Bioassay for prolactin: densitometric analysis on polyacrylamide gels of milk protein production by mammary explants in vitro.

Mammary explants from mice in midpregnancy cultured in a synthetic medium containing insulin and hydrocortisone respond to the addition of prolactin by the synthesis of milk proteins. The secretory response in alveolar lumina has served as a histologic endpoint in a sensitive bioassay for prolactin developed by Kleinberg and Frantz (J Clin Invest 50: 1557, 1971). This report demonstrates that the quantitative densitometric analysis of stained milk proteins (caseins) made in response to prolactin is a useful modification of that bioassay. Electrophoretic analysis of the proteins extracted from the explants permits a quantitative estimate of casein content. The amount of casein present after 5 days of incubation was found to be a measure of the prolactin concentration in the medium. No radioactive isotopes are used. The use of electrophoretic analysis has practical advantages over the histologic scoring used earlier, and has approximately the same range of sensitivity.