Protein kinase A-dependent and -independent stimulation of exocytosis by cAMP in mouse pancreatic B-cells

1. The mechanisms by which cAMP stimulates Ca2+-dependent insulin secretion were investigated by combining measurements of whole-cell Ca2+ currents, the cytoplasmic free Ca2+ concentration ([Ca2+](i)) and membrane capacitance in single mouse B-cells maintained in tissue culture. 2. Cyclic AMP stimulated exocytosis > 4-fold in whole-cell experiments in which secretion was evoked by intracellular dialysis with a Ca2+-EGTA buffer with a [Ca2+](i) of 1.5 mu M. This effect was antagonized by inhibitors of protein kinase A (PICA). 3. Photorelease of cAMP from a caged precursor potentiated exocytosis at Ca2+ concentrations which were themselves stimulatory (greater than or equal to 60 nM), but was without effect in the complete absence of Ca2+. 4. Elevation of intracellular cAMP (by exposure to forskolin) evoked a 6-fold PKA-dependent enhancement of the maximal exocytotic response (determined as the maximum increase in cell capacitance that could be elicited by a train of depolarizations) in perforated-patch whole-cell recordings. 5. Exocytosis triggered by single depolarizations in standard whole-cell recordings was strongly potentiated by cAMP, but in this case the effect was unaffected by PKA inhibition. 6. When exocytosis was triggered by Ca2+ released from Ca2+-NP-EGTA ('caged Ca-2+,Ca-), cAMP exerted a dual stimulatory effect on secretion: a rapid (initiated within 80 ms) PKA-independent phase and a late PKA-dependent component. 7. We conclude that cAMP stimulates insulin secretion both by increasing the release probability of secretory granules already in the readily releasable pool and by accelerating the refilling of this pool.