Objective: 

Insulin secretion from the pancreatic beta cell is controlled by a variety of modulatory pathways and glucose-induced insulin secretion can be amplified. These amplification pathways may utilise an increase in intracellular cAMP that acts in PKA-dependent or PKA-independent manner as well as activation of PKC pathway. In this study we assessed the role of cAMP and PKC-dependent amplifying pathways in Ca²⁺ -dependent exocytosis in pancreatic beta cells.

Methods: 

The whole-cell patch-clamp was used simultaneously with slow photo-release of caged Ca²⁺ to elicit membrane capacitance change (Cm).

Results: 

The control cells exposed to slow Ca²⁺ uncaging the Cm increased in a reproducible Ca²⁺- dependent manner. The Ca²⁺-dependency of the rate of the initial Cm change followed the saturation kinetics with half-maximal value (EC50) of 2.9 ± 0.2 mM. After clamping the cytosol at 200 mM cAMP or inclusion of a specific PKA activator the Ca²⁺-dependency of the initial Cm has been shifted to significantly lower EC50. Alternatively the specific activation of Epac2 increased the rate of exocytosis without changing the sensitivity to [Ca²⁺]i. The activation of PKC significantly increased the calcium sensitivity comparing to the cells where PKC was inhibited.

Conclusions: 

Our findings suggest that cAMP modulates the rate of exocytosis in pancreatic beta cells mainly through PKA-dependent pathway by sensitizing the insulin releasing machinery to [Ca²⁺]i. On the other hand activation of PKC appears to be important mechanism to prevent the fusion of the secretory vesicles at insufficient [Ca²⁺]i.