Immunohistochemical colocalization of type II angiotensin receptors with somatostatin in rat pancreas

Abstract

Earlier studies indicate that binding sites of type II angiotensin (AT 2) receptors are detected all over the pancreas, as well as in the pancreatic exocrine cell line AR4-2J. However, lack of corresponding functional AT 2 receptor responses can be detected in the exocrine pancreas. The aim of present study is to determine the protein expression of AT 2 receptors in the pancreas by probing with an AT 2 receptor-specific antibody, and to examine the role of AT 2 receptors in the regulation of pancreatic endocrine hormone release. In Western protein analysis of adult rat tissues, expression of AT 2 receptor-immunoreactive bands of 56, 68, and 78 kDa was detected in the adrenal, kidney, liver, salivary glands, and pancreas. In adult rat pancreas, strong immunoreactivity was detected on cells that were located at the outer region of Langerhans islets. Immunohistochemical studies indicated that AT 2 receptors colocalized with somatostatin-producing cells in the endocrine pancreas. Consistent with the findings in adult pancreas, abundant expression of AT 2 receptors was also detected in immortalized rat pancreatic endocrinal cells lines RIN-m and RIN-14B. To examine the role of AT 2 receptors on somatostatin secretion in the pancreas, angiotensin-stimulated somatostatin release from pancreatic RIN-14B cells was studied by an enzyme immunoassay in the absence or presence of various subtype-selective angiotensin analogues. There was a basal release of somatostatin from RIN-14B cells at a rate of 8.72±4.21 ng/10 6 cells ( n=7). Angiotensin II (1 nM–10 μM) stimulated a biphasic somatostatin release in a dose-dependent manner with an apparent EC 50 value of 49.3±25.9 nM ( n=5), and reached maximal release at 1 μM angiotensin II (982±147.34% over basal secretion; n=5). Moreover, the AT 2 receptor-selective angiotensin analogue, CGP42112, was 1000 times more potent than the AT 1 receptor-selective angiotensin analogue, losartan, in inhibiting angiotensin II-stimulated somatostatin release. These results suggest that angiotensin may modulate pancreatic hormone release via regulation of somatostatin secretion.