REDUCED LEVELS OF MESSENGER-RIBONUCLEIC-ACID FOR CALCIUM-CHANNEL, GLUCOSE TRANSPORTER-2, AND GLUCOKINASE ARE ASSOCIATED WITH ALTERATIONS ININSULIN-SECRETION IN FASTED RATS

Expression of the genes for voltage-dependent calcium channels (VDCCs) ,glucose transporter-2 (GLUT2), and glucokinase was studied in pancrea tic islets obtained from normal rats after periods of fasting and refe eding using a competitive polymerase chain reaction procedure. A 72-h fast induced about a 3-fold decrease in the beta-cell/neuroendocrine t ype VDCC alpha(1)-subunit and GLUT2 messenger RNA (mRNA) levels and ab out 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 a mong fed, fasted, and refed rats. We also studied insulin secretion fr om the isolated perfused pancreata obtained from these animals. We fou nd 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 drama tic decrease in insulin secretory responses during the application of an L-type VDCC agonist, Bay K8644 (1 mu M). Furthermore, fasting resul ted in a significant decrease in both Ca-45(2+) uptake by the isolated islets and insulin release from the islets. A strong positive correla tion was observed between glucose-induced Ca-45(2+) uptake and insulin output among the animals studied. On the other hand, after a 24-h ref eeding, significant increases in the insulin secretory response to glu cose 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 r eductions in expression of at least in part the beta-cell/neuroendocri ne type VDCC in addition to two components of the glucose-sensing appa ratus, including glucokinase and GLUT2, and the reduction in mRNA for insulin.