Insulin immunoreactive sites demonstrated in the Golgi apparatus of pancreatic B cells.

Insulin immunoreactive sites were localized in the Golgi apparatus of pancreatic B cells by light and electron microscopy. Identification of the Golgi apparatus by immunofluorescence required the prior degranulation of B cells with glibenclamide to reduce the insulin immunostaining due to secretory granules. In such cells, insulin immunofluorescence revealed brightly stained, crescent-shaped strands with form and location super-imposable on that of Golgi complexes seen in thin sections of the same cells. With the electron microscope, the insulin immunoreactive sites revealed by the protein A/gold technique were localized in the cisternae and vesicles of the Golgi apparatus of glibenclamide-treated and control B cells and over maturing and mature secretory granules. The quantitative evaluation of the intensity of the insulin immunoreactive sites in the Golgi apparatus revealed a density of sites 4 times more than cellular background values. The demonstration of insulin immunoreactivity in the Golgi apparatus provides direct evidence for the involvement of this compartment in the transport and maturation of proinsulin into insulin.