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

Abstract

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.