Insulin was used to deplete the adrenalin stores of rat adrenal medulla cells. Release of secretion was observed to occur by exocytosis. In addition, during the stages of massive release of secretory granules, the insulin-treated preparations showed greatly enhanced endocytic uptake of horseradish peroxidase. The tracer was taken up within vesicles, tubules, multivesicular bodies, and dense bodies. From acid phosphatase studies and from previous work it appears that many of the structures in which peroxidase accumulates are lysosomes or are destined to fuse with lysosomes. Subsequent to the period of intense exocytosis and endocytosis, there is a transient accumulation of lipid droplets in the adrenalin cells. The cells then regranulate, with new granules forming near the Golgi region. These results suggest that under the conditions used, much of the membrane that initially surrounds secretory granules is degraded after release of the granules.

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