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The pigeon crop-sac is a well-known target organ of prolactin (PRL). Pukac and Horseman (Endocrinology 114: 1718, 1984) reported that injections of the hormone caused changes in the expression of several specific proteins in that organ. We investigated the possibility that measuring the amounts of three of these proteins might improve the precision of the local pigeon crop-sac assay for PRL. Pigeons were given four injections of phosphate-buffered saline or different doses of ovine (o) PRL for 2 days and were killed on day 3. The DNA and total protein content of their crop-sac mucosal epithelial cells were measured and a supernatant of homogenized epithelial cells was processed by SDS-PAGE. The amounts of the three specific proteins in the stained gels were measured by densitometry. Treatment with oPRL increased the concentration of two of these proteins in the crop mucosal tissue (CP 14 and CP 25) and decreased that of the third one (CP 17). Measurement of total protein or of changes in the individual protein bands gave dose-response relationships with poor indices of precision (lambda greater than 0.3). Measurement of DNA content gave an assay endpoint with a reasonably good lambda value (0.16) but when the changes in the amounts of CP 14 and CP 25 were expressed as ratios to the changes occurring in CP 17, very precise assay endpoints (lambda = 0.06-0.08) were obtained.

A pigeon crop-sac bioassay for prolactin was developed applying a morphometric approach for quantification of the epithelial cell proliferation induced by the hormone. A small male dove, indigenous from South America (Zenaida auriculata) previously castrated was used as experimental animal. The total dose of prolactin was divided into 4 daily injections administered systemically into the pectoral muscles. The number of the proliferated cell layers used as the bioassay end point was analyzed statistically. This procedure yields a highly sensitive bioassay with an excellent precision index, particularly well suited for quantification of the different molecular forms of pituitary PRL.