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Measurement of Prolactin in Milk by Radioimmunoassay¹

Department of Animal Sciences, Purdue University, Lafayette, Indiana 47907

ABSTRACT

Double-antibody radioimmunoassay procedures previously developed for measuring prolactin in blood plasma were used for prolactin measurements in milk. Substantial quantities of immunoreactive prolactin were in milk from cows, goats, and sheep. Milk from all three species produced assay responses qualitatively similar to those produced by standard bovine prolactin (i.e., parallel inhibition curves). While prolactin concentrations varied considerably between individual milk samples, those in cow and goat milk were substantially less than reported for rat milk. Samples of milk from all species, except cows, could be frozen and stored without appreciable decreases in prolactin.

An additional validation for radioimmunoassay to measure prolactin in milk was from results of a series of in vitro experiments: (a) rat milk and cow milk each crossreacted in the heterologous radioimmunoassay for the hormone of the opposite species only slightly more than would be predicted from their content of native prolactin as measured by homologous assay; (b) cow milk did not significantly affect the final precipitation step in the assay; (c) incubation of iodine-131 labeled prolactin with 50 µl cow milk under assay conditions did not decrease appreciably its binding to excess anti-prolactin. Incubation with 100 and 200 µl milk did decrease binding enough to result in erroneous over-estimations of prolactin concentration. None of these in vitro experiments provided results which invalidated either the experimental finding of substantial prolactin in milk or accurate quantification of the hormone.

¹ Journal Paper No. 5326, Purdue University Agricultural Experiment Station, Lafayette, Indiana 47907.

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