The Stimulus-Secretion Coupling of Glucose-Induced Insulin Release: IX. The Participation of the Beta Cell Web

The rat pancreatic beta cell is surrounded by an ectoplasmic band of microfilamentous material located just beneath the plasma membrane. This cell web extends into the core of microvillous processes. Alteration of such a microfilamentous network by cytochalasin B (10 μg./ml.) results in a shortening or disappearance of the microvillous processes, a prominent margination of secretory granules and a redistribution of the cell web with the appearance of large and heterogenous masses of granular material extending into the cytoplasm. These ultrastructural changes are associated with a reversible enhancement of insulin release induced by glucose, theophylline or sulfonylurea in isolated islets of Langerhans. However, cytochalasin B fails to affect the basal rate of insulin release observed, respectively, at low glucose or calcium concentrations or in the presence of deuterium oxide. Cytochalasin B does not significantly modify the stimulant action of glucose on calcium accumulation and insulin biosynthesis in the isolated islets. These findings suggest that the microfilamentous cell web plays an important role in the active process of insulin secretion by controlling the access of insulin-containing secretory granules to the cell membrane.