Calcium-activated factors in pancreatic islets that inhibit actin polymerization

Pancreatic islet cytosol in combination with 50 μm or lower Ca²⁺ markedly inhibits action polymerization when added to actin under conditions in which actin usually polymerizes. Inhibition by either cytosol or Ca²⁺ alone is minimal and heated cytosol is ineffective in inhibiting actin polymerization. Two inhibitory factors with relative molecular weights of about 200,000 and less than 40,000 can be separated by gel permeation chromatography. The larger factor can inhibit actin polymerization almost 100% in the presence of Ca²⁺. Pancreatic beta cells contain a well-described microfilamentous cell web beneath the plasma membrane hypothesized to be a barrier to insulin granule movement. An hypothesis on the function of the factors is that when beta cell calcium increases, as it does in response to stimuli for insulin secretion, calcium interacts with the factors to locally disrupt a microfilamentous barrier to insulin granule extrusion.