Study of hexose transport, glycerol phosphate shuttle and Krebs cycle in islets of adult rats injected with streptozotocin during the neonatal period

Abstract

At 3–4°C, the transport of 3-O- methyl- d - glucose (30 mM) was severely impaired in islets prepared from adult rats injected with streptozotocin during the neonatal period. However, at 37°C, the first and second phase of glucose-stimulated insulin release were decreased to the same relative extent in perifused islets of diabetic, as compared to control, animals. Moreover, the time-related increase in the oxidative response of the islets to 16.7 mM d-glucose was less pronounced in diabetic than control rats. The activity of the mitochondrial FAD-linked glycerophosphate dehydrogenase in islet homogenates of diabetic rats only represented one-fifth of that found in control rats, whereas the activity of the cytosolic NAD-glycerophosphate dehydrogenase was comparable in both types of rats. This coincided with the fact that a rise in d-glucose concentration from 2.8 to 16.7 mM failed to increase significantly l-[2- 3H]glycerol conversion to 3HOH in islets from diabetic rats, in contrast to the situation found in control animals. The activity of 2-ketoglutarate dehydrogenase in islet homogenates when expressed per μg protein was not different in control and diabetic rats. Likewise, the ratio between d-[6− 14C]glucose oxidation and d-[3,4- 14C]glucose oxidation and the capacity of either a non-metabolized analog of l-leucine or 3-phenylpyruvate to preferentially stimulated d-[6− 14C]glucose oxidation relative to d-[5- 3H]glucose utilization were both unaffected in islets from diabetic rats. These findings argue against the existence of a primary defect in the Krebs cycle of diabetic rats. It is proposed that, despite an obvious alteration of the hexose transport system in the islet cells of diabetic rats, the preferential impairment of the B-cell secretory response to d-glucose, as distinct from other secretagogues, in this model of non-insulin-dependent diabetes is mainly attributable to the low activity of FAD-linked glycerophosphate dehydrogenase, resulting in a decreased metabolic flow through the glycerol phosphate shuttle and a reduced rate of aerobic glycolysis.