Dopaminergic inhibition of growth hormone and prolactin release during continuous in vitro perifusion of normal and adenomatous human pituitary.

An in vitro technique for continuous perifusion of human pituitary has been developed to study the effect of dopamine on hormone release. Pituitary tissue was obtained from 2 patients with prolactinomas, 3 patients with acromegaly and 2 patients with disseminated malignancy whose pituitaries were normal. During perifusion prolactin release was reversibly inhibited from both normal and adenomatous glands, confirming the direct effect of dopamine on the pituitary. Rebound secretion following inhibition suggested that dopamine acted upon hormone release rather than synthesis under these conditions. Growth hormone (GH) release in vivo is stimulated by levodopa in normal subjects but inhibited in acromegaly. In vitro, however, we have demonstrated inhibition of GH release by dopamine, regardless of whether the pituitary was normal or adenomatous. This finding does not support the view that dopamine receptors are acquired by tumour cells as a result of adenomatous dedifferentiation. It is suggested that the paradoxical GH response to levodopa in acromegaly can be explained by a dual action of dopamine at hypothalamic and pituitary levels. According to this hypothesis GH release is stimulated by a hypothalamic effect, whereas inhibition occurs at pituitary level. The net effect of these opposing stimuli in vivo may depend upon the sensitivity of dopamine receptors at the prevailing concentration of GH.