[Studies on mechanisms of prolactin synthesis and secretion using perfusion system of rat pituitary cells]

The direct effect of TRH, dopamine, and hypothalamic extract on prolactin (PRL) synthesis and release were studied in an in vitro perfusion system using female rat pituitary cells and rat pituitary tumor cells (GH3 cells). The cells were exposed to tritium-leucine containing medium during perfusion, and total amounts of released PRL were measured by radioimmunoassay (IR-PRL), while those of newly synthesized PRL by counting immunoprecipitated radioactivity of tritium-labeled PRL (IP-PRL). IP-PRL began to be released immediately after exposure to tritium-leucine, and gradually increased to a plateau within five hours. After this period, calculated specific activity (SA; IP-PRL/IR-PRL) was unchanged by stimulation of TRH, but when TRH (10(-10)M-10(-8)M) was applied in the phase of increasing SA, decreased SA of released PRL was observed, indicating that TRH released previously stored PRL preferentially. Hypothalamic extract showed PRL releasing effect similar to TRH in point of decreasing SA. Continuous administration of TRH evoked pulsatile IR-PRL release, followed by a decrease to basal level, and had no effect on SA except pulsatile decrease coincident with pulses of IR-PRL. Dopamine inhibited PRL release without change of SA and was presumed to affect both newly synthesized and previously stored PRL. However, brief interruption of dopamine infusion evoked PRL release with a decrease of SA, indicating the release of previously stored PRL. In contrast with the heterogenous PRL responsiveness to TRH stimulation in normal pituitary cells, GH3 cells, a clonal strain of rat pituitary tumor cells, released PRL in response to TRH with a constantly increasing rate of SA. Thus the concept of functionally heterogenous subpopulations in normal pituitary mammotrope cells was supported.