Episodic prolactin release after removal of dopamine inhibition in vitro

Prolactin (PRL) release following the removal of tonic dopamine (DA) inhibition was investigated in superfused cultures of dispersed rat anterior pituitary cells. The removal of DA (500 nM) inhibition resulted in a rapid and sustained increased in PRL release as measured by RIA which occurred in an episodic manner. The first PRL pulse reached a maximum 15 to 30 min. after DA removal (20.6 +/- 1.1 min). The second pulse occurred 40 to 45 min. after DA removal and the mean interpulse period was 21.7 +/- 1.2 min. The amplitude of the first pulse was 1.6 to 13.5 higher than the value observed in the presence of DA. It was followed by a nadir that was 1.1 to 8.0 fold higher than the basal value. The second pulse was of slightly lower amplitude, reaching a maximum of 2.3 to 8.0 fold over the control value and 1.25 to 2.5 fold over the preceding nadir. This episodic pattern was observed in 16 independent experiments performed with cells cultured at a density of 0.7-2.8 x 10(6) cells/coverslip. We hypothesize that lactotrophs in vitro have an inherent 20 minute secretory rhythm as has been observed under certain circumstances in vivo. Tonic dopamine inhibition restrains the otherwise random secretory rhythms of lactotrophs thereby synchronizing the secretory phase of the cells upon its removal. Consistent with this hypothesis episodic PRL release was not observed when DA was constantly present or absent.