ABSTRACT
It is generally accepted that translocation of calcium ions forms an essential step in the sequence that couples receptor activation to the mechanisms that execute exocytosis. This chapter summarizes some aspects of the role of Ca2+ in secretion responses and its sensitivity to alterations in intracellular pH, cAMP, and cGMP concentrations. Unstimulated cells maintain a low cytosolic free Ca2+ concentration in spite of a large, inward directed Ca2+ gradient across the plasma membrane. Possible mechanisms might involve voltage-dependent Ca2+ channels, receptor-operated channels, and lipid mediators of calcium mobilization, which might act as "Ca2+ ionophores." Contradictory results regarding the role of Ca2+ and the C-kinase in secretion were also obtained in neutrophils. This Ca2+ dependent enzyme plays a central role in stimulus-response coupling in platelets and its activation optimum lies in the alkaline pH range. The ability of Ca2+ to trigger a secretory response depends on the presence of MgATP2- in the medium.
