Identification of the docked granule pool responsible for the first phase of glucose-stimulated insulin secretion

The mechanisms underlying the first phase of glucose-stimulated insulin rel ease, the deterioration of which marks the early stages of both type 1 and type 2 diabetes, are essentially unknown. Among many hypotheses, one holds that the first phase is due to a readily releasable pool of insulin-contain ing granules, We used current knowledge of the mechanisms of exocytosis and the proteins involved in docking granules at the plasma membrane to test t his hypothesis, A docked pool of readily releasable granules was identified by immunoprecipitation of the plasma membrane protein syntaxin with a spec ific antibody and by co-immunoprecipitation of soluble N-ethylmaleimide-sen sitive factor attachment protein-25 (SNAP-25) and the granule proteins syna ptobrevin and synaptotagmin. The four SNARE proteins co-immunoprecipitated each other, thus identifying the core complex associated with docked granul es, Using co-immunoprecipitation as a marker for docked granules, we found that the docked pool was rapidly discharged during the first phase of gluco se-stimulated insulin release and refilled during the second phase, Other s ecretagogues also released the pool, whereas the physiological inhibitor no repinephrine blocked its release. Further studies on the nature of this poo l of granules should shed light on the causes of its deterioration in the e arly stages of diabetes and the reasons for deficient insulin release.