Immunocytochemical Characterization of Secretory Granule Maturation in Pancreatic A-Cells*

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Abstract
Antigenic sites related to glucagon and glucagon precursors were characterized by ultrastructural immunocytochemistry in secretory compartments of the [hamster] pancreatic A-cell, i.e., Golgi cisternae, condensing granules in Golgi cisternae, coated immature secretory granules and noncoated mature secretory granules. The C-terminal glucagon immunoreactivity was low in all these compartments except in the noncoated mature secretory granules. By contrast, N-terminal glucagon and glicentin immunoreactivities were high at the condensing granule stage and, respectively, increased (N-terminal) or decreased (glicentin) until the mature secretory granule stage. Apparently, glucagon precursors and glucagon coexist at different concentrations in secretory compartments of the A-cell; the condensing granules in Golgi cisternae and immature granules consist predominantly of glucagon precursors; and the removal of the peptide masking the C-terminal of the glucagon molecule occurs between the coated and the mature granule stages.