The Pro12Ala polymorphism of the peroxisome proliferator-activated receptor (PPAR)-␥ 2 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-␥ 2 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/m 2 ) and matched for age, BMI, waist-to-hip ratio, and sex. The isotopically (infusion of d 5 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 (EC 50 ). This was directly determined during a threestep 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 mol ⅐ kg ؊1 ⅐ min ؊1 ⅐ pmol ؊1 ⅐ l -1 in the control group and 0.129 ؎ 0.008 mol ⅐ kg ؊1 ⅐ min ؊1 ⅐ pmol ؊1 ⅐ l -1 in the X/Ala group (P ‫؍‬ 0.003). The EC 50 was 56 ؎ 2 pmol/l in the control group and 44 ؎ 3 pmol/l in the X/Ala group (P ‫؍‬ 0.001). The EC 50 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-␥ 2 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. Diabetes 50: 876 -881, 2001

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.

OBJECTIVE -Studies on insulin sensitivity and insulin secretion in subjects with a familial predisposition for type 2 diabetes mellitus (T2DM) traditionally produce inconsistent results. This may be due to small sample size, subject selection, matching procedures, and perhaps lack of a measure of physical fitness. RESEARCH DESIGN AND METHODS -In the present study, we specifically tested the hypothesis that a family history of T2DM is associated with reduced VO 2max , measured by incremental bicycle ergometry, independent of insulin sensitivity estimated from an oral glucose tolerance test (OGTT; n ϭ 424) and measured by a euglycemic-hyperinsulinemic clamp (n ϭ 185). Subjects included in the study were young (34 Ϯ 10 years), healthy, and normal glucose tolerant with either a first-degree relative (FDR) with T2DM (n ϭ 183), a second-degree relative with T2DM (n ϭ 94), or no family history of T2DM (control subjects, n ϭ 147). BMI, percent body fat, waist-to-hip ratio (WHR), and habitual physical activity (HPA; standard questionnaire) were comparable among groups. FDRs had significantly lower VO 2max than control subjects: 40.5 Ϯ 0.6 vs. 45.2 Ϯ 0.9 ml O 2 /kg lean body mass, P ϭ 0.01 after adjusting for sex, age, BMI, HPA, and insulin sensitivity (euglycemic-hyperinsulinemic clamp).RESULTS -BMI, percent body fat, waist-to-hip ratio (WHR), and habitual physical activity (HPA; standard questionnaire) were comparable among groups. FDRs had significantly lower VO 2max than control subjects: 40.5 Ϯ 0.6 vs. 45.2 Ϯ 0.9 ml O 2 /kg lean body mass, P ϭ 0.01 after adjusting for sex, age, BMI, HPA, and insulin sensitivity (euglycemic-hyperinsulinemic clamp). Insulin sensitivity per se was not affected by family history of T2DM after adjusting for age, sex, BMI, and percent body fat (P ϭ 0.76). The appropriateness of ␤-cell function for the individual insulin sensitivity (disposition index: product of a validated secretion parameter [OGTT] and sensitivity [clamp]) was significantly lower in FDRs (87 Ϯ 4 units) versus control subjects (104 Ϯ 6 units, P ϭ 0.02 after adjusting for sex, age, and BMI). Analyses of the larger OGTT group produced essentially the same results.CONCLUSIONS -In conclusion, these data are compatible with the hypothesis that familial predisposition for T2DM impairs maximal oxygen consumption in skeletal muscle. Because habitual physical activity was not different, genetic factors may be involved. Conceivably, reduced VO 2max precedes skeletal muscle insulin resistance, providing a partial explanation for discrepancies in the literature. Diabetes Care 26:2126 -2132, 2003F amilial predisposition is an important risk factor for type 2 diabetes mellitus (T2DM). Studies performed to identify the underlying mechanisms have produced conflicting results. Whereas some primarily demonstrated impaired insulin secretion in subjects with a family history of T2DM, others reported reduced insulin sensitivity as the main finding. To date, no consensus has been reached (1,2). Issues such as differences in experimental techn...

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.

The relation between increased availability of nonesterified fatty acids (NEFA) and impaired muscle glucose disposal is well established for insulin resistant states. Increased lipolysis in subcutaneous and visceral adipose tissue is commonly assumed to be the source of these NEFAs [1,2]. In addition to the well-known fat depots, however, muscle has been identified as tissue containing relevant amounts of lipids. These were shown to be located not only extramyocellularly but also intramyocellularly [2±4]. Moreover, a statistically significant correlation between the intramyocellular lipid content (by magnetic resonance spectroscopy) and insulin resistance (decreased metabolic clearance rate during a hyperinsulinaemic euglycaemic clamp) in normal glucose tolerant subjects was recently shown indicating an important role of muscular lipids for glucose homeostasis Diabetologia (1999) Abstract 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 = 1.0mUḱ g ±1´m in ±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 nonesterified fatty acids, could represent a potential mechanism involved in the pathogenesis of impaired glucose disposal, i. e. insulin resistance, in muscle. [Diabetologia (1999