Heterogeneity of activity of the prolactin-releasing factor in the bovine hypothalamo-neurohypophysial complex.

Fresh samples of bovine hypothalamic median eminence or hypophysial stalk extracted with 0.1 M-HCl and fractionated on Sephadex G-100 had two prolactin-releasing factor (PRF) components, the major one in the void volume (PRF-A) and the minor one in an elution position (partition coefficient (Kav) = 0.333) immediately after PRF-A (PRF-B). Similar fractionation of bovine posterior pituitary extracts showed two PRF peaks: PRF-A and another in a more retarded position than PRF-B (Kav = 0.645; PRF-C). Storage of bovine stalk extract at -20 degrees C for 1 year caused a shift from higher molecular weight PRF into PRF-C. Thyrotrophin-releasing hormone stimulated prolactin secretion, but its dose-response slope and chromatographic mobility were quite different from those for bovine tissue extracts. Secretion of prolactin induced by extracts of bovine hypophysial stalk is calcium dependent and is inhibited by dopamine.