A specific and sensitive homologous radioimmunoassay (RIA) for Xenopus prolactin (xPRL) was developed. PRL isolated from X. laevis pituitary glands was used for generating antiserum in a rabbit, for radioligand and for the standard. Pituitary homogenates and plasma from adult Xenopus produced displacement curves parallel to the xPRL standard. Plasma from hypophysectomized Xenopus showed negligible cross-reactivity. Purified PRLs from other amphibian species such as the bullfrog (Rana catesbeiana) and toad (Bufo japonicus) gave inhibition curves which did not parallel the standard. Ovine PRL, mouse PRL, newl (Cynops pyrrhogaster) PRL, bullfrog GH, and bullfrog LH showed no inhibition of binding even at relatively high doses in this RIA. The sensitivity of the RIA was 0.122 ± 0.005 ng xPRL/100 μl assay buffer. Intraassay and interassay coefficients of variation were 2.46 and 6.65%, respectively. Histological studies of Xenopus adenohypophyses revealed that the cells which reacted immunologically with the antiserum against xPRL corresponded to those positively stained with antiserum against bullfrog PRL. A preliminary application of this homologous RIA for xPRL was performed by evaluating plasma and pituitary PRL levels in subadult and adult Xenopus of both sexes.
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