TIME-COURSE OF RELEASE OF CATECHOLAMINES FROM INDIVIDUAL VESICLES DURING EXOCYTOSIS AT ADRENAL-MEDULLARY CELLS

The time course of extrusion of the vesicular contents during exocytos is has been examined at adrenal medullary cells with carbon-fiber micr oelectrodes. Two electrochemical techniques were used: cyclic voltamme try and amperometry. Spikes obtained by amperometry had a faster time course than those measured by cyclic voltammetry, consistent with the different concentration profiles established by each technique. Howeve r, the experimental data obtained with both techniques were temporally broadened with respect to dispersion of an instantaneous point source by diffusion. Measurements with the electrode firmly pressed against the cell surface established that the temporal broadening is a result of a rate-limiting kinetic step associated with extrusion of the vesic ular contents at the cell surface. The data do not support a rate-limi ting process due to restricted efflux from a small pore. When combined with previous results, the data suggest that the rate-limiting step f or chemical secretion from adrenal medullary cells during exocytosis i s the dissociation of catecholamines from the vesicular matrix at the surface of the cell.