ANALYSIS OF DIFFUSIONAL BROADENING OF VESICULAR PACKETS OF CATECHOLAMINES RELEASED FROM BIOLOGICAL CELLS DURING EXOCYTOSIS

Secretion of catecholamines is observed as a series of current spikes when measured at the surface of a bovine adrenal medullary cell in culture with a carbon-fiber microelectrode operated in the amperometric mode. Prior work has shown that these spikes are due to detection of concentrated packets of catecholamines which are released from individual vesicles after their fusion with the cell membrane, a process known as exocytosis. The shape of the individual current spikes, detected with a 5-mum spacing between the hemispherical cell and the electrode, has been compared to the shape generated by a theoretical model. The model consists of an instantaneous point source of material on a surface which subsequently diffuses to a disk that consumes the emitted material. The pertinent diffusion conditions have been evaluated with finite difference and random walk digital simulations. The two methods give identical results when the point source is located on a plane. The more realistic simulation geometry, emission from a hemispherical surface, was evaluated with the random walk method. The simulations allow a set of criteria to be established to evaluate diffusion-controlled broadening of spike shape. The broad range of spike widths observed experimentally and their individual shapes measured with 5-mum cell-electrode spacing are consistent with diffusion from point sources randomly distributed across a hemispherical surface. The data can be described with the diffusion coefficient for catecholamines in free solution. The model enables evaluation of signal-to-noise losses and correction for diffusional losses which are dependent on electrode radius. Through evaluation of spike areas and correction for diffusive losses, an apparent vesicular concentration of catecholamines can be determined.