ABSTRACT
Expression of the genes for voltage-dependent calcium channels (VDCCs), glucose transporter-2 (GLUT2), and glucokinase was studied in pancreatic islets obtained from normal rats after periods of fasting and refeeding using a competitive polymerase chain reaction procedure. A 72-h fast induced about a 3-fold decrease in the beta-cell/neuroendocrine type VDCC alpha 1-subunit and GLUT2 messenger RNA (mRNA) levels and about a 2-fold decrease in insulin and glucokinase mRNA levels compared to those in fed and refed rats. No significant differences were found in beta-actin and the cardiac-type VDCC alpha 1-subunit mRNA levels among fed, fasted, anf refed rats. We also studied insulin secretion from the isolated perfused pancreata obtained from these animals. We found an elevated threshold and decreased insulin release in response to a stepwise increase in glucose concentrations in the isolated perfused pancreata obtained from fasted rats. Fasting also resulted in a dramatic decrease in insulin secretory responses during the application of an L-type VDCC agonist, Bay K8644 (1 microM). Furthermore, fasting resulted in a significant decrease in both 45Ca2+ uptake by the isolated islets and insulin release from the islets. A strong positive correlation was observed between glucose-induced 45Ca2+ uptake and insulin output among the animals studied. On the other hand, after a 24-h refeeding, significant increases in the insulin secretory response to glucose and Bay K8644 were found, with a normalization in mRNA levels for these components. It, thus, appears that the alterations in beta-cell sensitivity to glucose that occur with fasting and refeeding are the result of complex metabolic alterations in the islet associated with reductions in expression of at least in part the beta-cell/neuroendocrine type VDCC in addition to two components of the glucose-sensing apparatus, including glucokinase and GLUT2, and the reduction in mRNA for insulin.
