A pigeon crop sac radioreceptor assay for prolactin

Ovine prolactin, labelled with 125I by either lactoperoxidase or a mild chloramine T method, bound to receptors from the pigeon crop sac mucosa cells of prolactin-injected pigeons. Binding was demonstrated in a crude homogenate of mucosal cells removed from the crop by scraping and in a subcellular fraction in which 5'-nucleotidase activity was enhanced two- to threefold. The binding was specific, dependent on time, temperature and the concentration of receptors and had a dissociation constant of 7 X 10(-10) mol/l. The binding capacity of the crop tissue was 71 fmol/mg membrane protein. Nine purified preparations of prolactin from four species were assayed by local pigeon crop sac bioassay and by radioreceptor assay. The two methods were highly correlated (r = 0.934). The regression equation was radioreceptor assay = 1.22 bioassay--0.18 indicating a 1 : 1 correspondence between the two methods for prolactin purified from sheep, rat, horse and pig anterior pituitary glands.