Exocytosis and endocytosis in juxtaglomerular cells

The cellular events related to secretion of renin are not well understood. Here we review some of the evidence that has led to the current understandi ng of renin secretion as a process that involves exocytosis as the predomin ant mode of secretion. This is based on the observation of occasional fusio n events between secretory granules and cell membrane and measurement of in termittent secretion of renin from single afferent arterioles, with a renin content of each secretion episode that corresponds to the renin content of one secretory granule. More recently it has been demonstrated that the aff erent arterioles lose a large number of renin granules after acute stimulat ion without changing the average granular volume. Current electrophysiologi cal techniques have now permitted direct measurements of cell membrane capa citance in juxtaglomerular (JG) cells as a measure of net addition (exocyto sis) or removal (endocytosis) of membrane material. With this technique we have shown that cAMP, which is a vasodilator and stimulates renin secretion , enhances net exocytosis at low concentrations, while at higher concentrat ions membrane retrieval processes are also stimulated. We suggest that both exocytosis and endocytosis are regulated processes in the JG-cells and bot h may be important for the long-term control of renin secretion at the sing le cell level.