Thyrotrophin releasing hormone and related peptides in canine tissues

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Abstract
Thyrotrophin releasing hormone (TRH)-immunoreactive peptides have been quantified in canine serum, hypothalamus, liver, pancreas, adrenal, thyroid, prostate, testis, epididymis and semen by TRH radioimmunoassay, SP-Sephadex C-25 cation exchange chromatography, Sephadex G-10 exclusion chromatography and high pressure liquid chromatography. The total concentration of TRH and TRH-like peptides was highest in the hypothalamus, followed by liver, adrenal, pancreas, thyroid, prostate, epididymis, testis and serum. All of the TRH immunoreactivity (TRH-IR) within extracts of the hypothalamus was due to TRH. On the other hand, nearly all of the TRH-IR of extracts of liver, thyroid, prostate, epididymis, testis and semen was due to TRH-homologous peptides. Adrenal and pancreatic extracts contained a greater proportion of TRH in relation to the TRH-homologous peptides. Extracts of dog serum and semen were found to contain a TRH-binding substance which reduced the retention of added TRH by cation exchangers. The half-time of disappearance (t½) of synthetic TRH incubated at 23 °C in 10% (w/v) homogenates in 0·15 m-NaCl–0·05 m-phosphate buffer, pH 7·5, ranged from 22±10 (s.d.) min for liver to 120 ±58 min for thyroid. The short t½ for TRH added to dog liver homogenates contrasted with a previous report that dog liver is essentially free of TRH-degrading activity.