Isolated live pancreatic acinar cells in physiological buffer, when imaged by using AFM (8, 19), reveal at the apical plasma membrane a group of circular "pits" measuring 0.4–1.2 µm in diameter, punctuated by smaller "depressions" within. Each depression averages 100–150 nm in diameter (Fig. 1), and typically three to four depressions are located within a pit. The basolateral membranes of acinar cells are, however, devoid of either pits or depressions. High-resolution AFM images of depressions in live cells further reveal a cone-shaped morphology (Fig. 2). The depth of each depression cone measures 15–30 nm. Similarly, both growth hormone (GH)-secreting cells of the pituitary gland and the chromaffin cell also possess pits and depression structures on their plasma membranes (7, 9), suggesting the universal presence of fusion pores in secretory cells.
Regulation and dynamics of depressions
Exposure of pancreatic acinar cells to a secretagogue (mastoparan) results in a time-dependent increase (20–35%) in depression diameter, followed by a return to resting size following completion of secretion (Fig. 3
). However, no demonstrable change in pit size is detected during this time. Enlargement of depression diameter and an increase in its relative depth following exposure to secretagogues correlated with increased secretion. Exposure of pancreatic acinar cells to cytochalasin B, a fungal toxin that inhibits actin polymerization, results in 15–20% decrease in depression size and a consequent 50–60% loss in secretagogue-induced secretion. Results from these studies suggested that depressions are the fusion pores in pancreatic acinar cells. Furthermore, these studies demonstrated the involvement of actin in regulation of the structure and function of depressions.
Analogous to pancreatic acinar cells, examination of resting GH-secreting cells of the pituitary (7) and chromaffin cells of the adrenal medulla (9) also reveal the presence of pits and depressions on the cell plasma membrane. Depressions in resting GH cells measure 154 ± 4.5 nm (mean ± SE) in diameter. Exposure of the GH cell to a secretagogue resulted in a 40% increase (215 ± 4.6 nm; P diameter but no appreciable change in pit size.