Transient dopamine withdrawal differentially potentiates thyrotropin releasing hormone-induced release of prolactin of various ages

Transient removal of dopamine (DA) potentiates the prolactin (PRL) releasing action of thyrotropin releasing hormone (TRH) in cultured lactotrophs. We have determined if this potentiating action on PRL release is related to the intracellular processing of PRL by studying this phenomenon in cells in which PRL was pulse-labeled at varying times. Anterior pituitaries from ovariectomized estradiol-treated rats were dispersed and cultured on plastic coverslips. Cells were incubated for 24 h in media containing 500 nM DA. Cells were exposed to three consecutive 20-min incubation periods. During the first period all cells were maintained in 500 nM DA, during the second period half of the cells were removed from DA inhibition and during the third period DA inhibition was restored and the cells challenged with 100 nM TRH. In experiments involving labeling of PRL, cells were incubated in leucine-free media containing 100 microCi/ml [3H] leucine for 1 h prior to treatment, for 1 h starting 5 h prior to treatment or for 16 h starting 24 h prior to treatment. Total PRL (by radioimmunoassay) and radiolabeled PRL (by liquid scintillation after electrophoretic isolation) were determined both in media and cells. The withdrawal of DA alone resulted in a 2-fold stimulation of total PRL release. The efficacy of TRH to release total PRL was also increased 2- to 4-fold after transient DA withdrawal. Whereas release of 1-hour-old labeled PRL was not affected by DA withdrawal, 5-hour-old and 8- to 24-hour-old [3H] PRLs were stimulated 2.6- and 2-fold, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)