ENDOTHELIN IS AN AUTOCRINE REGULATOR OF PROLACTIN SECRETION

The aim of this study was to establish the cellular source of ET-like peptides affecting PRL secretion. Fluorescence double label immunocyto chemistry and confocal laser scanning microscopy were used to demonstr ate cellular colocalization for PRL and endothelin-1 (ET1)-like immuno reactivities in the anterior lobe of the pituitary gland of rats. An E T-specific reverse hemolytic plaque assay was applied to demonstrate t hat lactotrophs are capable of releasing ET-like peptides. A PRL-speci fic reverse hemolytic plaque assay was used to assess the influence of the released endogenous ETs on PRL secretion. ETA-specific receptor a ntagonists BQ123 and BQ610, and endothelin convertase enzyme inhibitor y peptide, [(22)Val]big ET1-(16-38), increased PRL secretion, whereas the ETB receptor-specific antagonist BQ788 was ineffective. The ETA an tagonist BQ123-induced increase in PRL secretion followed a bell-shape d dose-response curve in cells obtained from female rats, whereas it f ollowed a sigmoid curve in males. Frequency distribution of PRL plaque sizes using logarithmically binned data revealed two subpopulations o f lactotrophs with differential responsiveness to endogenous ETs. Thes e data demonstrate that a large proportion of lactotrophs is capable o f expressing and secreting ET-like peptides in biologically significan t quantities. As low pituitary cell density in reverse hemolytic plaqu e assay minimizes cell to cell communications, these findings constitu te direct proof of autocrine regulation of PRL secretion by ET-like pe ptides.