Relationship between Radioreceptor Assay and Radioimmunoassay Estimates of Prolactin in Rat Pituitary Tissue, Incubation Medium, and Serum: Effects of Dialysis on Measurements of the Hormone*

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Abstract
The relationships between radioreceptorassay (RRA) and RIA estimates of PRL in rat pituitary tissue, in medium in which rat adenohypophyses were incubated, and in serum were determined. The RRA involved l25I-labeled ovine PRL and hepatic PRL receptors from female rats which bore a PRL-secreting pituitary tumor (MtTWl5). A homologous RIA from rat PRL was used for comparison. Highly significant correlations were obtained between both assays with the three types of samples, with r values of 0.90,0.97, and 0.94, respectively. Regression analyses revealed that the RRA estimates of PRL in pituitary homogenate and incubation medium were slightly but significantly higher than those obtained with the RIA, but the RRA measured about 3.5 times more PRL in serum than did the RIA. The results indicate that the RRA gives estimates of PRL in rat serum that are, relative to RIA measurements, quantitatively similar to those obtained previously by bioassay. However, the RRA is subject to some degree of nonspecific interference from cross-reacting materials because serum from hypophysectomized rats with little or no radioimmunoassayable active PRL showed significant activity in the RRA. Dialysis reduced the amount of radioreceptor assayable and radioimmunoassayable active PRL in serum to a greater degree than it did with pituitary homogenate or incubation medium, and it eliminated the nonspecific cross-reactivity from most of the serum samples from hypophysectomized rats. Overall, the results also indicate that, with respect to RRA and RIA detectability, PRL in pituitary tissue and incubation medium differs from the hormone in serum. (Endocrinology 106: 61, 1980)