Stimulation of insulin release by glucose in associated with an increase in the number of docked granules in the β-cells of rat pancreatic islets

Electron microscopy and quantitative stereological techniques were used to study the dynamics of the docked granule pool in the rat pancreatic beta-cell. The mean number of granules per beta-cell was 11,136. After equilibration in RPMI containing 5.6 mmol/l glucose, 6.4% of the granules (similar to700) were docked at the plasma membrane (also measured as [means +/- SE] 4.3 +/- 0.6 docked granules per 10 mum of plasma membrane at the perimeter of the cell sections). After a 40-min exposure to 16.7 mmol/l glucose, 10.2% of the granules (similar to1,060) were docked (6.4 +/- 0.8 granules per 10 mum of plasma membrane). Thus, the docked pool increased by 50% during stimulation with glucose. Islets were also exposed to 16.7 mmol/l glucose in the absence or presence of 10 mumol/l nitrendipine. In the absence and presence of nitrendipine, there were 6.1 +/- 0.7 and 6.3 +/- 0.6 granules per 10 mum of membrane, respectively. Thus, glucose increased granule docking independently of increased [Ca2+]i and exocytosis. The data suggest a limit to the number of docking sites. As the rate of docking exceeded the rate of exocytosis, docking is not rate limiting for insulin release. Only with extremely high release rates, glucose stimulation after a 4-h incubation with a high concentration of fatty acid-free BSA, was the docked granule pool reduced in size.