It was shown that during cell spreading, and more recently also during phagocytosis, cell area increases over time until plasma membrane reservoir becomes completely sequestered, which also leads to an increase in membrane tension. Ginsenoside-F5 tension on the membrane is then compensated by the exocytosis of a vesicle pool. Altogether, these findings put membrane tension as a very important regulator of some biological processes. Considering this scenario and the fact that cholesterol removal enhances cell surface tension, we should expect that in this condition we would also observe exocytosis of an intracellular membrane reservoir. In fact, previous studies have shown that cholesterol sequestration through cyclodextrin treatment alters the regulation of several exocytic events, such as the release of synaptic vesicles in the peripheral and central nervous system or sperm acrosome and insulin secretion. Recently, our group, as well as Chen and co-workers, have also shown that membrane cholesterol sequestration leads to lysosomal exocytosis in cardiomyocytes and fibroblasts. Here we showed that lysosomal exocytosis occurs when cholesterol is removed from a fibroblast cell line, concomitantly with the increase in surface cell tension, measured by OT. Therefore, lysosome secretion may be a reaction triggered by the need to restore 
basal surface tension values. However the exact mechanism by which cholesterol sequestration and surface tension induces these exocytic events were still not clear. Membrane fusion events, such as synaptic vesicle and lysosomal exocytosis as well as other types of vesicle secretion, are usually regulated by calcium and occur through a mechanism dependent on proteins from the SNARE complex. Some of these proteins are known to be partitioned in cholesterol-dependent clusters, such as membrane rafts, at which sites vesicles fuse. It is possible that raft disorganization could Labetalol hydrochloride change the distribution and/or function of SNARE proteins, disturbing the exocytic events regulated by these proteins. In fact, SNARE redistribution upon cholesterol sequestration was reported for sperm cells and PC12 cells. In the case of PC12 cells, it was shown that SNARE localization in rafts act as negative regulators of synaptic vesicle secretion, and reducing SNAP 23 partitioning to raft sites enhanced vesicle exocytosis. Interestingly, SNAP 23 is one of the SNARE complex proteins involved in lysosomal fusion events. Therefore, SNARE re-localization could be a possible explanation for lysosome exocytosis triggered upon MbCD treatment. However, it is well accepted that, in regulated exocytosis, calcium is important for the final fusion event, especially for docked/primed vesicles. Even though SNAREs are most likely already partially zippered in this state, full zippering is believed to occur only when calcium is present. Calcium binding to synaptotagmin would trigger fusion either by activating SNAREs or by lowering the activation energy barrier for fusion. In our previous work with cardiomyocytes we have demonstrated that cholesterol sequestration leads to lysosomal exocytosis independently of extracellular calcium. Here we show that this cholesterol-induced lysosomal exocytosis in cardiomyocytes is also independent of intracellular calcium. However, this study also proposed that, for the cells analyzed, these exocytic events were dependent on extracellular calcium.This apparent discrepancy in relation to our previous published data is probably due to the pair cyclodextrin-cell utilized. Also, since MbCD presents more affinity for cholesterol then HPbC, the lysosomal exocytosis triggered by MbCD possibly shows a different pattern in relation to the exocytosis provoked by HPbC incubation.