Calcium-phospholipid interactions in secretory cells: a new perspective on stimulus-secretion coupling

Recent evidence suggests that stimuli delivered to the cell surface may alter Ca availability by promotion membrane phospholipid turnover. Due to the importance of Ca in stimulus-secretion coupling, the study of membrane phospholipid turnover could provide valuable information on the cellular mechanisms of evoked secretion. In the adrenal cortex, Ca is an obligatory requirement for adrenocorticotropin (ACTH)-induced steroid production and release, and recent findings revealed the presence of a Ca-dependent phospholipase A2 localized to the surface of isolated feline adrenocortical cells. This phospholipase is activated by ACTH to promote the release of arachidonic acid from phospholipids and thereby stimulate prostaglandin formation. Further investigations into the fate of radiolabeled arachidonic acid described a rapid and specific turnover of arachidonic acid within a phosphatidylinositol (PI) pool that is independent of changes in de novo synthesis and is characterized as a Ca-dependent hydrolysis of PI, followed by a rapid, selective reacylation of lysoPI. A similar deacylation-reacylation reaction involving arachidonyl PI is also found in the rabbit neutrophil when challenged with the formyl-methionyl peptide, F-Met-Leu-Phe, or the Ca-selective ionophore A23187. The relevance of this reaction to secretory phenomena is indicated by its requirement for Ca, its rapid onset, and dose-response curves that paralleled those of the secretory response. The Ca-dependent activation of arachidonyl PI turnover triggered by the activation of membranous phospholipase A2 occupies a critical position in the train of events associated with the activation of the secretory process, and one or another of the products of this reaction-prostaglandins, arachidonic acid, and/or lysophospholipids-may participate in the cellular processes that accompany the discharge of secretory product.