Prolactin release inhibiting factor (PIF) activity present in acetic acid extracts of 470 000 pig hypothalami was purified by gel filtration on Sephadex G 25, extraction with phenol, chromatography on carboxymethylcellulose, countercurrent distribution, rechromatography on Sephadex and partition chromatography. PIF activity was followed by inhibition of prolactin release in vitro from rat or sheep pituitaries, and occasionally in vivo by infusion into a hypophyseal portal vessel of the rat. Some of the highly purified fractions which powerfully inhibited the release of prolactin in vitro in doses smaller than 0.5 μg/ml were found to contain up to 15% of noradrenaline and 2% of dopamine. Catecholamines were identified by fluorometric analyses after separation on alumina, and by R(F) on column chromatography, as well as by TLC. The magnitude of inhibition of release of prolactin was related to noradrenaline content. Synthetic noradrenaline and dopamine, in doses of 10-100 ng also strongly inhibited the release of prolactin in vitro. The results indicate that catecholamines, purified from hypothalamic tissue or synthetic, inhibit the release of prolactin by an action exerted directly on the pituitary gland. Whether catecholamines represent a physiological prolactin release inhibiting hormone remains to be determined.
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