SURFACE DYNAMICS IN LIVING ACINAR-CELLS IMAGED BY ATOMIC-FORCE MICROSCOPY - IDENTIFICATION OF PLASMA-MEMBRANE STRUCTURES INVOLVED IN EXOCYTOSIS

The dynamics at the plasma membrane resulting from secretory vesicle d ocking and fusion and compensatory endocytosis has been difficult to o bserve in living cells primarily due to limited resolution at the ligh t microscopic level. Using the atomic force microscope, np have been a ble to image and record changes in plasma membrane structure at ultrah igh resolution after stimulation of secretion from isolated pancreatic acinar cells. ''Pits'' measuring 500-2000 nm and containing 3-20 depr essions measuring 100-180 nm in diameter were observed only at the api cal region of acinar cells. The time course of an increase and decreas e in ''depression'' size correlated with an increase and decrease of a mylase secretion from live acinar cells. Depression dynamics and amyla se release were found to be regulated in part by actin. No structural changes were identified at the basolateral region of these cells. Our results suggest depressions to be the fusion pores identified earlier in mast cells by freeze-fracture electron microscopy and by electrophy siological measurements. The atomic force microscope has enabled us to observe plasma membrane dynamics of the exocytic process in living ce lls in real time.