2-Bromo-alpha-ergocryptine causes degradation of prolactin in primary cultures of rat pituitary cells after chronic treatment.

Abstract

Bromocriptine (2-bromo-a-ergocryptine) inhibited prolactin release when added to primary cultures of dispersed pituitary glands from male rats.The total amount of prolactin (intracellular plus extracellular) was reduced to less than 20% of control values after 6 days.The amount of intracellular prolactin was increased in treated cultures and the total amount of hormone was the same in control and treated cultures for the first 8 h that prolactin release was inhibited.After 4 days with bromocriptine, treated cultures did not contain more intracellular prolactin than control cultures.The rate of accumulation of total prolactin from 3 to 4 days after bromocriptine was added varied from 6-fold less than controls to no accumulation at all.The apparent rate of synthesis at this time, measured by the incorporation of [3H]leucine for 1 or 2 h, was only 1.4-to 2.8-fold less in bromocriptine-treated cultures.After longer labeling periods, when the rate of accumulation of [3H]prolactin may increasingly reflect degradation as well as synthesis, the difference in the rate of accumulation became 6-to 7-fold less than control.Stability of [3H]prolactin was measured by incubating the cells for 2 h with [3H]leucine and then adding 3 pg/ml of cycloheximide and 100-fold excess leucine.[3H]Prolactin was stable for 9 h in control cultures, but in bromocriptine-treated cultures there was a 40% decrease by 9 h.Therefore, at least some of the decrease in prolactin accumulation caused by bromocriptine results from degradation of the hormone.Secretion of prolactin from the anterior pituitary gland is primarily under negative control by the hypothalamus (1) and dopamine appears to be the hypothalamic inhibitory factor (2).The evidence is that dopaminergic agonists inhibit prolactin secretion in intact animals (3, 4), and direct inhibition of prolactin secretion has been demonstrated by adding these compounds to pituitary halves in organ culture or to primary cultures of dispersed pituitary glands (5, 6).In addition, the inhibitory activity found in hypothalamic extracts has properties of catecholamines (7).Ergot alkaloids block prolactin secretion (5,6).These compounds appear to act at the dopamine receptor in the pituitary gland since the inhibition of prolactin release by ergot alkaloids is prevented by dopaminergic antagonists (5, 6).Furthermore, the binding of ergot alkaloids to pituitary membranes can be blocked by dopaminergic agonists and antagonists (6).