Interaction effect between common polymorphisms in PPARgamma2 (Pro12Ala) and insulin receptor substrate 1 (Gly972Arg) on insulin sensitivity

The Pro12Ala polymorphism in the peroxisome proliferator-activated receptor (PPAR) gamma2 gene is associated with a reduced risk of type 2 diabetes. A beneficial effect on insulin sensitivity is reported in some but not all populations. It is possible that this genetic variant produces a characteristic phenotype only against a certain genetic background. We therefore tested the hypothesis that carriers of the Ala allele of PPARgamma2 exhibit a different phenotype against the background of the Gly972Arg polymorphism in the insulin receptor substrate (IRS) 1. We determined insulin sensitivity in the four combinations defined by the absence or presence of the polymorphic allele (healthy, glucose tolerant subjects), by the oral glucose tolerance test (OGTT; using a validated index, n=318) and hyperinsulinemic clamp ( n=201). Insulin sensitivity was not or was only marginally different between Pro/Pro and X/Ala in the overall population. Interestingly, using the OGTT index, insulin sensitivity was significantly greater in X/Ala (PPARgamma2) + X/Arg (IRS-1) than in Pro/Pro (PPARgamma2) + X/Arg (IRS-1). On the other hand, insulin sensitivity was similar in the X/Ala (PPARgamma2) + Gly/Gly (IRS-1 972) and the Pro/Pro (PPARgamma2) + Gly/Gly (IRS-1). The results were practically identical using insulin sensitivity from the clamp. In conclusion, the Arg972 (IRS-1) background produced a marked difference in insulin sensitivity between X/Ala and Pro/Pro (PPARgamma) which was not present in the whole population or against the Gly972 (IRS-1) background. This suggests that the Ala allele of PPARgamma2 becomes particularly advantageous against the background of an additional, possibly disadvantageous genetic polymorphism. Allowing for gene-gene interaction effects may reveal novel information regarding metabolic effects of genetic variants.

Association of the T-G polymorphism in adiponectin (exon 2) with obesity and insulin sensitivity: interaction with family history of type 2 diabetes

The adipocyte-derived hormone adiponectin seems to protect from insulin resistance, a key factor in the pathogenesis of type 2 diabetes. Genome-wide scans have mapped a susceptibility locus for type 2 diabetes and the metabolic syndrome to chromosome 3q27, where the adiponectin gene is located. A common silent T-G exchange in nucleotide 94 (exon 2) of the adiponectin gene has been associated with increased circulating adiponectin levels. Metabolic abnormalities associated with the G allele have not been reported. We therefore assessed whether this polymorphism alters insulin sensitivity and/or measures of obesity using the Tübingen Family Study database (prevalence of the G allele, 28%). In 371 nondiabetic individuals, we found a significantly greater BMI in GG + GT (25.5 +/- 0.7 kg/m(2)) compared with TT (24.1 +/- 0.3 kg/m(2); P = 0.02). Insulin sensitivity (determined by euglycemic clamp, n = 209) was significantly lower in GG + GT (0.089 +/- 0.007 units) compared with TT (0.112 +/- 0.005 units; P = 0.02). This difference disappeared completely on adjustment for BMI. Because our population contains a relatively high proportion of first-degree relatives of patients with type 2 diabetes, we stratified by family history (FHD). Much to our surprise, the genotype differences in BMI and insulin sensitivity in the whole population were attributable entirely to differences in the subgroup without FHD, whereas in the subgroup with FHD, the G allele had absolutely no effect. Moreover, individuals without FHD had a significantly lower BMI than individuals with FHD (25.2 +/- 0.4 vs. 26.2 +/- 0.5 kg/m(2); P = 0.01), which was not the case for the GG + GT subgroup without FHD (27.0 +/- 0.9 kg/m(2); NS). This suggests that in individuals without familial predisposition for type 2 diabetes, the adiponectin polymorphism may mildly increase the obesity risk (and secondarily insulin resistance). In contrast, in individuals who are already burdened by other genetic factors, this small effect may be very hard to detect.

Effects of intravenous and dietary lipid challenge on intramyocellular lipid content and the relation with insulin sensitivity in humans

An increased intramyocellular lipid (IMCL) content, as quantified by (1)H-magnetic resonance spectroscopy ((1)H-MRS), is associated with reduced insulin sensitivity. At present, it is unclear which factors determine IMCL formation and how rapidly IMCL accumulation can be induced. We therefore studied the impact of hyperinsulinemia and elevated circulating nonesterified fatty acid (NEFA) levels on IMCL formation and insulin sensitivity. We further evaluated the influence of a high-fat diet on IMCL storage. In the infusion protocol, 12 healthy male subjects underwent a 6-h hyperinsulinemic-euglycemic glucose clamp with concomitant infusion of Intralipid plus heparin. IMCL was quantified by (1)H-MRS in soleus (SOL) and tibialis anterior (TA) muscle at baseline and then every hour. IMCL levels started to increase significantly after 2 h, reaching a maximum of 120.8 +/- 3.4% (SOL) and 164.2 +/- 13.8% (TA) of baseline after 6 h (both P < 0.05). In parallel, the glucose infusion rate (GIR) decreased progressively, reaching a minimum of 60.4 +/- 5.4% of baseline after 6 h. Over time, the GIR was strongly correlated with IMCL in TA (r = -0.98, P < or = 0.003) and SOL muscle (r = -0.97, P < or = 0.005). In the diet protocol, 12 male subjects ingested both a high-fat and low-fat diet for 3 days each. Before and after completion of each diet, IMCL levels and insulin sensitivity were assessed. After the high-fat diet, IMCL levels increased significantly in TA muscle (to 148.0 +/- 16.9% of baseline; P = 0.005), but not in SOL muscle (to 114.4 +/- 8.2% of baseline; NS). Insulin sensitivity decreased to 83.3 +/- 5.6% of baseline (P = 0.033). There were no significant changes in insulin sensitivity or IMCL levels after the low-fat diet. The effects of the high-fat diet showed greater interindividual variation than those of the infusion protocol. The data from the lipid infusion protocol suggest a functional relationship between IMCL levels and insulin sensitivity. Similar effects could be induced by a high-fat diet, thereby underlining the physiological relevance of these observations.

Two novel prevalent polymorphisms in the hormone-sensitive lipase gene have no effect on insulin sensitivity of lipolysis and glucose disposal

Free fatty acids released during triglyceride lipolysis play an important role in obesity-associated insulin resistance of glucose disposal. Individual sensitivity of lipolysis to the suppressive effect of insulin varies greatly among healthy subjects. It is possible that genetic factors contribute to this variation. Among the many proteins involved in the regulation of lipolysis, hormone-sensitive lipase (HSL) represents a prime candidate for genetic variants contributing to the biological variation of insulin sensitivity of lipolysis. We determined the insulin sensitivity of lipolysis (suppression of isotopically [primed-continuous infusion of d5 glycerol] measured glycerol rate of appearance) and of glucose disposal, using a three-step (n = 20) or standard (n = 53) hyperinsulinemic euglycemic clamp in 73 healthy, unrelated subjects. To assess the possible role of genetic polymorphisms, we directly sequenced the coding region of the HSL gene and the noncoding exon B from these subjects. We identified two silent mutations and three amino acid polymorphisms: Arg262Met (prevalence, 5%), Glu620Asp (prevalence, 31%) and Ser681Ile (prevalence, 22%). The latter two are located in the regulatory domain of HSL but neither had a significant impact on insulin sensitivity of lipolysis or glucose disposal (with and without adjustment for obesity and age as covariates; all P values > 0.20). We conclude that a number of genetic polymorphisms in HSL exist, some of which are highly prevalent. Neither of the polymorphisms we identified in the coding region, however, contributed measurably to the biological variation of insulin sensitivity in our lean, healthy population.

The prevalent Gly1057Asp polymorphism in the insulin receptor substrate-2 gene is not associated with impaired insulin secretion

Disruption of the insulin receptor substrate-2 was shown to cause type 2 diabetes in mice. This could be largely attributed to abnormal beta-cell development. In humans, a prevalent polymorphism in insulin receptor substrate-2 (Gly1057Asp) was not found be associated with type 2 diabetes in linkage and association studies. We tested the hypothesis that an extreme challenge of the beta cell might reveal subtle abnormalities in carriers of this polymorphism undetected by conventional insulin secretion tests. Therefore, in addition to assessing beta-cell function by oral glucose tolerance testing (n = 318, normal glucose tolerance), we measured the secretory response to maximal stimulation by hyperglycemia (10 mM), glucagon-like peptide-1, and arginine administered in an additive fashion (n = 77, nondiabetic). The allelic frequency of the Asp allele was approximately 37%. Neither the beta-cell function indices from the oral glucose tolerance test nor the secretory response during the hyperglycemic clamp differed measurably between carriers and controls. Moreover, maximal plasma C-peptide concentrations in response to the combined glucose, glucagon-like peptide-1, and arginine stimulus was not different between Gly/Gly (10,745 +/- 1,186 pmol/liter) and X/Asp (10,800 +/- 490 pmol/liter, P = 0.99). In conclusion, our findings strongly suggest that the Gly1057Asp polymorphism in insulin receptor substrate-2 is not associated with beta-cell dysfunction. The normal maximal insulin secretory response makes it unlikely that this common polymorphism results in abnormal beta-cell development.

The prevalent Gly1057Asp polymorphism in the insulin receptor substrate-2 gene is not associated with impaired insulin secretion

Disruption of the insulin receptor substrate-2 was shown to cause type 2 diabetes in mice. This could be largely attributed to abnormal beta-cell development. In humans, a prevalent polymorphism in insulin receptor substrate-2 (Gly1057Asp) was not found be associated with type 2 diabetes in linkage and association studies. We tested the hypothesis that an extreme challenge of the beta cell might reveal subtle abnormalities in carriers of this polymorphism undetected by conventional insulin secretion tests. Therefore, in addition to assessing beta-cell function by oral glucose tolerance testing (n = 318, normal glucose tolerance), we measured the secretory response to maximal stimulation by hyperglycemia (10 mM), glucagon-like peptide-1, and arginine administered in an additive fashion (n = 77, nondiabetic). The allelic frequency of the Asp allele was approximately 37%. Neither the beta-cell function indices from the oral glucose tolerance test nor the secretory response during the hyperglycemic clamp differed measurably between carriers and controls. Moreover, maximal plasma C-peptide concentrations in response to the combined glucose, glucagon-like peptide-1, and arginine stimulus was not different between Gly/Gly (10,745 +/- 1,186 pmol/liter) and X/Asp (10,800 +/- 490 pmol/liter, P = 0.99). In conclusion, our findings strongly suggest that the Gly1057Asp polymorphism in insulin receptor substrate-2 is not associated with beta-cell dysfunction. The normal maximal insulin secretory response makes it unlikely that this common polymorphism results in abnormal beta-cell development.

Functional significance of the UCSNP-43 polymorphism in the CAPN10 gene for proinsulin processing and insulin secretion in nondiabetic Germans

Recently, an association of the G allele in UCSNP-43 of calpain 10 with type 2 diabetes and decreased glucose disposal was reported. Calpain 10 is also expressed in pancreatic islets. It is not known, however, whether and how this polymorphism contributes to the biological variation of beta-cell function. We studied 73 nondiabetic subjects from the southwest region of Germany (G/G, n = 41; G/A, n = 29; and A/A, n = 3) using a modified hyperglycemic clamp (10 mmol/l glucose, added glucagon-like peptide 1, final arginine bolus). The genotype distribution was not different between subjects with normal glucose tolerance (n = 56) and those with impaired glucose tolerance (n = 17; P = 0.74, chi2 test). First-phase insulin secretion (adjusted for sex and insulin sensitivity from hyperglycemic clamp) was greater in G/G (2,747 +/- 297 pmol/min) than in G/A + A/A (1,612 +/- 156 pmol/min, P = 0.003). Insulin secretion in response to arginine (adjusted for insulin sensitivity) was also greater in G/G (9,648 +/- 1,186 pmol/min) than in G/A + A/A (5,686 +/- 720 pmol/min, P = 0.04). The acute poststimulus proinsulin-to-insulin ratio was lower in G/G (1.6 +/- 0.4% first phase; 1.6 +/- 0.2% arginine) than in G/A + A/A (4.0 +/- 0.5% first phase, P < 0.001; 2.5 +/- 0.4% arginine, P = 0.03). In conclusion, it appears unlikely that any association of the UCSNP-43 polymorphism alone with type 2 diabetes involves impairment of insulin secretion in our population of German Caucasians. This may be entirely different with specific haplotype combinations.

Effect of the pattern of elevated free fatty acids on insulin sensitivity and insulin secretion in healthy humans

In order to investigate whether the pattern of elevated free fatty acids (FFAs) has any effect on insulin sensitivity and insulin secretion in humans, we produced 2 distinct serum FFA patterns (PT 1 and 2) by infusing 6 healthy volunteers with 2 different lipid emulsions plus heparin for 24 hours. A hyperglycemic clamp (approx. 8 mM, 140 min) was performed before and 5 and 24 hours after both lipid infusions to determine insulin sensitivity and insulin secretion simultaneously. Total FFAs had increased comparably by 24 hours (2020+/-268 microM in PT 1) and (1812+/-154 microM in PT 2, p =0.24). Serum PT 1 contained 66% saturated FFAs plus monoenes and 34% polyenes, while PT 2 contained 80% saturated FFAs plus monoenes and 20% polyenes. Thus, the ratio of polyunsaturated to saturated plus monoenes was about 0.5 in PT 1 vs. 0.25 in PT 2. In PT 1, the insulin sensitivity index (ISI) decreased by 20 +/- 7% and 27 +/- 10% from basal after 5 and 24 hours, respectively. In PT 2, the ISI decreased significantly more after 5 (41+/-7%, p = 0.008) and 24 hours (52+/-6%, p = 0.005). In contrast, different phases of insulin secretion did not change during the lipid infusion and did not vary between the two FFA profiles. In conclusion, these findings provide preliminary evidence that the composition of elevated serum FFAs influenced insulin sensitivity in humans. The FFA pattern low in polyunsaturated FFAs reduced insulin sensitivity more than the pattern high in polyunsaturated FFAs. In contrast, no effect on insulin secretion was observed.

Determining significant fold differences in gene expression analysis

A typical use for RNA expression microarrays is comparing the measurement of gene expression of two groups. There has not been a study reproducing an entire experiment and modeling the distribution of reproducibility of fold differences. Our goal was to create a model of significance for fold differences, then maximize the number of ESTs above that threshold. Multiple strategies were tested to filter out those ESTs contributing to noise, thus decreasing the requirements of what was needed for significance. We found that even though RNA expression levels appears consistent in duplicate measurements, when entire experiments are duplicated, the calculated fold differences are not as consistent. Thus, it is critically important to repeat as many data points as possible, to ensure that genes and ESTs labeled as significant are truly so. We were successfully able to use duplicated expression measurements to model the duplicated fold differences, and to calculate the levels of fold difference needed to reach significance. This approach can be applied to many other experiments to ascertain significance without a priori assumptions.

[Polymorphism of pro12Ala in peroxisome proliferator activated receptor gamma 2 (PPAgamma2): beta cell function and insulin sensitivity]

BACKGROUND AND OBJECTIVE

The peroxisome proliferator-activated receptor isoform gamma (PPAR gamma) is a key regulator in lipid and glucose homoeostasis. A common polymorphism (Pro12Ala in PPAR gamma 2, prevalence ca. 25%) was shown to be associated with a decreased risk of type 2 diabetes. Generally, both beta-cell dysfunction and insulin resistance contribute to the development of type 2 diabetes. Therefore, the aim of the present study was to assess the mechanism by which the Ala allele of this polymorphism contributes to the reduced risk for type 2 diabetes.

PATIENTS AND METHODS

We studied 51 subjects without (Pro/Pro) and 26 subjects with this polymorphisms (X/Ala) (both groups non-diabetic) by a modified hyperglycaemic clamp which permitted determination of both insulin secretion (in response to glucose, GLP-1 and arginine) and insulin sensitivity.

RESULTS

None of the various phases of insulin secretion was significantly different between the 2 genotype groups (all p values > 0.13). In contrast, insulin sensitivity was significantly greater in X/Ala (0.19 +/- 0.03 U) compared to Pro/Pro (0.14 +/- 0.01 U, p = 0.04). In a two-dimensional assessment of insulin sensitivity and secretion, the homozygous alanine carriers appeared to have the most favourable constellation.

CONCLUSION

These simultaneously obtained data for insulin secretion and sensitivity strongly suggest that the mechanism by which the Ala allele contributes to a risk reduction for type 2 diabetes most likely involves an increase in insulin sensitivity.

Avoidance of hypoglycemia restores hypoglycemia awareness by increasing beta-adrenergic sensitivity in type 1 diabetes

BACKGROUND

Lack of awareness of hypoglycemia is a major limiting factor in the management of type 1 diabetes.

OBJECTIVE

To examine whether reduction in the number of episodes of hypoglycemia restores hypoglycemia awareness by influencing beta-adrenergic sensitivity in patients with type 1 diabetes.

DESIGN

Controlled interventional study.

SETTING

Research unit and outpatient diabetes clinic of a university hospital.

PATIENTS

10 men with type 1 diabetes and hypoglycemia unawareness (mean age [+/-SD], 46 +/- 16 years; mean duration of diabetes, 20 +/- 10 years).

INTERVENTION

Strict avoidance of hypoglycemia.

MEASUREMENTS

beta-Adrenergic sensitivity was measured by isoproterenol testing before and at 2 and 4 months after strict avoidance of hypoglycemia. Hypoglycemia awareness and catecholamine response were measured by performing hypoglycemic clamp (glucose level, 3 mmol/L [54 mg/dL]) before and after 4 months of avoidance of hypoglycemia.

RESULTS

After 4 months, the mean (+/-SE) number of episodes of hypoglycemia (glucose level < 3.9 mmol/L [70 mg/dL]) decreased from 8.4 +/- 0.9 to 1.4 +/- 0.3 per week (P < 0.001). Hemoglobin A(1c) values increased from 0.068 +/- 0.003 (6.8% +/- 0.3%) to 0.077 +/- 0.003 (7.7% +/- 0.3%) (P < 0.001). Autonomic symptom scores during hypoglycemic clamp increased from 1.8 +/- 0.6 to 3.3 +/- 0.7 (P = 0.004) and did not significantly differ from those of normal participants (4.7 +/- 0.8) (P > 0.2). Although catecholamine responses to hypoglycemia were unchanged, the dose of isoproterenol necessary to increase heart rate by 25 beats/min (IC(25)) decreased from 1.96 +/- 0.43 microgram before treatment to 1.09 +/- 0.17 microgram after 4 months of treatment (P = 0.01), a value similar to that in normal participants (0.85 +/- 0.20 microgram) (P > 0.2). Improvements in beta-adrenergic sensitivity (change in IC(25)(-1)) were correlated with improvements in autonomic symptoms (r = 0.65; P = 0.04).

CONCLUSIONS

Avoidance of hypoglycemia in patients with type 1 diabetes who have hypoglycemia unawareness seems to restore hypoglycemia awareness, primarily by increasing beta-adrenergic sensitivity.

Effect of experimental elevation of free fatty acids on insulin secretion and insulin sensitivity in healthy carriers of the Pro12Ala polymorphism of the peroxisome proliferator--activated receptor-gamma2 gene

The transcription of many genes involved in lipid metabolism is regulated by the peroxisome proliferator-activated receptor-gamma (PPAR-gamma). The Pro12Ala polymorphism in the PPAR-gamma2 gene has been associated with reduced transcriptional activity in vitro and increased insulin sensitivity in vivo. Although PPAR-gamma has been demonstrated in human beta-cells, it is unknown whether the Pro12Ala polymorphism plays a role in insulin secretion. Moreover, it is also unknown if and how the effect of free fatty acids (FFAs) on insulin secretion and insulin sensitivity is modulated by the presence of this polymorphism. We therefore performed hyperglycemic clamps (8 mmol/l, 140 min, 5 g arginine bolus at min 120) in 10 healthy subjects with the (X/Ala) polymorphism and in 10 subjects without the polymorphism (Pro/Pro) basally and after 5 h infusion of Intralipid plus heparin. FFA concentrations increased from 473 +/- 61 micromol/l to 1,732 +/- 163 micromol/l in the Pro/Pro and from 372 +/- 46 micromol/l to 1,630 +/- 96 micromol/l in the X/Ala group (P = 0.68). Basally, neither insulin sensitivity nor insulin secretion were significantly different between the two groups. During infusion of Intralipid, first-phase insulin secretion remained unchanged in both groups (P = 0.21). In the Pro/Pro group, second-phase insulin secretion remained unchanged (444 +/- 67 vs. 471 +/- 93 pmol/min) and the response to arginine increased from 5,007 +/- 41 to 6,072 +/- 732 pmol/min. In contrast, in the X/Ala group, there was a decrease of both second-phase insulin secretion (533 +/- 58 to 427 +/- 48 pmol/min, P = 0.02 vs. Pro/Pro) and in the response to arginine (from 7,518 +/- 1,306 to 6,458 +/- 1,040 pmol/min, P = 0.014 vs. Pro/Pro). The insulin sensitivity index decreased comparably in Pro/Pro and X/Ala (to 71 +/- 8 vs. 74 +/- 9% of basal, P = 0.8). In conclusion, these results provide evidence that the Pro12Ala polymorphism in the PPAR-gamma2 gene might be involved in a differential regulation of insulin secretion in response to increased FFAs in humans.

The Gly972Arg polymorphism in the insulin receptor substrate-1 gene contributes to the variation in insulin secretion in normal glucose-tolerant humans

The Gly972Arg polymorphism in the insulin receptor substrate (IRS)-1 was found in some studies to have a higher prevalence in type 2 diabetic subjects than in control subjects. Previously, transfection of IRS-1 with this polymorphism into insulin-secreting cells resulted in a marked reduction of glucose-stimulated insulin secretion compared with the wild-type transfected cells. In the present study, we compared insulin secretion in well-matched normal glucose-tolerant subjects with and without this polymorphism. Several validated indexes of beta-cell function from the oral glucose tolerance test were significantly lower in X/Arg (n = 31) compared with Gly/Gly (n = 181) (P between 0.002 and 0.05), whereas insulin sensitivity (measured with a euglycemic clamp) was not different. During a modified hyperglycemic clamp, insulin secretion rates were significantly lower in Gly/Arg (n = 8) compared with Gly/Gly (n = 36) during the first phase (1,711+/-142 vs. 3,014+/-328 pmol/min, P = 0.05) and after maximal stimulation with arginine (5,340+/-639 vs. 9,075+/-722 pmol/min, P = 0.03). In summary, our results suggest that the Gly972Arg polymorphism in IRS-1 is associated with decreased insulin secretion in response to glucose but not with insulin sensitivity. It is possible that this polymorphism causes insulin resistance at the level of the beta-cell and contributes to the polygenic etiology of type 2 diabetes.

Pro12Ala polymorphism in the peroxisome proliferator-activated receptor-gamma2 gene is associated with increased antilipolytic insulin sensitivity

The Pro12Ala polymorphism of the peroxisome proliferator-activated receptor (PPAR)-gamma2 is associated with reduced transcriptional activity in vitro and increased insulin sensitivity in humans in vivo. The mechanism by which this polymorphism influences insulin sensitivity in humans is unclear. PPAR-gamma2 is mainly expressed in adipocytes, and free fatty acids released from adipose tissue are key mediators of peripheral insulin resistance. Therefore, we examined insulin suppression of lipolysis in 51 subjects without (Pro/Pro) and 17 subjects with the polymorphism (X/Ala). Both groups were lean (BMI <27.0 kg/m2) and matched for age, BMI, waist-to-hip ratio, and sex. The isotopically (infusion of d5 glycerol) determined glycerol rate of appearance was used as an index of lipolysis. Insulin sensitivity of lipolysis was expressed as the insulin concentration resulting in half-maximal suppression (EC50). This was directly determined during a three-step hyperinsulinemic-euglycemic clamp (n = 21) or estimated indirectly during a standard hyperinsulinemic-euglycemic clamp (n = 47). The insulin sensitivity index (ISI) of glucose disposal was 0.095+/-0.006 micromol x kg(-1) x min(-1) x pmol(-1) x l(-1) in the control group and 0.129+/-0.008 micromol x kg(-1) x min(-1) x pmol(-1) x l(-1) in the X/Ala group (P = 0.003). The EC50 was 56+/-2 pmol/l in the control group and 44+/-3 pmol/l in the X/Ala group (P = 0.001). The EC50 of lipolysis and ISI was significantly correlated (r = 0.42, P = 0.002). In conclusion, in lean subjects, the Pro12Ala polymorphism is associated with increased insulin sensitivity of glucose disposal and suppression of lipolysis. This result suggests that an altered transcriptional activity of PPAR-gamma2 in X/Ala subjects either causes a more efficient suppression of lipolysis in adipose tissue, which in turn results in improved insulin-stimulated glucose disposal in muscle, or, alternatively, beneficially affects insulin signaling in both tissues independently of one another.

Evidence against a rate-limiting role of proinsulin processing for maximal insulin secretion in subjects with impaired glucose tolerance and beta-cell dysfunction

In subjects with impaired glucose tolerance (IGT) insulin secretion is impaired. Increased proinsulin/insulin (PI/I) ratios suggest that there is also reduced processing of proinsulin to insulin in this condition. The PI/I ratio in the insulin secretory granule is ideally assessed by plasma measurements in response to acute stimulation of insulin secretion. In the present study we tested the hypothesis that maximal stimulation of insulin secretion results in exhaustion of the proinsulin conversion pathway to insulin. We therefore determined the PI/I ratio in 11 normal glucose-tolerant subjects (NGT) and 11 subjects with IGT in response to glucose (squarewave hyperglycemic clamp, 10 mmol/L), glucagon-like peptide-1 (GLP-1; primed-continuous infusion), and arginine given during the continued GLP-1 infusion. In IGT, insulin levels were significantly lower during the first phase (144 +/- 20 vs. 397 +/- 119 pmol/L; P = 0.02), at the end of the GLP infusion (2142 +/- 350 vs. 5430 +/- 1091 pmol/L; P: = 0.002), and in response to arginine (3983 +/- 375 vs. 8663 +/- 1430 pmol/L; P = 0.005). In response to glucose, the minimum PI/I ratio was significantly higher in IGT (3.4 +/- 0.6%) than in NGT (1.4 +/- 0.5%; P = 0.02), suggesting defective proinsulin processing in this condition. In subjects with IGT, the PI/I ratio decreased significantly after GLP-1 priming (1.7 +/- 0.2%; P = 0.02) and after arginine given during GLP-1 (1.4 +/- 0.2%; P = 0.007) and was not significantly different from those values in NGT (1.3 +/- 0.2% and 1.3 +/- 0.2%, respectively; both P = NS). In conclusion, during maximal stimulation of insulin secretion in subjects with IGT, the PI/I ratio in plasma decreased significantly and was not different from that in normal controls. This strongly argues against the hypothesis that defective processing of proinsulin to insulin represents a major component of the beta-cell dysfunction in IGT.

Clinical features of insulin resistance and beta cell dysfunction and the relationship to type 2 diabetes

Type 2 diabetes mellitus is a heterogeneous disorder characterized by varying degrees of impaired insulin secretion and insulin resistance. The metabolic manifestations of insulin resistance include (1) reduced insulin-stimulated glucose uptake, (2) reduced insulin-suppression of endogenous glucose production, and (3) reduced antilipolysis. All of these mechanisms contribute to the hyperglycemis of T2DM, both post-absorptively and postprandially. In addition, insulin resistance is involved in decreaswed insulin-induced vasodilation, dyslipidemia, and platelet hyperaggregability. The pathogenesis of T2DM involves a combination of genetic and environmental factors. Monogenic causes account for only a minority of insulin resistance and beta cell dysfunction. Among environmental factors the most important are obesity, reduced physical activity, and age. Obesity-associated insulin resistance is thought to be mediated mainly by FFAs whose clearance is reduced in subjects with T2DM. A number of clinical tests have been developed to assess insulin sensitivity and beta cell function in vivo. The euglycemic hyperinsulinemic clamp and the hyperglycemic clamp, respectively, represent the gold standard procedures. Recently, indices calculated parameters of the OGTT have been proposed as surrogates for assessing both insulin sensitivity and beta cell function in clinical situations and epidemiologic studies.

Renal gluconeogenesis: its importance in human glucose homeostasis

Studies conducted over the last 60 years in animals and in vitro have provided considerable evidence that the mammalian kidney can make glucose and release it under various conditions. Until quite recently however, it was generally believed that the human kidney was not an important source of glucose except during acidosis and after prolonged fasting. This review will summarize early work in animals and humans, discuss methodological problems in assessing renal glucose release in vivo, and present results of recent human studies that provide evidence that the kidney may play a significant role in carbohydrate metabolism under both physiological and pathological conditions.

A novel use of the hyperinsulinemic-euglycemic clamp technique to estimate insulin sensitivity of systemic lipolysis

The aim of the present study was to assess whether a standard hyperinsulinemic-euglycemic clamp can provide an estimate for the antilipolytic insulin sensitivity. For this purpose, we infused 9 non-obese, healthy volunteers with [2H5]glycerol and used the glycerol rate of appearance (Ra) in plasma as an index for systemic lipolysis during a standard (1 mU/kg x min, 120 min) and a 3-step (0.1, 0.25, 1.0 mU/kg x min) hyperinsulinemic-euglycemic clamp. The insulin concentration, which half-maximally suppressed lipolysis (EC50) in the three-step clamp, was considered to be the gold standard for the antilipolytic insulin sensitivity. Glycerol Ra decreased from 1.53+/-0.11 micromol/kg x min to 0.60+/-0.09 micromol/kg x min (p <0.001) during the standard clamp. The decrease in Ra at most time points during the standard clamp significantly correlated with the EC50. The highest correlation for the % decrease of glycerol Ra from baseline was found at 60 min (r = 0.96, p < 0.001) making this parameter a useful index for the antilipoytic insulin sensitivity. Neither plasma glycerol nor plasma free fatty acid (FFA) concentrations were significantly correlated with the EC50. In conclusion, the standard hyperinsulinemic-euglycemic clamp in combination with isotopic determination of glycerol Ra provides a reasonable estimate for the antilipolytic insulin sensitivity. In healthy subjects, the parameter best suited to estimate the insulin EC50 (by linear correlation) was the percentage decrease of glycerol Ra at 60 min.

Effects of troglitazone on cellular differentiation, insulin signaling, and glucose metabolism in cultured human skeletal muscle cells

To determine the immediate effect of thiazolidinediones on human skeletal muscle, differentiated human myotubes were acutely (1 day) and myoblasts chronically (during the differentiation process) treated with troglitazone (TGZ). Chronic TGZ treatment resulted in loss of the typical multinucleated phenotype. The increase of muscle markers typically observed during differentiation was suppressed, while adipocyte markers increased markedly. Chronic TGZ treatment increased insulin-stimulated phosphatidylinositol (PI) 3-kinase activity and membranous protein kinase B/Akt (PKB/Akt) Ser-473 phosphorylation more than 4-fold. Phosphorylation of p42/44 mitogen-activated protein kinase (42/44 MAPK/ERK) was unaltered. Basal glucose uptake as well as both basal and insulin-stimulated glycogen synthesis increased approximately 1.6- and approximately 2.5-fold after chronic TGZ treatment, respectively. A 2-fold stimulation of PI 3-kinase but no other significant TGZ effect was found after acute TGZ treatment. In conclusion, chronic TGZ treatment inhibited myogenic differentiation of that human muscle while inducing adipocyte-specific gene expression. The effects of chronic TGZ treatment on basal glucose transport may in part be secondary to this transdifferentiation. The enhancing effect on PI 3-kinase and PKB/Akt involved in both differentiation and glycogen synthesis appears to be pivotal in the cellular action of TGZ.

Acute stimulation of leptin concentrations in humans during hyperglycemic hyperinsulinemia. Influence of free fatty acids and fasting

OBJECTIVE

To assess the acute regulation of leptin concentrations by insulin, glucose and free fatty acids (FFAs).

DESIGN

Four protocols: saline control experiment (CON); hyperglycemic clamps (approximately 8.3 mmol/l, 120 min) after an overnight fast (12 FAST); after a 36 h fast (36 FAST); and after a 36 h fast during which Intralipid/heparin was given over the last 24 h (36 FAST+FFA).

SUBJECTS

Lean, young, healthy volunteers; control group (n=6), experimental group (n=6).

MEASUREMENTS

Serum leptin concentrations.

RESULTS

Glucose and insulin concentrations were similar during the three clamp protocols. Average FFAs during the last 60 min of the clamp were 671+/-68 microM (CON),109+/-15 microM (12 FAST), 484+/-97 microM (36 FAST) and 1762+/-213 microM (36 FAST+FFA). Leptin concentrations decreased similarly during 36 FAST and 36 FAST+FFA. Leptin concentrations at 120 min (expressed as percentage of mean basal value) were 0.82+/-0.02 (CON), 0.93+/-0.08 (12 FAST) (P=0.29), 1.19+/-0.06 (36 FAST) (P<0.01) and 1.44+/-0.12 (36 FAST+FFA) (P<0.01).

CONCLUSION

During a one-day fast leptin concentrations decrease regardless of maintainance of an isocaloric balance. During acute hyperinsulinemic hyperglycemia leptin concentrations increase only after a preceding fast. This increase was most pronounced during simultaneous elevation of FFAs. Overall, our findings are compatible with the hypothesis that leptin secretion may be coupled to triglyceride synthesis rather than to the absolute lipid content of the adipocyte. International Journal of Obesity (2001) 25, 138-142

Pathophysiology and pharmacological treatment of insulin resistance

Diabetes mellitus type 2 is a world-wide growing health problem affecting more than 150 million people at the beginning of the new millennium. It is believed that this number will double in the next 25 yr. The pathophysiological hallmarks of type 2 diabetes mellitus consist of insulin resistance, pancreatic beta-cell dysfunction, and increased endogenous glucose production. To reduce the marked increase of cardiovascular mortality of type 2 diabetic subjects, optimal treatment aims at normalization of body weight, glycemia, blood pressure, and lipidemia. This review focuses on the pathophysiology and molecular pathogenesis of insulin resistance and on the capability of antihyperglycemic pharmacological agents to treat insulin resistance, i.e., a-glucosidase inhibitors, biguanides, thiazolidinediones, sulfonylureas, and insulin. Finally, a rational treatment approach is proposed based on the dynamic pathophysiological abnormalities of this highly heterogeneous and progressive disease.

Stimulatory effect of increased non-esterified fatty acid concentrations on proinsulin processing in healthy humans

AIMS/HYPOTHESIS

To assess the effect of increased concentrations of non-esterified fatty acids (NEFA) on proinsulin processing in healthy humans.

METHODS

We did a hyperglycaemic clamp (130 min duration, 8 mmol/l glucose, with a 5-g arginine bolus at min 120) before and after a 5-h infusion of Intralipid/heparin in 14 healthy subjects. Of the subjects eight underwent a saline control experiment. The proinsulin:insulin (PI:I) ratio immediately after the arginine bolus (122.5 to 125 min) was considered to provide an estimate for the conversion of proinsulin to insulin in the beta cell.

RESULTS

Concentrations of NEFA were 757 +/- 86 micromol/l and 1669 +/- 134 micromol/l (p < 0.001) after the 5-h infusion of saline or Intralipid, respectively. Insulin secretion rates were no different between the Intralipid and saline infusions (p = 0.73). There was no statistically significant difference for either the proinsulin concentration or the PI:I ratio during glucose stimulation alone (0 to 120 min). In response to arginine, in contrast, proinsulin remained unchanged during the saline infusion (from 31 +/- 6 to 29 +/- 7 pmol/l, p = 0.50) but decreased during 5 h of lipid infusion from (21 +/- 3 to 15 +/- 2 pmol/l, p = 0.02). The PI:I ratio in response to the arginine bolus was higher during the saline infusion (2.0 +/- 0.2% vs 1.7 +/- 0.2%, p = 0.04) but decreased during the Intralipid infusion (from 1.6 +/- 0.2% to 1.2 +/- 0.1%, p = 0.04).

CONCLUSION/INTERPRETATION

The statistically significantly lower PI:I ratio in response to arginine during experimentally increased concentrations of NEFA suggests that NEFA increase the conversion of proinsulin to insulin in humans in vivo.

Suppression of systemic, intramuscular, and subcutaneous adipose tissue lipolysis by insulin in humans

In addition to sc and visceral fat deposits, muscle has been shown to contain relevant amounts of lipids whose breakdown is subject to hormonal regulation. The aim of the present study was to determine insulin dose-response characteristics of systemic, sc adipose tissue and muscle lipolysis in humans. We used a combination of isotopic (primed continuous infusion of [d5]glycerol) and microdialysis techniques (catheters placed in the anterior tibial muscle and sc abdominal adipose tissue) during a three-step hyperinsulinemic-euglycemic clamp (insulin infusion, 0.1, 0.25, 1.0 mU/kg x min) in 13 lean, healthy volunteers. The glycerol rate of appearance was used as the index for systemic lipolysis; interstitial glycerol concentrations were used as the index for muscle and sc adipose tissue lipolysis. The insulin concentrations resulting in a half-maximal suppression (EC50) of systemic lipolysis, adipose tissue, and muscle lipolysis were 51, 68, and 44 pmol/L, respectively (between one another, P < 0.001). For each compartment there were significant correlations between the EC50 and the insulin sensitivity index for glucose disposal (r > 0.67; P < 0.05). However, lipolysis (as percent of baseline) was similar during the first two insulin infusion steps, but was significantly lower in adipose (22+/-2%) than in muscle (53+/-4%; P < 0.001) during step 3. Although we have no direct measurement of interstitial insulin concentrations, we conclude that based on the EC50 values, muscle is more sensitive with respect to the net effect of circulating insulin (transendothelial transport plus intracellular action) on lipolysis than sc adipose tissue in terms of exerting its full suppression within the physiological insulin range. This could be important in muscle for switching from preferential utilization of free fatty acids to glucose in the postprandial state. Inadequate suppression of im lipolysis resulting in excessive local availability of free fatty acids may represent a novel mechanism contributing to the pathogenesis of impaired glucose disposal, i.e. insulin resistance, in muscle.

The PPARgamma2 polymorphism pro12Ala is associated with better insulin sensitivity in the offspring of type 2 diabetic patients

Recently, a highly prevalent polymorphism of the PPARgamma2-receptor (Pro12Ala) was described and found to be associated with reduced transcriptional activity. Both human and animal studies suggested that this polymorphism may be associated with increased insulin sensitivity. However, an effect independent of other factors known to influence insulin sensitivity has yet to be demonstrated. Therefore, we compared insulin sensitivity using the hyperinsulinemic-euglycemic clamp technique in 37 subjects heterozygous for the PPARgamma2-Pro12Ala mutation and 37 control subjects negative for the PPARgamma2-Pro12Ala. The control group was selected from 190 subjects by pair-matching for sex, BMI, fat distribution and body composition. In the group heterozygous for the polymorphism steady-state plasma insulin during the clamp was significantly lower (63.3 microU/ml +/- 2.8) than in the control group (74.9 microU/ml +/- 4.0, p = 0.02). While MCR of glucose was similar in the PPARgamma2-Pro12Ala group (8.1 ml/kg x min x 100 +/- 0.5) and the control group (7.6 ml/kg x min x 100 +/- 3.0, p = 0.7), the insulin sensitivity index was significantly higher in the PPARgamma2-Pro12Ala group (12.5 mg/kg x min x microU/ml +/- 0.9 vs. 9.7 mg/kg x min x microU/ml +/- 0.8, p = 0.039). In addition, an arbitrary lipolysis index (decrease in FFA divided by increase in insulin) was also found to be marginally higher in the PPARgamma2-Pro12Ala group (8.0 +/- 0.9) compared to the control group (6.1 +/- 0.7, p = 0.097). In conclusion, these data suggest that the PPARgamma2-Pro12Ala mutation is associated with better insulin sensitivity of glucose disposal and possibly, also of antilipolysis.

Characterisation of beta-cell dysfunction of impaired glucose tolerance: evidence for impairment of incretin-induced insulin secretion

AIMS/HYPOTHESIS

Our studies were undertaken to characterise the defective insulin secretion of impaired glucose tolerance (IGT).

METHODS

We studied 13 normal glucose tolerant subjects (NGT) and 12 subjects with IGT carefully matched for age, sex, BMI and waist-to-hip ratio. A modified hyperglycaemic clamp (10 mmol/1) with a standard 2-h square-wave hyperglycaemia, an additional glucagon-like-peptide (GLP)-1 phase (1.5 pmol x kg(-1) x min(-1) over 80 min) and a final arginine bolus (5 g) was used to assess various phases of insulin secretion rate.

RESULTS

Insulin sensitivity during the second phase of the hyperglycaemic clamp was low in both groups but not significantly different (0.12 +/- 0.021 in NGT vs 0.11 +/- 0.013 micromol x kg(-1) x min(-1) x pmol(-1) in IGT, p = 0.61). First-phase insulin secretion was lower in IGT (1467 +/- 252 vs 3198 +/- 527 pmol x min(-1), p = 0.008) whereas the second phase was not (677 +/- 61 vs 878 +/- 117 pmol x min(-1), p = 0.15). The acute insulin secretory peak in response to GLP-1 was absent in IGT subjects who only produced a late phase of GLP-1-induced insulin secretion rate which was lower (2228 +/- 188 pmol x min(-l)) than in NGT subjects (3056 +/- 327 pmol x min(-1), p = 0.043). Insulin secretion in response to arginine was considerably although not significantly lower in IGT subjects. The relative impairment (per cent of the mean rate for NGT subjects) was greatest for the GLP-1 peak (19 +/- 9%).

CONCLUSION/INTERPRETATION

In this Caucasian cohort a defective insulin secretion rate is essential for the development of IGT. The variable degrees of impairment of different phases of the insulin secretion rate indicate that several defects contribute to its abnormality in IGT. Defects in the incretin signalling pathway of the beta cell could contribute to the pathogenesis of beta-cell dysfunction of IGT and thus Type II (non-insulin-dependent) diabetes mellitus.

Insulin signaling and action in cultured skeletal muscle cells from lean healthy humans with high and low insulin sensitivity

The aim of these studies was to investigate whether insulin resistance is primary to skeletal muscle. Myoblasts were isolated from muscle biopsies of 8 lean insulin-resistant and 8 carefully matched insulin-sensitive subjects (metabolic clearance rates as determined by euglycemic-hyperinsulinemic clamp: 5.8 +/- 0.5 vs. 12.3 +/- 1.7 ml x kg(-1) x min(-1), respectively; P < or = 0.05) and differentiated to myotubes. In these cells, insulin stimulation of glucose uptake, glycogen synthesis, insulin receptor (IR) kinase activity, and insulin receptor substrate 1-associated phosphatidylinositol 3-kinase (PI 3-kinase) activity were measured. Furthermore, insulin activation of protein kinase B (PKB) was compared with immunoblotting of serine residues at position 473. Basal glucose uptake (1.05 +/- 0.07 vs. 0.95 +/- 0.07 relative units, respectively; P = 0.49) and basal glycogen synthesis (1.02 +/- 0.11 vs. 0.98 +/- 0.11 relative units, respectively; P = 0.89) were not different in myotubes from insulin-resistant and insulin-sensitive subjects. Maximal insulin responsiveness of glucose uptake (1.35 +/- 0.03-fold vs. 1.41 +/- 0.05-fold over basal for insulin-resistant and insulin-sensitive subjects, respectively; P = 0.43) and glycogen synthesis (2.00 +/- 0.13-fold vs. 2.10 +/- 0.16-fold over basal for insulin-resistant and insulin-sensitive subjects, respectively; P = 0.66) were also not different. Insulin stimulation (1 nmol/l) of IR kinase and PI 3-kinase were maximal within 5 min (approximately 8- and 5-fold over basal, respectively), and insulin activation of PKB was maximal within 15 min (approximately 3.5-fold over basal). These time kinetics were not significantly different between groups. In summary, our data show that insulin action and signaling in cultured skeletal muscle cells from normoglycemic lean insulin-resistant subjects is not different from that in cells from insulin-sensitive subjects. This suggests an important role of environmental factors in the development of insulin resistance in skeletal muscle.

A 60 minute hyperglycemic clamp is sufficient to assess both phases of insulin secretion

The hyperglycemic clamp is considered to be the gold standard for determining both first and second phase insulin secretion. In order to achieve a reasonable insulin plateau for the second phase, it has become common practice to clamp for 120 or even 180 minutes at 10 mM. It is unknown whether earlier insulin determinations would be sufficient to predict second phase insulin secretion. We therefore reviewed the hyperglycemic clamp data of 58 subjects with different degrees of glucose tolerance to assess whether one or more insulin concentrations determined at earlier time points of the clamp could predict second phase insulin secretion (insulin and C-peptide concentration at 120 minutes). The correlation coefficients between second-phase insulin secretion and plasma insulin or C-peptide at 60 min were 0.95 and 0.96, respectively (both p<0.00005). Averaging plasma insulin or C-peptide over 2 or more adjacent time points did not improve the correlation. In conclusion, a one-hour hyperglycemic clamp can provide the standard measurement of first phase insulin secretion plus a good approximation of second phase insulin secretion.

A novel hyperglycaemic clamp for characterization of islet function in humans: assessment of three different secretagogues, maximal insulin response and reproducibility

BACKGROUND

Characterization of beta-cell function in humans is essential for identifying genetic defects involved in abnormal insulin secretion and the pathogenesis of type 2 diabetes.

MATERIALS AND METHODS

We designed a novel test assessing plasma insulin and C-peptide in response to 3 different secretagogues. Seven lean, healthy volunteers twice underwent a 200 min hyperglycaemic clamp (10 mmol L-1) with administration of GLP-1 (1.5 pmol. kg-1. min-1) starting at 120 min and an arginine bolus at 180 min. We determined glucose-induced first and second-phase insulin secretion, GLP-1-stimulated insulin secretion, arginine-stimulated insulin response (increase above prestimulus, DeltaIarg) and the maximal, i. e. highest absolute, insulin concentration (Imax). Insulin sensitivity was assessed during second-phase hyperglycaemia. On a third occasion 6 subjects additionally received an arginine bolus at > 25 mM blood glucose, a test hitherto claimed to provoke maximal insulin secretion.

RESULTS

Insulin levels increased from 46 +/- 11 pM to 566 +/- 202 pM at 120 min, to 5104 +/- 1179 pM at 180 min and to maximally 8361 +/- 1368 pM after arginine (all P < 0.001). The within subject coefficients of variation of the different secretion parameters ranged from 10 +/- 3% to 16 +/- 6%. Except for second-phase which failed to correlate significantly with DeltaIarg (r = 0.52, P = 0.23) and Imax (r = 0.75, P = 0.053) all phases of insulin secretion correlated with one another. The insulin concentration after the arginine bolus at > 25 mM glucose (n = 6) was 2773 +/- 855 pM vs. 7562 +/- 1168 pM for Imax (P = 0.003).

CONCLUSION

This novel insulin secretion test elicits a distinct pattern of plasma insulin concentrations in response to the secretagogues glucose, GLP-1 and arginine and is highly reproducible and can be used for differential characterization of islet function.

Intensive insulin therapy combined with metformin in obese type 2 diabetic patients

Unlike other pharmacological therapies used in obese type 2 diabetic patients, metformin has been shown to improve glycemic control with lower insulin levels and not to involve weight gain. We therefore examined the effect of adjunct metformin in 13 severely obese type 2 diabetic patients (BMI 39.3 +/- 3.9 kg/m2) in suboptimal glycemic control pretreated with intensified insulin therapy. Patients were randomly assigned to either metformin or placebo treatment (double-blind) for 10 weeks and after a 2 week washout period received the opposite treatment, respectively, for 10 additional weeks. HbA1c decreased comparably during placebo (from 8.1 +/- 0.4 to 7.6 +/- 0.3%) and metformin (from 8.5 +/- 0.4 to 7.4 +/- 0.3%, p = 0.29 vs. placebo). Changes in fasting glucose levels were also not different between placebo (from 9.3 +/- 0.7 to 9.5 +/- 0.7 mM) and metformin (from 10.3 +/- 0.5 to 9.5 +/- 0.6 mM, p = 0.44 vs. placebo). Total exogenous insulin requirements decreased from 53 +/- 10 to 35 +/- 7 units during metformin treatment (p = 0.02 vs. placebo). Changes in fasting insulin levels during placebo and metformin treatments were not different (p = 0.11). Metformin had no effect on body weight and serum triglycerides but marginally decreased serum cholesterol levels (from 239 +/- 18 to 211 +/- 14 mg/dl, p = 0.005, p = 0.08 vs. placebo). During the oral glucose tolerance test no differences were observed in the areas under the curve for glucose and insulin while that for C-peptide showed a tendency to increase during metformin administration. We conclude that addition of metformin to insulin treatment in severely obese type 2 diabetic patients improves glycemia but not hyperinsulinemia in comparison to intensive insulin therapy alone. With adjunct metformin, approximately 30% less exogenous insulin is required. With respect to glycemia and lipids, adjunct metformin can be a reasonable treatment alternative in selected obese patients with type 2 diabetes already on intensive insulin therapy.

Use of the oral glucose tolerance test to assess insulin release and insulin sensitivity

OBJECTIVE

The oral glucose tolerance test (OGTT) has often been used to evaluate apparent insulin release and insulin resistance in various clinical settings. However, because insulin sensitivity and insulin release are interdependent, to what extent they can be predicted from an OGTT is unclear.

RESEARCH DESIGN AND METHODS

We studied insulin sensitivity using the euglycemic-hyperinsulinemic clamp and insulin release using the hyperglycemic clamp in 104 nondiabetic volunteers who had also undergone an OGTT. Demographic parameters (BMI, waist-to-hip ratio, age) and plasma glucose and insulin values from the OGTT were subjected to multiple linear regression to predict the metabolic clearance rate (MCR) of glucose, the insulin sensitivity index (ISI), and first-phase (1st PH) and second-phase (2nd PH) insulin release as measured with the respective clamps.

RESULTS

The equations predicting MCR and ISI contained BMI, insulin (120 min), and glucose (90 min) and were highly correlated with the measured MCR (r = 0.80, P < 0.00005) and ISI (r = 0.79, P < 0.00005). The equations predicting 1st PH and 2nd PH contained insulin (0 and 30 min) and glucose (30 min) and were also highly correlated with the measured 1st PH (r = 0.78, P < 0.00005) and 2nd PH (r = 0.79, P < 0.00005). The parameters predicted by our equations correlated better with the measured parameters than homeostasis model assessment for secretion and resistance, the delta30-min insulin/delta30-min glucose ratio for secretion and insulin (120 min) for insulin resistance taken from the OGTT.

CONCLUSIONS

We thus conclude that predicting insulin sensitivity and insulin release with reasonable accuracy from simple demographic parameters and values obtained during an OGTT is possible. The derived equations should be used in various clinical settings in which the use of clamps or the minimal model would be impractical.

Leptin levels in humans are acutely suppressed by isoproterenol despite acipimox-induced inhibition of lipolysis, but not by free fatty acids

Leptin secretion is complexly regulated in humans. Insulin has been shown to stimulate leptin secretion, whereas in vitro data suggest that catecholamines and free fatty acids (FFAs) inhibit leptin secretion. To dissect differential effects on leptin secretion, we performed two experimental protocols in 11 lean healthy subjects in addition to a saline infusion plus oral acipimox to suppress lipolysis (SAL + ACX) as a control experiment: (1) isoproterenol (approximately 30 ng/kg x min, to increase the heart rate by approximately 50 bpm) plus oral acipimox (ISO + ACX, 240 minutes) and (2) Intralipid (Pharmacia & Upjohn, Erlangen, Germany) plus heparin (LIP, 420 minutes). During SAL + ACX, FFAs decreased from 0.44 +/- 0.04 to 0.06 +/- 0.02 mmol/L (P = .001), while serum insulin and leptin remained unchanged. During ISO + ACX, FFAs decreased similarly from 0.41 +/- 0.13 to 0.09 +/- 0.02 mmol/L (P= .001), while insulin increased from 47 +/- 8 to a maximum of 116 +/- 15 pmol/L (P= .001) and serum leptin decreased acutely from 6.4 +/- 2.1 to a minimum of 5.4 +/- 1.8 ng/mL after 90 minutes (P = .003 vSAL + ACX). After 150 minutes, leptin returned to control levels. During LIP, the elevation of FFAs from 0.34 +/- 0.04 to 1.71 +/- 0.19 mmol/L (P = .001) had no effect on serum insulin or leptin concentrations (both P = nonsignificant). In conclusion, our results show that in humans, isoproterenol acutely suppresses leptin levels independently of increased FFAs, and elevated FFAs have no acute effect on leptin levels. The fact that an inhibition of leptin secretion occurred despite conditions that are known to suppress intracellular cyclic adenosine monophosphate (cAMP) levels, as demonstrated by suppressed lipolysis, suggests that signaling mechanisms other than those mediated by cAMP must be involved in modulating leptin secretion.

Reversal of hypoglycemia unawareness in a long-term type 1 diabetic patient by improvement of beta-adrenergic sensitivity after prevention of hypoglycemia

The purpose of this study was to assess the effect of strict avoidance of hypoglycemia on beta-adrenergic sensitivity in a type 1 diabetic patient with hypoglycemia unawareness and a diabetes duration of 55 yr. beta-Adrenergic sensitivity was determined by an isoproterenol test and was expressed as the lowest dose of isoproterenol that increases the heart rate by 25 beats/min (IC25). Plasma epinephrine and symptom responses to hypoglycemia were determined during a 3-h hypoglycemic (3 mmol/L) clamp. Initially, the patient had a near-normal counterregulatory plasma epinephrine response to hypoglycemia but reduced beta-adrenergic sensitivity (IC25, 2 microg) compared to 10 hypoglycemia aware, type 1 diabetic patients (0.65 +/- 0.14 microg) and 10 normal control subjects (1.13 +/- 0.21 microg). After 1 yr of strict avoidance of blood glucose levels below 4 mmol/L, the IC25 decreased to 0.25 microg, reflecting improved beta-adrenergic sensitivity. In conclusion, the reduced beta-adrenergic sensitivity in this patient was probably the reason for hypoglycemia unawareness and was reversed by strict avoidance of hypoglycemia.

Long term effect of a structured inpatient diabetes teaching and treatment programme in type 2 diabetic patients: influence of mode of follow-up

Structured diabetes teaching and treatment programmes (STTP) are increasingly offered for patients with diabetes to improve metabolic control. We prospectively studied the long term-effect of STTP on metabolic control and knowledge of diabetes in patients with type 2 diabetes. In addition, differences in the mode of follow-up by a university diabetes centre (UDC) versus general practitioner (GP) were assessed. Of the 64 patients with type 2 diabetes (61 +/- 10 years old, diabetes duration 11 +/- 7 years) included in the study 52 could be reevaluated after 2 years. Of those, 31 were followed up by the UDC and 21 by their GPs who received detailed follow-up instructions from the UDC. In all patients, HbA1c decreased from 9.1 +/- 0.3% before the programme to 8.3 +/- 0.3% 2 years after the programme (P = 0.004), whereas body mass index increased from 28.8 +/- 0.8 to 30.3 +/- 0.9 kg/m2 (P < 0.001). Patients had a significantly better knowledge of diabetes and diet 2 years after the programme. For all parameters tested, none of the changes differed between patients managed by the UDC versus those managed by their GP. However, patients who chose follow-up by the UDC were more obese and had a better knowledge of diabetes. In conclusion, the STTP for patients with type 2 diabetes was effective in improving the long-term glycaemic control and knowledge of diabetes. Moreover, with precise therapeutic goals and follow-up instructions given to patient and GP this improvement was independent of the mode of outpatient follow-up.

Lipolysis in skeletal muscle is rapidly regulated by low physiological doses of insulin

AIMS/HYPOTHESIS

Both patients with Type II (non-insulin-dependent) diabetes mellitus and normoglycaemic, insulin resistant subjects were shown to have an increased lipid content in skeletal muscle, which correlates negatively with insulin sensitivity. Recently, it was shown that during a hyperinsulinaemic euglycaemic clamp interstitial glycerol was reduced not only in adipose tissue but also in skeletal muscle. To assess whether lipolysis of muscular lipids is also regulated by low physiological concentrations of insulin, we used the microdialysis technique in combination with a 3-step hyperinsulinaemic glucose clamp.

METHODS

Nineteen lean, healthy subjects (12 m/7 f) underwent a glucose clamp with various doses of insulin (GC I = 0.1, GC II = 0.25 and GC III = 1.0 mU x kg(-1) x min(-1)). Two double lumen microdialysis catheters each were inserted in the paraumbilical subcutaneous adipose tissue and in skeletal muscle (tibialis anterior) to measure interstitial glycerol concentration (index of lipolysis) and ethanol outflow (index of tissue flow).

RESULTS

During the different steps of the glucose clamp, glycerol in adipose tissue was reduced to 81 +/- 7 % (GC I), 55 +/- 8 % (GC II) and 25 +/- 5 % (GC III), respectively, of basal. In contrast, glycerol in skeletal muscle declined to 73 +/- 5 % (GC I) and to 57 +/- 6 % (GC II) but was not further reduced at GC III. Tissue flow was higher in the skeletal muscle and remained unchanged in both compartments throughout the experiment.

CONCLUSION/INTERPRETATION

This study confirms the presence of glycerol release in skeletal muscle. Lipolysis in skeletal muscle and adipose tissue are suppressed similarly by minute and physiological increases in insulin but differently by supraphysiological increases. Inadequate suppression of intramuscular lipolysis resulting in increased availability of non-esterified fatty acids, could represent a potential mechanism involved in the pathogenesis of impaired glucose disposal, i. e. insulin resistance, in muscle. [Diabetologia (1999) 42: 1171-1174]

Important role of the kidney in human carbohydrate metabolism

Recent studies using a combination of isotope and balance techniques have shown that, in the postabsorptive state, the human kidney contributes substantially to overall glucose production and consumption. The kidney may contribute as much as the liver to gluconeogenesis and play an important role in the counterregulation of hypoglycemia. Furthermore, increased renal glucose production may contribute to fasting hyperglycemia found in type I and type II diabetes mellitus. Finally, loss of renal tissue as a consumer of glucose could explain the insulin resistance of uremia. We hypothesize that the human kidney may play a more important role in human carbohydrate metabolism than previously appreciated.

Role of glutamine in human carbohydrate metabolism in kidney and other tissues

Glutamine is the most abundant amino acid in the human body and is involved in more metabolic processes than any other amino acid. Until recently, the understanding of many aspects of glutamine metabolism was based on animal and in vitro data. However, recent studies using isotopic and balance techniques have greatly advanced the understanding of glutamine metabolism in humans and its role in glucose metabolism in the kidney and other tissues. There is now evidence that in postabsorptive humans, glutamine is an important glucose precursor and makes a significant contribution to the addition of new carbon to the glucose carbon pool. The importance of alanine for gluconeogenesis, viewed in terms of the addition of new carbons, is less than previously assumed. It appears that glutamine is predominantly a renal gluconeogenic substrate, whereas alanine gluconeogenesis is essentially confined to the liver. As shown recently, renal gluconeogenesis contributes 20 to 25% to whole-body glucose production. Moreover, glutamine has been shown not only to stimulate net muscle glycogen storage but also to stimulate gluconeogenesis in normal humans. Finally, in humans with type II diabetes, conversion of glutamine to glucose is increased (more so than that of alanine). The available evidence on the hormonal regulation of glutamine gluconeogenesis in kidney and liver and its alterations under pathological conditions are discussed.

Evidence for inhibition of leptin secretion by catecholamines in man

The regulation of leptin secretion is complex and not entirely understood in humans. Insulin has been shown to stimulate leptin secretion in humans, whereas in vitro data suggest that catecholamines inhibit leptin secretion. The present studies were therefore undertaken to examine the leptin response to hyperinsulinemia in the presence and absence of elevated plasma levels of endogenous catecholamines in humans. Leptin concentrations were determined during both a euglycemic and hypoglycemic hyperinsulinemic clamp study in 10 normal and 10 type I diabetic subjects. Serum leptin increased during the hyperinsulinemic euglycemic clamp in normal (from 6.1 +/- 0.9 to 7.2 +/- 1.1 ng/dl, p = 0.003) and diabetic subjects (from 6.2 +/- 1.4 to 7.8 +/- 1.8 ng/dl, p = 0.001). During hyperinsulinemic hypoglycemia leptin concentrations increased significantly in type 1 diabetic patients (from 5.6 +/- 1.1 to 7.6 +/- 1.7 ng/dl, p = 0.003) but remained unaltered in normals (from 5.5 +/- 0.7 to 5.7 +/- 0.9 ng/dl, p = 0.7). During hypoglycemia in all subjects the increase in leptin was negatively correlated with the increase in epinephrine (r = 0.60, p = 0.005) and positively with the decrease in free fatty acids (r = 0.71, p = 0.003). In conclusion our results indicate that catecholamines play a suppressive role in the regulation of leptin secretion.

Effects of glucagon on renal and hepatic glutamine gluconeogenesis in normal postabsorptive humans

Glutamine is an important gluconeogenic amino acid in postabsorptive humans. To assess the effect of glucagon on renal and hepatic glutamine gluconeogenesis, we infused six normal healthy postabsorptive subjects with glucagon at a rate chosen to produce circulating glucagon concentrations found during hypoglycemia and, using a combination of isotopic and net balance techniques, determined the systemic, renal, and hepatic glucose release and renal and hepatic production of glucose from glutamine. Infusion of glucagon increased systemic and hepatic glucose release (both P < .02), but had no effect on renal glucose release (P = .26). Systemic and hepatic glutamine gluconeogenesis increased from 0.45 +/- 0.3 and 0.11 +/- 0.02 micromol x kg(-1) x min(-1), respectively, to 0.61 +/- 0.04 (P = .002) and 0.31 +/- 0.03 micromol x kg(-1) x min(-1) (P = .001), respectively, whereas renal glutamine gluconeogenesis was unchanged (from 0.33 +/- 0.03 to 0.30 +/- 0.04 micromol x kg(-1) x min(-1), P = .20). The hepatic contribution to systemic glutamine gluconeogenesis increased from 25.2% +/- 6.2% to 51.6% +/- 5.5% (P = .002), while that of the kidney decreased from 74.8% +/- 6.2% to 48.4% +/- 5.5% (P = .003). Glucagon had no effect on the renal net balance, fractional extraction, or uptake and release of either glucose or glutamine. We thus conclude that glucagon stimulates glutamine gluconeogenesis in normal postabsorptive humans, predominantly due to an increase in hepatic glutamine conversion to glucose. Thus, under certain conditions such as counterregulation of hypoglycemia, the liver may be an important site of glutamine gluconeogenesis.

Effect of hypoglycemia on beta-adrenergic sensitivity in normal and type 1 diabetic subjects

OBJECTIVE

The purpose of this study was to assess the potential role of reduced tissue sensitivity to catecholamines in the pathogenesis of hypoglycemia unawareness in patients with type 1 diabetes.

RESEARCH DESIGN AND METHODS

The effect of a single episode of hypoglycemia on beta-adrenergic sensitivity was studied in 10 type 1 diabetic patients with apparently normal awareness of hypoglycemia (age 29 +/- 5 years, diabetes duration 13 +/- 8 years, HbA1c 7.3 +/- 0.9%) and 10 age-matched healthy control subjects. Beta-adrenergic sensitivity was measured with the isoproterenol test after a hyperinsulinemic euglycemic clamp and after a hyperinsulinemic hypoglycemic clamp. Beta-adrenergic sensitivity was expressed as the dose of intravenous isoproterenol that increased the heart rate by 25 beats/min (IC25).

RESULTS

During hypoglycemia, diabetic subjects had an impaired plasma epinephrine response compared with that of the control subjects (16.7 +/- 5.0 vs. 40.1 +/- 6.8 ng/ml, P = 0.02). In control subjects, the IC25 was lower after hypoglycemia than after euglycemia (0.83 +/- 0.22 vs. 1.13 +/- 0.21 microg, P = 0.02) indicating an increase in beta-adrenergic sensitivity. In diabetic subjects, on the other hand, the IC25 was greater after hypoglycemia than after euglycemia (1.00 +/- 0.26 vs. 0.65 +/- 0.14 microg, P = 0.04), indicating a decrease in beta-adrenergic sensitivity.

CONCLUSIONS

In normal subjects, a single episode of hypoglycemia increases beta-adrenergic sensitivity. In diabetic subjects, in contrast, hypoglycemia reduces beta-adrenergic sensitivity. These results provide evidence that in type 1 diabetic patients, some maladaptation of tissue sensitivity to catecholamines contributes to the development of hypoglycemia unawareness. A unifying hypothesis is presented for the pathogenesis of hypoglycemia unawareness in type 1 diabetic patients incorporating the concepts of both a reduced catecholamine response and reduced adrenergic sensitivity

Abnormal renal and hepatic glucose metabolism in type 2 diabetes mellitus

Release of glucose by liver and kidney are both increased in diabetic animals. Although the overall release of glucose into the circulation is increased in humans with diabetes, excessive release of glucose by either their liver or kidney has not as yet been demonstrated. The present experiments were therefore undertaken to assess the relative contributions of hepatic and renal glucose release to the excessive glucose release found in type 2 diabetes. Using a combination of isotopic and balance techniques to determine total systemic glucose release and renal glucose release in postabsorptive type 2 diabetic subjects and age-weight-matched nondiabetic volunteers, their hepatic glucose release was then calculated as the difference between total systemic glucose release and renal glucose release. Renal glucose release was increased nearly 300% in diabetic subjects (321+/-36 vs. 125+/-15 micromol/min, P < 0.001). Hepatic glucose release was increased approximately 30% (P = 0.03), but increments in hepatic and renal glucose release were comparable (2.60+/-0.70 vs. 2.21+/-0.32, micromol.kg-1.min-1, respectively, P = 0.26). Renal glucose uptake was markedly increased in diabetic subjects (353+/-48 vs. 103+/-10 micromol/min, P < 0.001), resulting in net renal glucose uptake in the diabetic subjects (92+/-50 micromol/ min) versus a net output in the nondiabetic subjects (21+/-14 micromol/min, P = 0.043). Renal glucose uptake was inversely correlated with renal FFA uptake (r = -0.51, P < 0.01), which was reduced by approximately 60% in diabetic subjects (10. 9+/-2.7 vs. 27.0+/-3.3 micromol/min, P < 0.002). We conclude that in type 2 diabetes, both liver and kidney contribute to glucose overproduction and that renal glucose uptake is markedly increased. The latter may suppress renal FFA uptake via a glucose-fatty acid cycle and explain the accumulation of glycogen commonly found in the diabetic kidney.