Inhomogeneity of surface labelling of B-cells at prospective sites of exocytosis.

Using ferritin-labelled Ricinus communis agglutinin to detect lectin-binding sites of the pancreatic B-cell surface, we show that limited regions of the plasma membrane are deprived of lectin-binding sites over marginated secretory granules. Such deprived regions increased during glucose stimulation of B-cells in monolayer culture: 56 +/- 8 of them were found in high (300 mg/100 ml) glucose as compared to only 27 +/- 5 in low (50 mg/100 ml) glucose (p less than 0.005). In addition, non-membrane, intracytoplasmic bridges were detected between the plasma membrane and the membrane of the marginated granule suggesting the involvement of cell web components in promoting the change in surface labelling.