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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 Vo2max, measured by incremental bicycle ergometry, independent of insulin sensitivity estimated from an oral glucose tolerance test (OGTT; n = 424) and measured by a euglycemic-hyperinsulinemic clamp (n = 185). Subjects included in the study were young (34 ± 10 years), healthy, and normal glucose tolerant with either a first-degree relative (FDR) with T2DM (n = 183), a second-degree relative with T2DM (n = 94), or no family history of T2DM (control subjects, n = 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 Vo2max than control subjects: 40.5 ± 0.6 vs. 45.2 ± 0.9 ml O2/kg lean body mass, P = 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 Vo2max than control subjects: 40.5 ± 0.6 vs. 45.2 ± 0.9 ml O2/kg lean body mass, P = 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 = 0.76). The appropriateness of β-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 ± 4 units) versus control subjects (104 ± 6 units, P = 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 Vo2max precedes skeletal muscle insulin resistance, providing a partial explanation for discrepancies in the literature.

Delta-6 Desaturase, one of the rate-limiting enzymes, catalyzes the conversion of linoleic acid (C18:2 omega6) into gamma-linolenic acid (C18:3 omega6), arachidonic acid (C20:4 omega6), and further metabolites. Recently, it has been shown that human Delta-6 desaturase is expressed not only in liver but in a variety of human tissues, including muscle. Skeletal muscle is a major site of insulin action, and insulin sensitivity may be related to the fatty acid composition of muscle lipids. We examined the effects of troglitazone on the regulation of Delta-6 desaturase gene expression in human muscle cell cultures obtained from muscle biopsies (n = 15). Delta-6 Desaturase mRNA and peroxisome proliferator-activated receptor gamma2 (PPARgamma2) mRNA were quantified by two-step RT-PCR, and the activity of the Delta-6 desaturase enzyme was estimated by gas chromatographic analysis of the omega 6-C18:3/C18:2 fatty acids ratio. In cells treated with 11.5 micromol troglitazone for 4 days, PPARgamma2 mRNA levels were significantly increased (301.0 +/- 51.5%, P < 0.05) and Delta-6 desaturase mRNA levels were significantly decreased (41.7 +/- 5.9%, P < 0.0005) compared with the untreated controls. In accordance with the decrease of Delta-6 desaturase mRNA, there was a significant decrease in the omega6-C18:3/C18:2 ratio down to 47.4 +/- 7.5% in cholesterol esters, 54.2 +/- 7.4% in phospholipids, 56.7 +/- 6.5% in nonesterified fatty acids, and 67.7 +/- 5.9% in triglycerides. The troglitazone-induced decrease in Delta-6 desaturase mRNA is associated with a change in the unsaturated fatty acid composition of the muscle cells. These results add new aspects to the known thiazolidinedione effects on lipid metabolism.

Insulin resistance plays an important role in the pathogenesis of type 2 diabetes; however, the multiple mechanisms causing insulin resistance are not yet fully understood. The aim of this study was to explore the possible contribution of intramyocellular lipid content in the pathogenesis of skeletal muscle insulin resistance. We compared insulin-resistant and insulin-sensitive subjects. To meet stringent matching criteria for other known confounders of insulin resistance, these individuals were selected from an extensively metabolically characterized group of 280 first-degree relatives of type 2 diabetic subjects. Some 13 lean insulin-resistant and 13 lean insulin-sensitive subjects were matched for sex, age, BMI, percent body fat, physical fitness, and waist-to-hip ratio. Insulin sensitivity was determined by the hyperinsulinemic-euglycemic clamp method (for insulin-resistant subjects, glucose metabolic clearance rate [MCR] was 5.77+/-0.28 ml x kg(-1) x min(-1) [mean +/- SE]; for insulin-sensitive subjects, MCR was 10.15+/-0.7 ml x kg(-1) x min(-1); P

Using the euglycemic-hyperinsulinemic glucose clamp and the human forearm technique, we have demonstrated that the improved glucose disposal rate observed after the administration of an angiotensin-converting enzyme (ACE) inhibitor such as captopril may be primarily due to increased muscle glucose uptake (MGU). These results are not surprising because ACE, which is identical to the bradykinin (BK)-degrading kininase II, is abundantly present in muscle tissue, and its inhibition has been observed to elicit the observed metabolic actions via elevated tissue concentrations of BK and through a BK B2 receptor site in muscle and/or endothelial tissue. These findings are supported by several previous studies. Exogenous BK applied into the brachial artery of the human forearm not only augmented muscle blood flow (MBF) but also enhanced the rate of MGU. In another investigation, during rhythmic voluntary contraction, both MBF and MGU increased in response to the higher energy expenditure, and the release of BK rose in the blood vessel, draining the working muscle tissue. Inhibition of the activity of the BK-generating protease in muscle tissue (kallikrein) with aprotinin significantly diminished these functional responses during contraction. Applying the same kallikrein inhibitor during the infusion of insulin into the brachial artery significantly reduced the effect of insulin on glucose uptake into forearm muscle. This is of interest, because in recent studies insulin has been suggested to elicit its actions on MBF and MGU via the accelerated release of endothelium-derived nitric oxide, the generation of which is also stimulated by BK in a concentration-dependent manner. This new evidence obtained from in vitro and in vivo studies sheds new light on the discussion of whether BK may play a role in energy metabolism of skeletal muscle tissue.

The effect of bradykinin on glucose transporter translocation in isolated rat heart was compared with the effect of insulin. Hearts from male obese (fa/fa) Zucker rats were perfused under normoxic conditions and constant pressure in a classic Langendorff preparation with 12 mmol/l glucose as substrate, and a set of functional parameters was measured simultaneously. Bradykinin was administered at a concentration (10−11 mmol/l) that did not increase coronary flow. Insulin was used at a concentration (8 × 10−8 mmol/1) known to maximally stimulate glucose transport in this model. After 15 min of perfusion with insulin or bradykinin, subcellular membrane fractions of the heart were prepared, and distribution of glucose transporter protein (GLUT1 and GLUT4) in fractions enriched with surface membranes (transverse tubules [TTs] and sarcolemmal membranes [PMs]) and with low-density microsomal membranes (LDMs) were determined by immunoblotting with the respective antibodies. Both glucose transporter isoforms were translocated after stimulation with insulin (increased transporter protein content in the PM+TT-enriched fraction with a concomitant decrease in the LDM-enriched fraction) and, to a smaller extent, also with bradykinin. These data suggest that in hearts of insulin-resistant obese (fa/fa) Zucker rats, bradykinin interacts with or facilitates the translocation process of both GLUT1 and GLUT4.