cAMP modulates exocytotic kinetics and increases quantal size in chromaffin cells

The role of cAMP/cAMP-dependent protein kinase (PKA) on the late phase of e xocytosis has been studied by amperometry on Ba2+-stimulated single bovine chromaffin cells. Forskolin (FSK) increases the intracellular CAMP levels i n a concentration-dependent manner. Forskolin (100 nM) does not increase th e number of exocytotic events, although it significantly increases the net granule content of catecholamines (CA), which is accompanied by a slowing o f the process of degranulation. These effects are reversible, occur within 15 to 60 s, and are not due to newly synthesized CA. Isoprenaline, pituitar y adenylate cyclase-activating polypeptide-38 or dB-cAMP reproduce FSK effe cts as does cholera toxin. The inhibition of phosphodiesterases with 3-isob utyl-1-methylxanthine mimics and potentiates the effect of FSK and isoprena line. Rolipram and okadaic acid also produce a drastic increase in net gran ule content of CA, whereas H-89 attenuates the FSK response. These data ind icate that cyclic AMP/PKA might favor the granule aggregation before its fu sion with cell membrane and slow the late step of the exocytotic process.