Exocytosis of catecholamine (CA)-containing and CA-free granules in chromaffin cells

Recent evidence suggests that endocytosis in neuroendocrine cells and neuro ns can be tightly coupled to exocytosis, allowing rapid retrieval from the plasma membrane of fused vesicles for future use. This can be a much faster mechanism for membrane recycling than classical clathrin-mediated endocyto sis. During a fast exo-endocytotic cycle, the vesicle membrane does not ful ly collapse into the plasma membrane; nevertheless, it releases the vesicul ar contents through the fusion pore. Once the vesicle is depleted of transm itter, its membrane is recovered without renouncing its identity. In this r eport, we show that chromaffin cells contain catecholamine-free granules th at retain their ability to fuse with the plasma membrane. These catecholami ne-free granules represent 7% of the total population of fused vesicles, bu t they contributed to 47% of the fusion events when the cells were treated with reserpine for several hours. We propose that rat chromaffin granules t hat transiently fuse with the plasma membrane preserve their exocytotic mac hinery, allowing another round of exocytosis.