Rafael Nesher

Glucotoxicity and β-Cell Failure in Type 2 Diabetes Mellitus

Type 2 diabetes mellitus is increasing worldwide with a trend of declining age of onset. It is characterized by insulin resistance and a progressive loss of beta-cell function. The ability to secrete adequate amounts of insulin is determined by the functional integrity of beta-cells and their overall mass. Glucose, the main regulator of insulin secretion and production, exerts negative effects on beta-cell function when present in excessive amounts over a prolonged period. The multiple metabolic aberrations induced by chronic hyperglycemia in the beta-cell include increased sensitivity to glucose, increased basal insulin release, reduced response to stimulus to secrete insulin, and a gradual depletion of insulin stores. Inadequate insulin production during chronic hyperglycemia results from decreased insulin gene transcription due to hyperglycemia-induced changes in the activity of beta-cell specific transcription factors. Hyperglycemia may negatively affect beta-cell mass by inducing apoptosis without a compensatory increase in beta-cell proliferation and neogenesis. The detrimental effect of excessive glucose concentrations is referred to as glucotoxicity. The present review discusses the role of glucotoxicity in beta-cell dysfunction in type 2 diabetes mellitus.

Cyclic nucleotide levels in the perfused rat heart subjected to hypoxia

Isolated rat hearts were subjected to hypoxic perfusion on a recirculating Langendorff apparatus. Following a 30-min-period of aerobic stabilization the hearts were perfused for 30 min with media equilibrated with 84% N2, 12% O2 and 4% CO2. At the end of the hypoxic period myocardial concentrations of cyclic AMP and cyclic GMP were determined by radioimmunoassay. Exposure to hypoxia resulted in a significant increase in cyclic AMP (p<0.01) and a decrease in cyclic GMP (p<0.05) as compared to hearts perfused for 60 min with media gassed with 96% O2, 4% CO2.

Insulin secretion in obese and non-obese NIDDM

Both the insulin response to glucose and the sensitivity to insulin show large variation in the normal population. Many subjects have either a markedly low insulin response or low sensitivity to insulin, with nevertheless normal glucose tolerance. For such subjects to become diabetic, insulin secretion or insulin action must further deteriorate with time, or other factors are added which tip the balance towards diabetes. Most evidence to date indicates that reduced beta-cell responsiveness and reduced insulin sensitivity co-exist in subjects prior to developing NIDDM. Both insulin secretion and insulin action are genetically controlled and influenced by intrauterine and neonatal factors. Insulin secretion and insulin action vary inversely in a closely linked manner; inability to fully compensate for changes in one variable may generate a functional deficit in glucose homeostasis. Subjects combining low functions would run a proportionately larger risk of decompensating the glucose tolerance and be more vulnerable, in terms of diabetes susceptibility, to factors that further reduce insulin output or insulin action. Careful analysis of existing data prompts us to ascribe a dominating role to the impairment of insulin secretion in the pathogenesis of IGT and NIDDM. Patients with NIDDM also exhibit increased proportions of proinsulin and proinsulin conversion intermediates. We used hyperinsulinaemic diabetic and non-diabetic Psammomys obesus to study the possible relationship between steady-state pancreatic insulin stores and the proportion of proinsulin-related peptides in the plasma and the pancreas. A marked increase in these peptides was associated with 90% reduction in insulin stores of the pancreas. After food deprivation, the depletion of pancreatic insulin in the diabetic animals was partially corrected, and the proinsulin/insulin ratio normalized. In contrast, non-diabetic psammomys showed only 50% reduction in pancreatic insulin stores under non-fasting conditions, with no change in proinsulin/insulin ratio. These findings suggest that in the diabetic Psammomys obesus, pancreatic capacity for storage/production of insulin is limited; the metabolic consequences of this limitation are amplified by increased secretory demand secondary to insulin resistance, thus facilitating the establishment of hyperglycaemia, which may in itself further exacerbate the pancreatic dysfunction.

The effect of intermittent hypoxia on RNA synthesis in the isolated rat heart

Abstract To determine whether hypoxia per se could initiate myocardial RNA synthesis, isolated rat hearts were subjected to varying degrees of hypoxic perfusion in a modified Langendorff apparatus. Hearts were perfused for 30 min with media equilibrated with gas mixtures containing 96, 48, 24, 12 and 0% O 2 , 4% CO 2 , balance N 2 ; then for an additional 30 min with aerobic (96% O 2 , 4% CO 2 ) media containing [ 14 C]-uridine and incorporation into RNA determined. Hearts which showed significantly decreased performance during hypoxic perfusion (24% O 2 or less) exhibited significant increases in uridine incorporation into RNA post-hypoxia. To assess the effect of hypoxia in the absence of contraction, hearts were arrested with media containing a high potassium concentration (26 mEq/l). Potassium-arrest during the aerobic labelling period only did not prevent increased incorporation of uridine following 30 min of hypoxia (12% and 0% O 2 ). Potassium-arrest during hypoxia as well as the labelling period prevented increased uridine incorporation in all but anoxia (0% O 2 ) exposed hearts. The myocardial content of high energy compounds (creatine phosphate, ATP, ADP, and AMP) was determined after 30 min of hypoxia in the absence and presence of potassium arrest. Only those hearts in which significant decreases in high energy phosphate content were observed, exhibited significant increases in uridine incorporation. It is concluded that intermittent hypoxia per se initiates events leading to increases in RNA synthesis, but only if energy depletion occurs.

Arginine and Clonidine Stimulation Tests for Growth Hormone Deficiency Revisited - Do We Really Need So Many Samples?

Growth hormone (GH) reserve is defined biochemically by the peak serum concentration after stimulation with a known secretagogue. Arginine and clonidine stimulation tests are currently performed with 5 timed blood samples. We evaluated the diagnostic utility of taking fewer samples by retrospectively analyzing 289 tests (202 arginine and 87 clonidine) performed in a single hospital. 123/202 (60.9%) arginine tests and 46/87 (52.9%) clonidine tests had at least one sample above 10 ng/ml. These were defined as negative for GH deficiency and studied further. For arginine tests, three samples taken at 0, 45 and 90 would have provided an acceptable false positive rate of 4.5%. For clonidine tests, two samples taken at 60 and 90 provided a false positive rate of 4.3%. Addition of either a 0 or 120 sample further reduced the false positive rate to 2.2%. Both the arginine and clonidine stimulation tests can be reliably performed with fewer samples.

RNA polymerases in isolated rat heart nuclei and their response to cyclic nucleotides

Isolated rat, heart nuclei were employed to study RNA polymerase activities and their response to cyclic nucleotides. Nuclei incubated at low ionic strength in the presence of Mg 2+ exhibited a rapid decline in RNA polymerase activity. Physiological levels of cyclic AMP stimulated RNA polymerase under these conditions. Actinomycin D (0.2 to 10 μg ml −1 ) blocked the stimulating effect of cyclic AMP on RNA polymerase. Cyclic GMP (10 −7 to 10 −4 m ) did not affect RNA synthesis under the same incubation conditions. Nuclei incubated at high ionic strength (0.6 m ammonium sulfate) in the presence of Mn 2+ exhibited linear RNA synthesis for at least 40 min. Nuclei incubated with 0.12 m ammonium sulfate in the presence of Mn 2+ exhibited two consucutive peaks of RNA polymerase activity as revealed by pulse labelling. The early peak of RNA polymerase activity was inhibited by alpha amanitin (0.04 or 0.25 μg ml −1 ) to a greater extent than was the later peak. Neither cyclic AMP nor cyclic GMP affected RNA polymerase activity at intermediate (0.12 m ) or high (0.6 m ) ionic strength in the presence of Mn 2+ .

THE SPINY MOUSE: IN SEARCH OF AN ANIMAL MODEL FOR PATHOGENESIS OF TYPE 2 DIABETES MELLITUS

ABSTRACT We review the data suggesting that the spiny mouse Acomys cahirinus may be a suitable model for the early stages of the development of Type 2 diabetes mellitus in man. When raised under laboratory conditions, A. cahirinus develops glucose intolerance, obesity, and varying degrees of insulin resistance. As in Type 2 diabetes, it exhibits a reduced glucose clearance rate (Kg) following an IV glucose challenge, deficient β-cell sensitivity to glucose (Km), but no change in its capacity to secrete insulin (Vmax). Its islets are well granulated with processed insulin, but, as in Type 2 diabetes, an intrinsic defect in β-cell stimulus-secretion coupling hinders the ability to meet the increasing peripheral demand for insulin caused by the developing obesity. In vitro studies with isolated islets resemble whole animal studies: a reduced early-phase insulin response to glucose (decreased Vmax), delayed and deficient late-phase response, and increased Km indicative of reduced sensitivity to glucose. We de...

THE METHODOLOGY FOR STUDYING COORDINATED CALCIUM CONCENTRATION CHANGES IN A PANCREATIC β CELL LINE

Intracellular calcium concentration was imaged in beta Tc cells with the aid of Fluo-3 indicator and the Meridian ACAS 570 interactive laser cytometry. This cell line does not respond by an elevation in [Ca2+]in to increase in extracellular [glucose], but does respond to 10 microM forskolin. It was found that forskolin increases the mean [Ca2+]in and produces calcium spikes. Time series analysis was performed on individual pixels. Autocorrelation revealed that forskolin induces oscillation in [Ca2+]in. Cross-correlation analysis showed that all the intracellular pixels along the line scan are highly correlated, indicating that the increase in [Ca2+]in encompasses the entire cell.