Dopamine Decreases 7315a Tumor Cell Prolactin Release Induced by Calcium Mobilization*

Abstract

The rat pituitary tumor 7315a secretes PRL [prolactin] and ACTH. Although dopamine has no effect on unstimulated PRL release from this tumor, dopamine decreases the adenylate cyclase activity in tumor cell homogenates in a manner similar to that in normal pituitary tissue. However, it was observed that under basal conditions, 7315a tumor cells have an abnormal Ca metabolism because basal PRL release from tumor cells is not modified by the Ca channel blocker D-600 [methoxyverapamil] and is only moderately decreased by low Ca, treatments that markedly decrease normal pituitary PRL release; D-600 had no effect on basal 7315a tumor Ca uptake, but blocked the increase in Ca uptake due to the Ca-channel activator maitotoxin; increasing the medium Ca2+ concentration above 5 mM increases 7315a PRL release, whereas this treatment decreases PRL release from normal pituitary cells. Maitotoxin and the Ca ionophore A23187 increased 7315a tumor cell PRL release in a manner similar to that in normal pituitary cells. Because dopamine blocks PRL release induced by maitotoxin, A23187, or elevated medium Ca concentration in 7315a tumor cells, the refractoriness of basal 7315a tumor cell PRL release to dopamine may be due to the abnormal Ca balance of the tumor cells under basal conditions.