Release of chromaffin granular content on interaction with plasma membranes

Cell-free interaction between bovine adrenal medullary plasma membranes and chromaffin granules results in the release of the granular content, a process specifically controlled by micromolar concentrations of calcium and therefore regarded as a putative model for exocytosis. The density gradient distribution of interacted organelles (1 min at 37 degrees C) was compared with the distribution of noninteracted material (kept at 0 degree C). Self-generating Percoll gradients, in which intact chromaffin granules were separated from plasma membranes and empty granule "ghosts," were used. The plasma membrane-induced release of the chromaffin granular matrix upon concomitant incubation was demonstrated by the disappearance of the "intact granule" peak accompanied by a proportional rise of the "granule ghost" peak. The degree of granule disappearance depended on the amount of plasma membranes added. The soluble content of the fraction that had disappeared was dispersed throughout the gradient. No shift of the granule peak toward intermediate densities was observed; this suggests that the loss of each granule's content is an all-or-none phenomenon rather than a partial release.