Annexin-I was demonstrated to specifically present in islets and not in exocrine tissues of the rat pancreas and to have a diffuse and homogeneous distribution in all islet cells in our previous study. In the present report, to clarify the functions of annexin-I in rat pancreatic islets, especially in β-cells, we investigated the role of annexin-I in insulin secretion. Immunoelectron microscopic analysis of pancreatic β-cells demonstrated that immunogold particles reactive to annexin-I were almost exclusively observed on most of the insulin-containing granules (∼ 90%) and less frequently located in cytosol and other organelles, such as the endoplasmic reticulum and mitochondria. The number of annexin-I gold particles located on insulin granules after oral glucose administration was significantly increased compared with that observed in fasted rats. Moreover, when the isolated islets were stimulated by a high concentration of glucose (20 mM), the phosphorylation of annexin-I was markedly enhanced, and it was synchronized to insulin secretion. This phosphorylation mainly occurred on serine residues. H-7 (100 μM), a potent inhibitor of protein kinase-C, inhibited the phosphorylation to about 90%. These findings suggest that annexin-I might be involved in the regulatory mechanism of glucose-induced insulin secretion in rat pancreatic islets via phosphorylation-dephosphorylation processes.
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