Differential calcium responses to the pituitary adenylate cyclase-activating polypeptide (PACAP) in the five main cell types of rat anterior pituitary

We have compared the effects of pituitary adenylate cyclase-activating polypeptide (PACAP-27) on the five main cell types of rat anterior pituitary in primary culture by monitoring changes in cytosolic Ca2+ concentration ([Ca2+]i) in single fura-2-loaded cells. Cells were typed by multiple sequential primary immunocytochemistry at the end of the Ca2+ measurements. PACAP-27 increased [Ca2+]i by three different mechanisms, each one dominant in a given cell type. These involved Ca2+ entry or release from the stores and mediation through different second messenger pathways: (1) stimulation of Ca2+ entry mediated by cAMP was the main mechanism in somatotrophs; (2) Ca2+ release from the intracellular Ca2+ stores mediated by phospholipase C (PLC) was the dominant modality in gonadotrophs; (3) stimulation of Ca2+ entry not mediated by cAMP was the main mechanism in lactotrophs. A minor fraction of somatotrophs (11%) may also use mechanism 3. Corticotrophs and thyrotrophs exhibited weak responses to PACAP (<10% of the cells responded), which in all cases were mediated by mechanism 1. Mechanism 3 represents a novel effect of PACAP which cannot be explained by interaction with the conventional PACAP receptor families.