We have examined the effect of alpha-SNAP on exocytosis in adrenal chromaffin cells by direct assay of exocytosis using patch-clamp capacitance analysis. Cells were recorded using the whole cell patch-clamp configuration and the cells dialysed with control pipette solution or with a pipette solution containing alpha-SNAP or the deletion mutant alpha-SNAP(41–295). The deletion mutant was found to be unable to bind to syntaxin allowing a test of the requirement for syntaxin-binding for any effect of alpha-SNAP on exocytosis. Following cell dialysis for 10 minutes, cells were depolarised five times at 2 minute intervals. At each depolarisation step cells dialysed with alpha-SNAP showed a significant increase in both the initial rate and extent of exocytosis which was seen as a rise in membrane capacitance. This increase in exocytosis was not observed with alpha-SNAP(41–295) which instead produced some inhibition of the extent but had no effect on the initial rate of exocytosis. These results show directly that alpha-SNAP has a specific and marked stimulatory effect on exocytosis in chromaffin cells.
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