The metabolic syndrome of fructose-fed rats: Effects of long-chain polyunsaturated ω3 and ω6 fatty acids. IV. D-glucose metabolism by isolated pancreatic islets

Abstract

Abstract. The major aim of the present study was to search for changes of D-glucose metabolism in isolated pancreatic islets possibly involved in the alteration of their secretory response to the hexose, as observed when comparing rats exposed for 8 weeks to diets containing either starch and sunflower oil or fructose and sunflower oil, as well as rats exposed to diets containing fructose, sunflower oil and either salmon oil or safflower oil. The substitution of starch by fructose in the diet affected unfavourably D-glucose phosphorylation by the isolated islets. In the fructose-fed rats, there was a close parallelism between D-[5-3 H]glucose utilization and the dietary ω3/ω6 fatty acid ratio. There was little to distinguish, however, between the four groups of rats in terms of D-[U-14 C]glucose oxidation. The paired ratio between D-[U-14 C]glucose oxidation and D- [5-3 H]glucose utilization, which always increased as the concentration of the hexose was raised from 2.8 to 8.3 and 16.7 mM, was tightly related, in the fructose-fed rats, to the HOMA index for insulin resistance. IntroductionIn the present series of reports dealing with the metabolic and hormonal effects of dietary long-chain polyunsaturated ω3 and ω6 fatty acids in rats exposed to a fructose-rich diet, emphasis was so far placed on i) the changes in glucose tolerance, insulino genic index and insulin resistance, as assessed during an intraperitoneal glucose tolerance test, ii) the time course of changes in such variables as body weight, food intake, plasma D-glucose and insulin concentrations over 8 weeks exposure to the four diets considered in this study, and iii) the insulin secretory behaviour and insulin content of pancreatic islets isolated from the rats at sacrifice (1-3). The present report concerns the metabolic fate of D-glucose in isolated pancreatic islets incubated in vitro at increasing concentrations of D-glucose. The major aim of such a study was to search for changes in D-glucose catabolism possibly involved in the alteration of the secretory response of the islets to this hexose. Materials and methodsPancreatic islets were collected after overnight starvation by the collagenase method, as described in the preceding article in this series. Groups of 20 islets were incubated for 90 min at 36˚C in 50 µl of a salt-balanced medium (4) containing 1.0 mg/ml bovine serum albumin and equilibrated against a mixture of O 2 /CO 2 (95/5, v/v 14 C]glucose (glucose oxidation) were measured by methods described elsewhere (5).All results are presented as mean values (± SEM) together with the number of individual observations (n). The statistical significance of differences between mean values was assessed by use of Student's t-test. Results D-[-U-14 C]glucose oxidation. As indicated in Table I, when expressed as D-glucose equivalent per islet over 90 min incubation, the oxidation of D-[-U-14 C]glucose progressively increased from 2.7±0.3 (n=49) at 2.8 mM D-glucose to 8.5±0.8 (n=49) and 15.7±2.1 (n=47) at 8.3 and 16.7 mM D-glucose, respectively. As...