Since a purified porcine hypothalamic preparation with high PIF activity as tested by an in vitro assay was found to contain a high content of catecholamines, PIF activity of this preparation and catecholamines were examined in vivo. The test samples were infused for 30 min into a hypophysial portal vessel of male rats anesthetized with urethan. Infusion of 30 ng and 300 ng of porcine PIF preparation in 5% glucose solution decreased serum prolactin levels significantly, as compared to the respective preinfusion levels. Infusion of 60 ng and 600 ng, but not 6 ng, of dopamine hydrochloride in glucose solution caused a significant reduction in prolactin levels. Administration of 6 ng, 60 ng, and 600 ng of norepinephrine bitartrate in 5% glucose also decreased serum prolactin levels. The magnitude of decrease in prolactin levels appeared to be dose related. When dopamine dydrochloride (600 ng) and norepinephrine bitartrate (600 ng) were dissolved in 0.9% saline and infused into the rat, their PIF activity was considerably diminished or not demonstrable at all. The results indicate that catecholamines suppress prolactin release in vivo as well as in vitro by directly acting on the pituitary gland. The PIF activity of the purified PIF preparation was related to the amount of catecholamines contained in that preparation.
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