and
TRH-Gly-IR concentrations in blood, hypothalamus, anterior and posterior pituitary, and thyroid in euthyroid, hypothyroid and
thyroxine (T4)-treated 250 g male Sprague-Dawley rats.
TRH-Gly-IR and TRH-IR were detected in all of these tissues. Highly significant positive correlations between whole blood
TRH-Gly-IR levels and the corresponding serum TSH values (p less than 0.01), whole blood TRH-IR versus serum TSH (p less than 0.01) and whole blood
TRH-Gly-IR versus whole blood TRH-IR (p less than 0.01) are consistent with cosecretion of TRH and TRH precursor
peptides into the circulation. Euthyroid rats injected with TRH IP (1 microgram/100 g b.wt.) and hypothyroid rats had 4-fold higher whole blood
TRH-Gly-IR levels compared to euthyroid controls (p less than 0.0005). Injection of TRH into euthyroid rats significantly increased the
TRH-Gly-IR concentration in the hypothalamus, anterior and posterior pituitary and thyroid. The increase in blood
TRH-Gly-IR following intravenous TRH may be due, in part, to partial saturation of TRH-degrading
enzymes in blood and cell membranes. The ratio of
TRH-Gly to TRH was significantly increased in the anterior pituitary by
hypothyroidism and TRH injection, suggesting that
thyroid hormones and TRH regulate the alpha-amidation of
TRH-Gly to form TRH in this tissue.
TRH-Gly levels of pooled pituitary and thyroid extracts quantitated by a combination of
TRH-Gly RIA and high performance liquid chromatography (HPLC) revealed several-fold increases following incubation at 60 degrees C. Heating at this temperature may block the alpha-amidation activity in extra-hypothalamic tissues but not the "
trypsin-like"
enzymes which cleave
prepro-TRH into
TRH-Gly-immunoreactive
peptides.