Regulation of 86Rb outflow from pancreatic islets. IV. Effect of cyclic AMP, dibutyryl-cyclic AMP and theophylline

Environmental modulation of the inhibitory action of D-mannoheptulose upon D-glucose metabolism in isolated rat pancreatic islets

In pancreatic islets prepared from fed rats and incubated at a low concentration (1.7 mM) of D-glucose, D-mannoheptulose (10.0 mM) virtually fails to affect the metabolism of the hexose. Likewise, in islets from starved rats, the relative extent of the inhibitory action of D-mannoheptulose upon D-glucose metabolism is much more marked at high (16.7 mM) than low (1.7 mM) hexose concentration. Nevertheless, despite decreasing the metabolism of D-glucose, starvation augments the sensitivity to D-mannoheptulose in the islets incubated at a low concentration of the hexose, D-galactose, but not D-fructose, also augments the inhibitory action of D-mannoheptulose upon D-glucose metabolism in islets prepared from fed rats and exposed to the low concentration of D-glucose. A comparable situation prevails in islets exposed to 2-ketoisocaproate. Forskolin, however, which decreases D-glucose catabolism in the islets from fed rats exposed to 1.7 mM D-glucose, fails to affect significantly the inhibitory action of D-mannoheptulose on D-glucose metabolism. It is proposed that hexoses transported by the same carrier as D-glucose and non-glucidic nutrient secretagogues somehow increase D-mannoheptulose uptake by the islet cells. The latter two conditions may be operative in islets exposed to a high concentration of D-glucose, this accounting for the exquisite sensitivity to D-mannoheptulose of glucose-stimulated islets.

Modulation of insulin secretion by feeding behavior and physical activity: possible beneficial effects on obese and aged rats

The high occurrence of diabetes in aged subjects is well known. In fact, the aging process is accompanied by obesity and presenting increased insulin release and lower peripheral responsiveness to this hormone. A recent study has shown that the changes in glucose utilization and insulin secretion during aging are abolished when obesity is somehow avoided. This fact justifies the effort to define new strategies to avoid the development of obesity during aging. In this review, beneficial effects of balanced diets, high frequency food intake and moderate exercise training on insulin secretion and its effect in rats are presented.

The stimulus-secretion coupling of glucose-induced insulin release. Environmental influences on L-glutamine oxidation in pancreatic islets

L-Glutamine at a near-physiological concentration (1.0mM) was rapidly taken up and metabolized in rat pancreatic islets. The rate of glutamine deamidation much exceeded that of glutamate conversion into 2-oxoglutarate, the latter conversion being mediated mainly by transamination reactions. The production of 14CO2 from L-[U-14C]glutamine, which reflected the generation of ATP through the metabolism of exogenous glutamine, appeared to be regulated by the redox state of nicotinamide nucleotides and the ATP content of the islet cells. The influence of environmental factors on glutamine oxidation was examined in order to identify ATP-requiring processes. Glutamine oxidation was decreased in the absence of extracellular Ca2+, under conditions aiming at inhibition of the (Na+ + NA+)-dependent ATPase and, provided that glucose was present in the incubation medium, by cycloheximide. These findings were interpreted to suggest that the handling of Ca2+ by the islet cells, the active transport of univalent cations and the biosynthesis of proinsulin represent three major ATP-consuming processes in this fuel-sensor organ.