The priming effect of glucose on insulin release does not involve redistribution of secretory granules within the pancreatic B-cell.

Short-term stimulation of the pancreatic B-cell with glucose produces a time-dependent potentiation of this cell, which markedly enhances the insulin response to a renewed stimulation with the hexose. To study if a redistribution of the B-cell secretory granules to a location close to the B-cell plasma membrane could underlie the priming effect of glucose, an investigation by ultrastructural morphometry was performed. After exposure of perfused rat pancreas to non-priming or priming concentrations of glucose, pale and dark B-cell secretory granules were distinguished and analysed both within a central and a peripheral zone of the B-cell. The pale secretory granules comprised 30-40% of the total granule population in the B-cell. Whereas no difference in diameter of the granules was observed, there was evidence for a greater numerical density of dark granules in the central than in the peripheral part of the B-cell. This finding may be in line with observations implying that newly synthesized insulin is released preferentially to older insulin. The present experiments did, however, not reveal any significant priming effect of glucose on the intracellular distribution of secretory granules in the pancreatic B-cell. The lack of morphological changes in the B-cell by glucose priming of insulin release should, rather, direct increased attention to the biochemical aspects of the priming phenomenon.