Morphometric Studies of Secretory Granule Distribution and Association With Microtubules in β-Cells of Rat Islets During Glucose Stimulation

Morphometric studies of β-cell granule positions and interactions with microtubules in areas of exocytosis were performed on islets of rats injected with tolbutamide for 72 h. This treatment depleted granules 85% below normal control values and rendered mapping of their positions and visualization of microtubules more feasible during electron microscopy. Islets were perifused with 2.8 or 16.7 mM glucose and were fixed at 4 min (acute phase) or 20 min (second phase). Photomicrographs of β-cell sections were digitized, and computer replicas were compared to computer-simulated cells of similar size and shape into which an identical number of granules was inserted randomly. Margination of granules into a zone within 1.5 μm of the plasma membrane was examined. At both 2.8 and 16.7 mM glucose, real cells marginated significantly more secretory granules than did simulated cell pairs. Within the zones of margination during 16.7 mM glucose perifusion, the number of secretory granules associated with microtubules was <6% in the acute phase and <1% in the second phase. These data suggest that glucose stimulation promotes movement of secretory granules into the β-cell periphery to a greater extent than can be attributed to chance alone. The role of microtubules in peripheral granule movement into regions of exocytosis appears to be minimal.