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 poly peptide (PACAP-27) on the five main cell types of rat anterior pituitary in primary culture by monitoring changes in cytosolic Ca2+ concentration ([Ca 2+](i)) in single fura-2-loaded cells. Cells were typed by multiple sequent ial 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 (P LC) was the dominant modality in gonadotrophs; (3) stimulation of Ca2+ entr y not mediated by cAMP was the main mechanism in lactotrophs. A minor fract ion of somatotrophs (11%) may also use mechanism 3. Corticotrophs and thyro trophs exhibited weak responses to PACAP (<10% of the cells responded), whi ch in all cases were mediated by mechanism 1. Mechanism 3 represents a nove l effect of PACAP which cannot be explained by interaction with the convent ional PACAP receptor families.