Measurement of Prolactin in Milk by Radioimmunoassay

Double-antibody radioimmunoassay procedures previously developed for measuring prolactin in blood plasma were used for prolactin measurements in milk.Substar~tial quantities of immunoreactive prolaetin were in milk from cows, goats, and sheep.Milk from all three species produced assay responses qualitatively similar to those produced by standard bovine prolaetin (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 ser/es of in vitro experiments: (a) rat milk and cow milk each erossreaeted in the heterologous radioimmunoassay for the hormone of the opposite species only slightly more than would be predicted from their content of native prolaetin as measured by homologous assay; (b) cow milk did not significantly affect the final precipitation step in the assay; (e) incubation of iodine-131 labeled prolactin with 50 /~1 cow milk under assay conditions did not decrease appreeiably its binding to excess anti-prolactin.Incubation with 100 and 200 pJ milk did decrease binding enough to result in erroneous overestimations of prolactin concentration.None of these in vitro experiments provided results which invalidated either the experimental finding of substantial prolactin in milk or accurate quantifleation of the bormo~e.