In Vitro Studies on Basal and Stimulated Prolactin Release by Rat Anterior Pituitary: A Possible Role for Calmodulin*

Abstract

The mechanism by which PRL is released from mammotrophs is a calcium-dependent process. Although calcium seems to function as a second messenger, its regulatory mechanism in PRL release has not been clarified. The binding of calcium to calmodulin and the activation of calmodulin-dependent enzymes have been suggested to be important steps during stimulus-secretion coupling in various cells. In the present work we investigated the in vitro effect of penfluridol, a potent neuroleptic that also possesses the ability to inhibit calmodulin's biological activity, on basal and stimulated PRL release. The effect of pimozide and haloperidol on basal PRL release was also investigated. Penfluridol, pimozide, and haloperidol inhibited basal PRL secretion in a dose-related manner, with the EC₅₀ ranging from 0.5–1 μM for penfluridol to 1–2 pM for pimozide and more than 3 μM for haloperidol. These concentrations are similar to those necessary for the inactivation of calmodulin-dependent enzymes in vitro. Ionophore A-23187, a compound whose ability to mobilize extracellular calcium is not affected by neuroleptics, stimulated PRL secretion in vitro. This effect, however, was blocked by penfluridol pretreatment. The site of action of penfluridol may occur after calcium mobilization, with calmodulin a possible target for penfluridol's inhibitory action on PRL secretion. TRH, K⁺, (Bu)₂cAMP, and theophylline, compounds that affect calcium mobilization, also significantly stimulated PRL release. The coincubation of varying concentrations of penfluridol with 70 nM TRH, 50 mM K⁺, 3 mM (Bu)₂cAMP, or 5 HIM theophylline resulted in a dose-related inhibition of secretagogue- stimulated PRL secretion. Perifusion of dispersed anterior pituitary cells with 1 μM penfluridol reduced the ability of 70 nM TRH to stimulate PRL release by approximately 50%, whereas removal of the penfluridol perifusion allowed the cells to again be fully responsive to TRH. These results are consistent with the hypothesis that calmodulin is involved in the stimulussecretion coupling of PRL.