Systemic treatment with sympatholytic dopamine agonists improves aberrant beta-cell hyperplasia and GLUT2, glucokinase, and insulin immunoreactive levels in ob/ob mice

Abstract

Sympatholytic dopamine agonist treatment utilizing bromocriptine and SKF38393 (BC/SKF) significantly lowers basal plasma insulin levels and normalizes basal and glucose-induced insulin secretion of the pancreatic beta cell in ob/ob mice. While BC/SKF has no significant effect on pancreatic islet cells directly, drug action is mediated via alterations in the hypothalamic-neuroendocrine axis, which drives metabolic changes in peripheral tissues leading to a marked reduction in hyperglycemia and hyperlipidemia and corrects autonomic control of islet function. To elucidate the nature of the functional response of islets to systemic BC/SKF treatment in ob/ob mice, we investigated the relative changes in the levels of functionally important beta-cell proteins in situ, as well as differences in the beta-cell turnover rate, following a 2-week drug treatment. Isolated islets from treated mice exhibit a 3.5-fold increase in insulin content (P <.01) that correlated with a 51% reduction in basal plasma insulin levels (P <.01) compared with vehicle-treated controls. Using quantitative immunofluorescence microscopy on pancreatic tissue sections, insulin and GLUT2 immunoreactivity of islet beta cells of BC/SKF-treated mice were significantly increased (approximately 2.3-fold and approximately 4.4-fold, respectively; P <.002) to the levels observed in islets of their lean littermates. Glucokinase (GK) immunoreactivity was greatly (75%) reduced in beta cells from ob/ob versus lean mice (P <.0001). A modest increase in GK immunoreactivity in beta cells of drug-treated mice was observed (approximately 1.6-fold; P <.05). Isolated islets from BC/SKF-treated mice exhibit a 42% reduction in DNA content compared with vehicle-treated controls (P <.01) to levels observed in lean mice, but without notable differences in islet size. In situ assays for mitosis and apoptosis, using 5-bromodeoxyuridine (BrdU) and terminal deoxyribotransferase (TdT)-UTP nick end labeling (TUNEL) staining techniques, respectively, were performed in pancreas of these mice to determine if beta cells show a reduction in hyperplasia following BC/SKF treatment. Accordingly, a pronounced decrease in replicating, BrdU-positive beta cells in the drug-treated mice compared with the control group was observed, but without differences in their TUNEL-staining patterns. Collectively, these data suggest that systemic sympatholytic dopaminergic therapy that attenuates hyperglycemia and hyperlipidemia improves islet function in ob/ob mice by improving aberrations in the beta cell's glucose-sensing apparatus, enhancing insulin storage and/or retention, and stabilizing hyperplasia, thus reducing basal insulin levels.