Immunohistochemical localization of monoamine oxidase type B in pancreatic islets of the rat.
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.
Huang
Yu-Hong
YH
Department of Anatomy, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan.
Ito
Akio
A
Arai
Ryohachi
R
D000818:Animals; D005455:Fluorescent Antibody Technique; D007150:Immunohistochemistry; D007515:Islets of Langerhans / Q000166:cytology / Q000201:enzymology* / Q000648:ultrastructure; D008297:Male; D008854:Microscopy, Electron; D008995:Monoamine Oxidase / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013347:Subcellular Fractions / Q000201:enzymology
D008995:Monoamine Oxidase