Activation of protein kinases and inhibition of protein phosphatases play a central role in the regulation of exocytosis in mouse pancreatic β cells

The mechanisms that regulate insulin secretion were investigated using capacitance measurements of exocytosis in single β cells maintained in tissue culture. Exocytosis was stimulated by voltage-clamp depolarizations to activate the voltage-dependent Ca²⁺ channels that mediate Ca²⁺ influx into the β cell. Under basal conditions, the exocytotic responses were small despite large Ca²⁺ currents. The exocytotic responses were dramatically increased (10- to 20-fold) by conditions that promote protein phosphorylation, such as activation of protein kinases A and C or inhibition of protein phosphatases. The stimulation of secretion was not due to an enhancement of Ca²⁺ influx and both peak and integrated Ca²⁺ currents were largely unaffected. Our data indicate that exocytosis in the insulin- secreting pancreatic β cell is determined by a balance between protein phosphorylation and dephosphorylation. They further suggest that although Ca²⁺ is required for the initiation of exocytosis, modulation of exocytosis by protein kinases and phosphatases, at a step distal to the elevation of Ca²⁺, is of much greater quantitative importance. Thus an elevation of Ca²⁺ may represent a permissive rather than a decisive factor in the regulation of the insulin secretory process.