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Immunohistochemical Localization of Monoamine Oxidase Type B in Pancreatic Islets of the Rat

Yu-Hong Huang, Akio Ito and Ryohachi Arai

Department of Anatomy, Shiga University of Medical Science, Otsu, Shiga, Japan (Y-HH,RA), and Department of Chemistry, Faculty of Science, Kyushu University, Fukuoka, Fukuoka, Japan (AI)

Correspondence to: Dr. R. Arai, Department of Anatomy, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan. E-mail: rarai{at}belle.shiga-med.ac.jp

Monoamine oxidase (MAO) is regarded as a mitochondrial enzyme. This enzyme localizes on the outer membrane of mitochondria. There are two kinds of MAO isozymes, MAO type A (MAOA) and type B (MAOB). Previous studies have shown that MAOB activity is found in the pancreatic islets. This activity in the islets is increased by the fasting-induced decrease of plasma glucose level. Islet B cells contain monoamines in their secretory granules. These monoamines inhibit the secretion of insulin from the B cells. MAOB is active in degrading monoamines. Therefore, MAOB may influence the insulin-secretory process by regulating the stores of monoamines in the B cells. However, it has not been determined whether MAOB is localized on B cells or other cell types of the islets. In the present study, we used both double-labeling immunofluorescence histochemical and electron microscopic immunohistochemical methods to examine the subcellular localization of MAOB in rat pancreatic islets. MAOB was found in the mitochondrial outer membranes of glucagon-secreting cells (A cells), insulin-secreting cells (B cells), and some pancreatic polypeptide (PP)-secreting cells (PP cells), but no MAOB was found in somatostatin-secreting cells (D cells), nor in certain other PP cells. There were two kinds of mitochondria in pancreatic islet B cells: one contains MAOB on their outer membranes, but a substantial proportion of them lack this enzyme. Our findings indicate that pancreatic islet B cells contain MAOB on their mitochondrial outer membranes, and this enzyme may be involved in the regulation of monoamine levels and insulin secretion in the B cells.

(J Histochem Cytochem 53:1149–1158, 2005)

Key Words: MAO • glucagon • insulin • somatostatin • pancreatic polypeptide • rat • pancreas • mitochondria • double-labeling • immunofluorescence • histochemistry • electron microscopic • immunohistochemistry

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