Zones of exocytotic release on bovine adrenal medullary cells in culture

Secretion of catecholamines from individual bovine adrenal medullary cells in culture was examined by amperometry with 1-micron radius carbon-fiber electrodes. Vesicular secretion is observed as a series of current spikes upon exposure to a secretagogue. The small size of the electrodes was exploited to map exocytotic release sites on the surface of bovine adrenal medullary cells. These studies reveal for the first time that release sites are spatially localized on endocrine cells in culture for a time scale of at least 15 min. Fluorescent monitoring by confocal microscopy of deposition of dopamine-beta-hydroxylase from the vesicular membrane into the plasma membrane during exocytosis confirms the existence of zones of exocytotic inactivity on the surface of the cell. Measurement of coincident spikes with two adjacent electrodes (2.5-microns radius) has allowed the spatial resolution for measurement of exocytosis to be defined as 2 microns from the projected circumference of the electrode on the surface of the cell. In this small domain, point-source release and diffusional broadening would result in narrow spikes (Schroeder, T. J., Jankowski, J. A., Kawagoe, K. T., Wightman, R. M., Lefrou, C., and Amatore, C. (1992) Anal. Chem. 64, 3077-3083), a feature not seen in the data. Thus, for adrenal medullary cells, release following vesicular fusion is not instantaneous, but is a prolonged event occurring over several milliseconds.