Direct effects of catecholamines, thyrotropin-releasing hormone, and somatostatin on growth hormone and prolactin secretion from adenomatous and nonadenomatous human pituitary cells in culture.

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Abstract

To determine the mechanism and the site of action of catecholamines as well as hormones including thyrotropin-releasing hormone (TRH)1 and somatostatin on pituitary hormone release in patients with acromegaly and in normal subjects, the effects of these substances on growth hormone (GH) and prolactin (PRL) secretion from adenomatous and nonadenomatous human pituitary cells in culture were examined. When dopamine (0.01-0.1 microM) or bromocriptine (0.01-0.1 microM) was added to the culture media, a significant inhibition of GH and PRL secretion from adenoma cells from acromegalic patients was observed. This inhibition was blocked by D2 receptor blockade with metoclopramide or sulpiride, but not by D1 receptor blockade. Similarly, dopamine suppressed GH and PRL release by nonadenomatous pituitary cells in a dose-dependent manner, which was again blocked by D2 receptor blockade. The minimum effective concentration of dopamine required for a significant inhibition of PRL secretion (0.01 microM) was lower than that for GH release (0.1 microM). Norepinephrine, likewise, caused a suppression of PRL secretion from adenomatous and nonadenomatous pituitary cells. This effect was blocked by sulpiride, phentolamine, however, was ineffective. When TRH was added to the media, both GH and PRL secretion were enhanced in adenoma cells, while only the stimulation of PRL release was observed in nonadenomatous pituitary cells. Coincubation of TRH and dopamine resulted in variable effects on GH and PRL secretion. Somatostatin consistently lowered GH and PRL secretion in both adenomatous and nonadenomatous pituitary cells and completely blocked the TRH-induced stimulation of GH and PRL secretion from adenoma cells. Opioid peptides (1 microM) failed to affect hormone release. These results suggest that no qualitative difference in GH and PRL responses to dopaminergic agonists or to somatostatin exists between adenoma cells of acromegalic patients and normal pituitary cells, and that the direct effect of catecholamines on GH and PRL secretion from human pituitary cells is mediated mainly through dopamine receptor activation.

Authors

M Ishibashi, T Yamaji