Thyrotropin-releasing hormone-like bioactivity in placenta: evidence for the existence of substances other than Pyroglu-His-Pro-NH2 (TRH) capable of stimulating pituitary thyrotropin release.

The purpose of this study was to investigate the chemical nature of the TRH-like bioactivity and immunoreactivity in extracts of human placenta. Methanolic extracts of human placenta contained nearly 36 ng TRH-like bioactivity/g dried tissue. The immunoreactivity of this extract was only 3 ng/g dried tissue. Two molar acetic acid extracts of human placenta yielded 9 ng TRH-like bioactivity/g dried tissue. The immunoreactivity of these acetic acid extracts, however, was 5.5 ng/g dried tissue. When these placental extracts were subjected to gel filtration chromatography, the bioactivity was found to reside in two fractions which were distinct from synthetic TRH. Furthermore, the immunoreactivity present in these placental extracts was also chromatographically distinct from that of synthetic TRH. In conclusion, these experiments confirm the presence of substantial quantities of materials in placenta which possess TRH-like immunoreactivity and bioactivity. These results also argue that the immunoreactive fractions are not bioactive and provide firm evidence that neither the TSH-releasing substances nor the TRH-like immunoreactivity found in human placenta are identical to pyroglu-His-Pro-NH2.