Colocalization of Calcium Entry and Exocytotic Release Sites in Adrenal Chromaffin Cells

"Snapshot" images of localized Ca2+influx into patch-clamped chromaffin cells were captured by using a recently developed pulsed-laser imaging system. Transient opening of voltage-sensitive Ca2+channels gave rise to localized elevations of Ca2+that had the appearance of either "hotspots" or partial rings found immediately beneath the plasma membrane. When the Ca2+imaging technique was employed in conjunction with flame-etched carbon-fiber electrodes to spatially map the release sites of catecholamines, it was observed that the sites of Ca2+entry and catecholamine release were colocalized. These results provide functional support for the idea that secretion occurs from "active zone"-like structures in neuroendocrine cells.