In normal isolated ␤-cells, the response to glucose is heterogeneous and characterized by an increasing number of secretory cells as glucose concentration rises (Pipeleers DG, Kiekens R, Ling Z, Wilikens A, Schuit F: Physiologic relevance of heterogeneity in the pancreatic beta-cell population. D i a b e t o l o g i a 37 (Suppl. 2):S 5 7 -S 6 4 , 1994). We hypothesized that fasting hyperinsulinemia in obesity might be explained by altered ␤-cell heterogeneity of signal transduction mechanisms, possibly involving exocytotic proteins. Insulin secretion from individual ␤-cells sorted according to the size of the islet donor (<125 µm, >250 µm, and intermediate diameter) was measured by reverse hemolytic plaque assay. ␤-cells from f a/f a rats were hypertrophied 25-40%, independent of donor islet size. This was accompanied by an increased proportion of secretory cells (recruitment) at 5.5-11.0 mmol/l glucose, increased secretion per cell at 2.8 mmol/l glucose, and decreased insulin content after acute glucose exposure without an increase in secretion per cell. Decreased expression of exocytotic (soluble N-ethylmaleimide-sensitive fusion protein receptor [SNARE]) proteins, vesicle-associated membrane protein isoform 2 (VAMP-2), synaptosomal protein of 25 kDa (SNAP-25), and syntaxin-1 and -2 in f a/f a ␤-cells may contribute to the failure to sustain excessive plaque size at higher glucose concentrations. Fasting hyperinsulinemia may be maintained by increased recruitment and an exaggerated secretory response in all f a-derived islet populations. Glucose regulates ␤-cell responsiveness in the short term, and these effects may involve altered expression of SNARE proteins. D i a b e t e s 48:997-1005, 1999

The hypothesis that a defect in glucose sensing by islets of fa/fa Zucker rats contributes to hyperinsulinemia in these animals was tested. Islets from lean and fa/fa rats were isolated by collagenase digestion and step-density gradient purification and then cultured overnight in Dulbecco's modified Eagle's medium containing 12.5 mM glucose. Obese rat islets were more sensitive to hypoglycemic glucose levels with half-maximal effective concentration (EC50) of 5.6 mM compared with an EC50 of 8.2 mM for lean rat islets. In contrast, responsiveness of both phenotypes to alpha-ketoisocaproate and quinine was similar. Mannoheptulose did not inhibit insulin secretion from fa/fa islets, although inhibitors of later events in the stimulus-secretion coupling pathway were normally inhibited by iodoacetate and diazoxide. Finally, starvation in vivo and culture of islets in low glucose concentrations (5 mM) in vitro both decreased glucose-stimulated insulin secretion from lean but not fa/fa rat islets. We conclude that fa/fa rat islets have an exaggerated insulin response to hypoglycemic stimuli, possibly as a result of a defect in B-cell glucokinase function.

To protect cells from toxicity, metal-sensitive cellular compartments must be insulated against essential but toxic metals [such as copper (Cu)] accumulated in excess of metabolic requirements. We measured Cu concentrations at the organ and hepatic subcellular levels in juvenile rainbow trout (Oncorhynchus mykiss) during exposure to sublethal waterborne Cu (40 microg/L) for 21 days. There was a time-dependent accumulation of Cu in the gill, liver, plasma, and carcass, with significant difference in Cu-exposed fish relative to the controls being evident by day 7. This significant accumulation of Cu was not associated with impaired growth. Copper concentrations in purity-tested liver subcellular fractions normalized to the liver protein concentration were in decreasing order: organelles > heat-stable proteins > nuclei-debris > NaOH-resistant granules > heat-labile proteins. As a proportion of the total, the majority of the hepatocellular Cu burden (60-68%) was associated with a metabolically active pool (organelles, nuclei-debris, and heat-stable proteins) and the remainder (32-40%) was associated with a metabolically detoxified pool (heat-stable proteins and NaOH-resistant granules) irrespective of the Cu-exposure regime. Because Cu concurrently accumulated in metabolically active and detoxified pools, we conclude that the spillover hypothesis of metal toxicity did not hold under the exposure conditions employed in this study. Moreover, these data suggest that rainbow trout can withstand significant above-background Cu accumulation in hepatic putative metal-sensitive compartments without chronic toxic effects at the organism level.

No abstract

No abstract