Confocal microscopy images (250-nm optical sections) of human umbilical vein endothelial cells (HUVEC) and quiescent and secretagogue-treated (+) AtT 20 cells. Cells were immunolabeled with monoclonal antimultimerin after transfection with the empty vector (control) or the multimerin expression vector. The upper panels show images taken at a higher magnification. Multimerin was identified in small granule-like structures (permeabilized cells) that were concentrated at the cell tips of stably transfected AtT 20 cells (arrows). Twenty minutes after stimulation with 10 mmol/L 8-Br-cAMP, transfected AtT 20 cells showed intense multimerin labeling in release patches (arrowheads) around the cells that were not evident in control cells, and this redistribution was associated with reduced multimerin granule labeling at the cell tips (permeabilized cells).