We have recently developed a combination of polarization and total internal reflection fluorescence microscopy (pTIRFM) to monitor changes in plasma membrane topology occurring after fusion of chromaffin granules. In this report, pTIRFM is further exploited to reveal two major findings in regards to the secretory pathway in bovine chromaffin cells. First, we show that changes in membrane topology are sometimes detected even prior to fusion. This occurs with high probability in a small subset of granules that appear in the evanescent field during the experiment. On these occasions, the plasma membrane invaginates with the movement just preceding the appearance of a granule in the evanescent field. Such events may represent a direct interaction of the granule with the plasma membrane. Second, we show that the topological fate of the post-fusion, granule/plasma membrane intermediate is regulated by divalent cation. When Sr2+ is used instead of Ca2+ to trigger exocytosis, membrane topology in the exocytotic region is stabilized with significant curvature and indentation.
