Mechanism of the prolactin rebound after dopamine withdrawal in rat pituitary cells.

To study the mechanism underlying the effect of dopamine withdrawal on prolactin release, continuous perfusion experiments were performed on rat lactotroph-enriched primary cultures. Removal of dopamine (10(-7) M) after a short-term application (15 min) produced a rebound of prolactin secretion, which was enhanced by pretreatment of the cell culture with 17 beta-estradiol (10(-8) M for 48 h). Ca2+ channel blockade by Co2+ (1 mM) abolished the rebound in prolactin release. An increase in intracellular adenosine 3',5'-cyclic monophosphate by either forskolin (5 microM) or 3-isobutyl-1-methylxanthine (100 microM) enhanced the prolactin rebound after dopamine withdrawal. Application of thyrotropin-releasing hormone (10(-7) M) increased the prolactin rebound after dopamine withdrawal with a maximum effect obtained by commencing treatment immediately after removal of dopamine. Pretreatment of cell cultures with pertussis toxin (100 ng/ml, for 10 h) totally abolished the effects of dopamine on prolactin secretion. The dopamine agonist bromocriptine (10(-9) M) significantly decreased prolactin secretion, but no rebound effect was observed after its removal. We conclude that the rebound of prolactin release after dopamine treatment involves the influx of Ca2+.