Nitric oxide modulates a late step of exocytosis

The effects of nitric oxide (NO) on the late phase of exocytosis have been studied, by amperometry, on Ba2+-stimulated chromaffin cells. Acute incubat ion with NO or NO donors (sodium nitroprusside, spermine-NO, S-nitrosogluta thione) produced a drastic slowdown of the granule emptying. Conversely, ce ll treatment with N-omega-nitro-L-arginine methyl ester (a NO synthase inhi bitor) or with NO scavengers (methylene blue, 2-(4-carboxy-phhenyl) -4,4,5, 5-tetramethyl-imidazoline-1-oxyl-3-oxide potassium) accelerated the extrusi on of catecholamines from chromaffin granules, suggesting the presence of a NO modulatory tone. The incubation with phosphodiesterase inhibitors (3-is obutyl-1-methylxanthine or zaprinast) or with the cell-permeant cGMP analog 8-bromo-cGMP, mimicked the effects of NO, suggesting the involvement of th e guanylate cyclase cascade. NO effects were not related to changes in intr acellular Ba2+ NO did not modify the duration of feet. Effects were evident even on pre-fusioned granules, observed under hypertonic conditions, sugge sting that the fusion pore is not the target for NO, which probably acts by modifying the affinity of catecholamines for the intragranular matrix, NO could modify the synaptic transmitter efficacy through a novel mechanism, w hich involves the regulation of the emptying of secretory vesicles.