Comparison between a radioimmunoassay and a radioreceptor assay for thyrotrophin-releasing hormone (TRH)

Two techniques for the assay of TRH, a radioimmunoassay (RIA) and a radioreceptor assay (RRA) were compared for sensitivity, specificity and the absolute values of TRH to which they lead. The radioimmunoassay of TRH was carried out with an antiserum obtained in the laboratory and having an affinity of the order of 1.3x10-9M. The radioreceptor assay uses a cultured antehypophysis strain (clone GH3); these cells have binding sites with a pronounced affinity for TRH (Ksub(m):4x10-9M); the latter stimulates the release of prolactin into the culture medium. The radioimmunoassay sensitivity (4pg) is 50 times higher than that of the radioreceptor (200pg). None of the hypothalamus hormones tested show any interference in the two techniques and neither do the TRH degradation products. On the other hand, methylated analogues of TRH, especially the compound [Nsup(tau)-Me-His]-TRH, can be recognized to varying degrees by the antiserum as well as by the GH3 cell receptors. The measurement of biological samples by each of these assay methods leads to comparable absolute values, provided that partial purification has been carried out beforehand. The two techniques are therefore complementary: although the radioimmunoassay is more sensitive, the radioreceptor assay can be used for correlating the binding of a substance on antehypophysis receptors with its biological activity on the release of prolactin. (author)