Exhibition of Specific Alterations in Activities and mRNA Levels of Rat Islet Glycolytic and Mitochondrial Enzymes in Three Different in Vitro Model Systems for Attenuated Insulin Release

We studied the possible relationships between the functional status of the β-cell and activities or mRNA contents of enzymes involved in the catabolism of glucose. Three different in vitro models with attenuated insulin response were used: rat islets cultured at a low glucose concentration, rat islets incubated in vitro with streptozocin, and fetal rat islets. The fetal and streptozocin-administered islets were compared with adult islets cultured in RPMI-1640 containing 11 mM glucose, and the effects of the in vitro glucose concentrations (3.3, 11, and 28 mM) were assessed on adult islets only. Cellular mRNA levels for the mitochondrial DNA-encoded cytochrome b and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were determined by Northern-blot analysis. Enzymatic activities of high-K_m (glucokinase) and low-K_m (hexokinase) glucose-phosphorylating enzymes and succinate–cytochrome c reductase were also determined. Islets cultured at 3.3 mM glucose displayed a decreased activity of glucokinase compared with islets cultured at 28 mM glucose (23.3 ± 12%), whereas there was no difference in hexokinase activity or the level of GAPDH mRNA. The activity of succinate–cytochrome c reductase was similar in islets cultured at the different glucose concentrations. The level of cytochrome b mRNA increased at 28 mM glucose compared with islets cultured at 11 mM glucose (140 ± 14%). Islets incubated with streptozocin and subsequently cultured for 7 days at 11 mM glucose exhibited a decreased level of cytochrome b mRNA (65 ± 5%) and no differences in the activities of glucokinase, hexokinase, succinate–cytochrome c reductase, or the level of GAPDH mRNA. Fetal islets displayed increased activity of hexokinase (570 ± 130%) and succinate–cytochrome c reductase (185 ± 42%) and a decreased level of cytochrome b mRNA (62 ± 11%) in comparison with adult islets cultured at similar conditions. No differences of glucokinase activity or the level of GAPDH mRNA were detected in these islets. The results suggest that the degree of expression of certain genes coding for enzymes in the oxidative metabolism of glucose may influence the ability of the α-cell to synthesize and secrete insulin.