Elevated basal insulin secretion under fasting conditions together with insufficient stimulated insulin release is an important hallmark of type 2 diabetes, but the mechanisms controlling basal insulin secretion remain unclear. Membrane rafts exist in pancreatic islet cells and spatially organize membrane ion channels and proteins controlling exocytosis, which may contribute to the regulation of insulin secretion. Membrane rafts (cholesterol and sphingolipid containing microdomains) were dramatically reduced in human type 2 diabetic and diabetic Goto-Kakizaki (GK) rat islets when compared with healthy islets. Oxidation of membrane cholesterol markedly reduced microdomain staining intensity in healthy human islets, but was without effect in type 2 diabetic islets. Intriguingly, oxidation of cholesterol affected glucose-stimulated insulin secretion only modestly, whereas basal insulin release was elevated. This was accompanied by increased intracellular Ca2+ spike frequency and Ca2+ influx and explained by enhanced single Ca2+ channel activity. These results suggest that the reduced presence of membrane rafts could contribute to the elevated basal insulin secretion seen in type 2 diabetes.
Analysis of mutant human fibroblasts deficient in a cell surface receptor for low density lipoproteins (LDL) has led to the delineation of an important, hitherto unrecognized, regulatory process for cholesterol metabolism. On normal cells, binding of LDL to this receptor regulates cholesterol metabolism by two mechanisms: (a) suppression of cholesterol synthesis and (b) facilitation of the rate of proteolytic degradation of the lipoprotein. In cells from homozygotes with the autosomal dominant disorder Familial Hypercholesterolemia, a nearly total reduction in LDL receptors results in two secondary abnormalities: (a) overproduction of cholesterol due to an inability of LDL to suppress the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase, the rate-controlling enzyme in cholesterol biosynthesis, and (b) impairment in the rate of proteolytic degradation of LDL. Cells from heterozygotes possess about 50 per cent of the normal number of LDL recpetors; this leads to a concentration-dependent defect in regulation, so that attainment of rates of cholesterol synthesis and LDL degradation equal to that in normal cells requires a two to three-fold higher concentration of extracellular LDL in the heterozygote. The identification of this genetic regulatory defect in fibroblasts of heterozygotes with Familial Hypercholesterolemia makes available an in vitro system for studying the molecular mechanism by which a dominant mutation affects gene expression in mammalian cells.
All-cause emergency readmission following AUGIB is frequent. It is related to rebleeding in one-fifth of cases and mortality is similar to that in index admissions. The presence of comorbid illness appears to predict readmissions. Reduced length of stay and bisphosphonate use appear to be important, potentially modifiable, predictors of postdischarge rebleeding.
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