A high concentration of dopamine preferentially permitted release of newly synthesized prolactin

We used continuous labelling (3H]leucine) of cultured adenohypophysial cells to investigate the relationship between the storage and release of newly synthesized and stored prolactin in response to dopamine (1 mumol/l) and thyrotropin-releasing hormone (TRH) (0.1 mumol/l) challenge. Newly synthesized prolactin was identified by the tritium radiation activity incorporated in prolactin. A maximal dose of dopamine (1 mumol/l) could not completely block prolactin release from a primary culture of lactotrophs. During 3 h of continuous labelling under maximal dopaminergic inhibition, newly synthesized prolactin was released which was of a significantly higher specific activity than control groups. In contrast, TRH stimulation produced results consistent with previous observations of the release of predominantly old, stored hormone. However, the absolute amount of the newly synthesized prolactin was increased by the TRH administration, and the increased release of the newly synthesized prolactin could be accounted for by increased levels of synthesis. Our results are consistent with the concept of the existence of a regulated route and a dopamine-insensitive constitutive route of prolactin release which predominantly encompasses newly synthesized hormone. However, the possibility that cellular heterogeneity or that non-dopaminergic prolactin-release inhibiting factor(s) (PIF) is responsible for this observed release cannot be ruled out.