Localization of beta-endorphin in rabbit pancreatic islets

To determine whether beta-endorphin might play a physiological role in regulating insulin secretion, we examined the rabbit pancreas for an endogenous source of beta-endorphin. Specific antibodies to beta-endorphin were used together with anti-insulin, anti-glucagon, and anti-somatostatin in single-labeled and double-labeled indirect immunofluorescence and in sequential avidin-biotin-peroxidase immunohistochemistry. Light microscopic examination of rabbit islets revealed beta-endorphin immunoreactivity primarily in the beta cells. Approximately 97% of the cells exhibiting beta-endorphin immunoreactivity contained immunoreactive insulin; whereas many, but not all, of the insulin-containing cells contained immunoreactive beta-endorphin. Strongly beta-endorphin-positive cells were usually weakly insulin-positive; and strongly insulin-positive cells were often beta-endorphin-negative. The beta-endorphin-positive cells that were insulin-negative were identified as immunoreactive glucagon- or somatostatin-containing cells. The intensity of the immunoreactive beta-endorphin appeared to be more heterogeneous than the staining intensities of other hormones. Together with the previously reported data on the effects of beta-endorphin on pancreatic hormone secretion, the results of this investigation suggest that beta-endorphin may play an important autocrine or paracrine role in the pancreatic islets.