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Upon exposure of pancreatic B cells to exogenous horseradish peroxidase (HRP), a population of secretory granules becomes HRP-labelled. In isolated islets of Langerhans, we studied the fate of HRP-labelled secretory granules during a pulse-chase experiment with HRP in order to assess their relationship with lysosomes containing secretory granule cores. These structures (crinophagic or multigranular bodies) were previously shown to be a site of insulin degradation (Orci et al., J cell biol 98 (1984) 222) [4]. After a 15-min pulse of peroxidase, the number and volume density of HRP-labelled secretory granules decreased over an 85-min chase period, during which the number and volume density of multigranular bodies labelled with HRP was significantly increased. At both time points, the surface density of HRP-labelled Golgi elements was very small compared with that of unlabelled ones. By autoradiography after a 5-min pulse of [3H]leucine and a 55-min chase, followed by a 15-min pulse of HRP and a 85-min chase, we could show that the majority of HRP-containing secretory granules were not radioactively labelled granules. These results suggest that: The low degree of HRP labelling of the Golgi makes it unlikely that secretory granules derive their HRP by budding from HRP-labelled cisternae. HRP-labelled SGs are preferentially transferred to MGBs (which become HRP-labelled) for prospective degradation. HRP labelling does not involve newly-formed mature secretory granules.
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