S(G), S(I), and EGP of exercise-trained middle-aged men estimated by a two-compartment labeled minimal model

Association between the PPARGC1A polymorphism and aerobic capacity in Japanese middle-aged men

OBJECTIVE

A lower frequency for the peroxisome proliferator-activated receptor γ coactivator 1α (PPARGC1A) Ser482 allele has been reported in elite-level endurance athletes among Caucasians, although this gene polymorphism has not been found to be associated with aerobic capacity in German, Dutch or Chinese populations. The purpose of the current study was to examine the associations between the Gly482Ser polymorphism and aerobic fitness in 112 Japanese middle-aged men.

METHODS

The PPARGC1A Gly482Ser polymorphism was identified according to a TaqMan(®) SNP genotyping assay. Habitual physical activity was objectively measured using an accelerometer. The lactate threshold (LT), an index of aerobic fitness, was measured based on a submaximal graded exercise test performed on an electric cycle ergometer. The association between the LT and the Gly482Ser polymorphism was assessed according to a multiple regression analysis and analysis of covariance, with adjustment for potential confounders (age, body mass index, cigarette smoking, physical activity level and regular exercise).

RESULTS

A significant association was observed between the PPARGC1A Gly482Ser polymorphism and LT, as carriers of the Ser482 had higher LT values than the Gly482 carriers.

CONCLUSION

The current results suggest that the PPARGC1A Ser482 allele is associated with a higher aerobic capacity in Japanese middle-aged men.

Continuous glucose monitoring during a 100-km race: a case study in an elite ultramarathon runner

The current case study intended to measure blood glucose fluctuation in 2 marathon runners during a 100-km race using a continuous glucose-monitoring system (CGMS) and investigate the relationship between glucose profile and change in running speed. Two experienced ultramarathon runners participated in this study. A CGMS glucose sensor was inserted into the subcutaneous abdominal tissue at 35 h before the 100-km race, and the glucose profile was monitored continuously until the end of the race. Race pace and energy intake during the race were recorded. Participants finished the race in 6h:51min:17s (runner A) and 8h:56min:04s (runner B), and the race-pace decrement ratios were 17.6% for runner A and 27.2% for runner B. The average relative intensity throughout the 100-km race was 89.9% ± 5.8% lactate threshold (LT) in runner A and 78.4% ± 8.6% LT in runner B. The total amount of carbohydrate intake during the race was 249 g and 366 g in runners A and B, respectively. Despite lower carbohydrate intake, runner A maintained a normal glucose level throughout the race, while runner B rapidly decreased blood glucose and became hypoglycemic after the 80-km point. These results suggest that elite ultramarathon runners may have the ability to prevent a large decrement in blood glucose level regardless of the amount of energy intake during the race to maintain higher relative running intensity.

Hepatic insulin sensitivity assessed by integrated model of hepatic and peripheral glucose regulation

BACKGROUND

The integrated model of hepatic and peripheral glucose metabolism incorporates a model for liver glucose metabolism into the two-compartment minimal model framework to describe endogenous glucose kinetics during a labeled intravenous glucose tolerance test (IVGTT). This model also provides a parametric description of endogenous glucose production (EGP). The present study extended the theoretical potential of the model by defining hepatic glucose effectiveness (hS(G)(2)) as the ability of glucose per se to inhibit EGP and hepatic insulin sensitivity (hS(1)(2)) as the ability of insulin to enhance glucose suppression of EGP.

METHODS

As a retrospective data base review of our previous study, we re-analyzed time courses of exogenous and endogenous glucose concentration during [6,6-(2)H(2)]glucose-labeled IVGTT (0.3 g/kg glucose), performed in 11 exercise-trained and 12 age-matched sedentary subjects. Model parameters of the two-compartment minimal model and of liver glucose metabolism were simultaneously identified to assess insulin sensitivity specific to stimulate glucose uptake (S(1)(2*)) and that specific to inhibit EGP (hS(1)(2)). The abilities of glucose per se to stimulate its own uptake (S(G)(2*)) and to inhibit EGP (hS(G)(2)) were also estimated.

RESULTS

Parameters of the integrated model were identified in all the subjects. Hepatic insulin sensitivity consisted of about one-third of total insulin sensitivity (S(1)(2*) + hS(1)(2)). Compared with the sedentary subjects, S(1)(2*), hS(1)(2), hS(G)(2) of the trained subjects were greater.

CONCLUSIONS

Because insulin resistance in liver and peripheral tissue may play a differential role in the pathogenesis of diabetes, this analysis can serve as a simple one-step approach to obtain metabolic indexes specific to EGP suppression and stimulating glucose uptake.

Hostility and minimal model of glucose kinetics in African American women

OBJECTIVE

To explore the underlying physiology of hostility (HOST) and to test the hypothesis that HOST has a greater impact on fasting glucose in African American (AA) women than it does on AA men or white men or women, using an intravenous glucose tolerance test (IVGTT) and the minimal model of glucose kinetics.

METHODS

A total of 115 healthy subjects selected for high or low scores on the 27 item Cook Medley HOST Scale underwent an IVGTT. Fasting nonesterified fatty acids (NEFA) levels were measured before the IVGTT. Catecholamine levels were measured 10 minutes into the IVGTT.

RESULTS

Moderation by group (AA women versus others) of HOST was found for glucose effectiveness (Sg, p = .02), acute insulin response (AIRg, p = .02), and disposition index (DI, p = .02). AA women showed a negative association between HOST and both Sg (beta = -0.45, p = .04) and DI (beta = -0.49, p = .02), controlling for age and body mass index. HOST was also associated with changes in epinephrine (beta = 0.39, p = .05) and fasting NEFA (beta = 0.44, p = .02) in the AA women. Controlling for fasting NEFA reduced the effect of HOST on both Sg and DI.

CONCLUSIONS

This study shows that HOST is related to decreased DI, a measure of pancreatic compensation for increased insulin resistance as well as decreased Sg, a measure of noninsulin-mediated glucose transport compared in AA women. These effects are partly mediated by the relationship of HOST to fasting NEFA.

A convenient LC-MS method for assessment of glucose kinetics in vivo with D-[13C6]glucose as a tracer

BACKGROUND

The isotope-labeled intravenous glucose tolerance test (IVGTT) combined with computer modeling is widely used to derive parameters related to glucose metabolism in vivo. Most of these methods involve use of either (2)H(2)-labeled or (13)C(1)-labeled D-glucose as a tracer with GC-MS to measure the isotope enrichment. These methods are challenging, both technologically and economically. We have developed a novel approach that is suitable for labeled-IVGTT studies involving a large cohort of individuals.

METHODS

The tracer, D-[(13)C(6)]glucose, is a low-cost alternative with the significant advantage that the sixth isotope of natural glucose has virtually zero natural abundance, which facilitates isotopomer analysis with <1% labeled glucose in the infusate. After deproteinization of plasma samples collected at various times, glucose is converted to a stable derivative, purified by solid-phase extraction (SPE), and analyzed by HPLC-electrospray ionization mass spectrometry to accumulate the isotope-abundance data for the A+2, A+3, and A+6 ions of the glucose derivative. A 2-pool modeling program was used to derive standard kinetic parameters.

RESULTS

With labeled-IVGTT data from 10 healthy male individuals, the values for insulin sensitivity, glucose effectiveness, and the plasma clearance rate estimated with the 2-pool minimal model compared well with values obtained via traditional methods.

CONCLUSIONS

The relative simplicity and robustness of the new method permit the preparation and analysis of up to 48 samples/day, a throughput equivalent to 2 complete IVGTT experiments, and this method is readily adaptable to existing 96 well-format purification and analytical systems.

Cell-specific effects of insulin receptor substrate-1 deficiency on normal and IGF-I-mediated colon growth

Insulin-like growth factor I (IGF-I) potently stimulates intestinal growth. Insulin receptor substrate-1 (IRS-1) mediates proliferative and antiapoptotic actions of IGF-I in cell lines, but its in vivo relevance in intestine is not defined. This study tested the hypothesis that there is cell type-specific dependence on IRS-1 as a mediator of IGF-I action. Length, mass, crypt cell proliferation, and apoptosis were measured in small intestine and colon of IRS-1-null mice and wild-type (WT) littermates and in colon of IRS-1-null or WT mice expressing IGF-I transgenes. Expression of IGF-I receptor and signaling intermediates was examined in intestine of WT and IRS-1-null mice, cultured intestinal epithelial cells, and myofibroblasts. Absolute IRS-1 deficiency reduced mucosal mass in jejunum and colon, but effects were more pronounced in colon. Muscularis mass was decreased in both segments. In IGF-I transgenics, IRS-1 deficiency significantly attenuated IGF-I-stimulated growth of colonic mucosa and abolished antiapoptotic but not mitogenic effects of IGF-I transgene on crypt cells. IGF-I-induced muscularis growth was unaffected by IRS-1 deficiency. In intestinal epithelial cells, IRS-1 was expressed at higher levels than IRS-2 and was preferentially activated by IGF-I. In contrast, IGF-I activated both IRS-1 and IRS-2 in intestinal myofibroblasts and IRS-2 activation was upregulated in IRS-1-null myofibroblasts. We conclude that the intestinal epithelium but not the muscularis requires IRS-1 for normal trophic actions of IGF-I and that IRS-1 is required for antiapoptotic but not mitogenic effects of IGF-I in the intestinal crypts in vivo.

Epithelial cells and their neighbors I. Role of intestinal myofibroblasts in development, repair, and cancer

Intestinal myofibroblasts are alpha-smooth muscle actin-positive stromal cells that exist as a syncytium with fibroblasts and mural cells in the lamina propria of the gut. Through expression and secretion of cytokines, chemokines, growth factors, prostaglandins, and basal lamina/extracellular matrix molecules, as well as expression of adhesion molecules and receptors for many of the same soluble factors and matrix, myofibroblasts mediate information flow between the epithelium and the mesenchymal elements of the lamina propria. With the use of these factors and receptors, they play a fundamental role in intestinal organogenesis and in the repair of wounding or disease. Intestinal neoplasms enlist and conscript myofibroblast factors and matrix molecules to promote neoplastic growth, carcinoma invasion, and distant metastases.

Integrated model of hepatic and peripheral glucose regulation for estimation of endogenous glucose production during the hot IVGTT

We have developed a new model to describe endogenous glucose kinetics during a labeled (hot) intravenous glucose tolerance test (IVGTT) to derive a time profile of endogenous glucose production (EGP). We reanalyzed data from a previously published study (P. Vicini, J. J. Zachwieja, K. E. Yarasheski, D. M. Bier, A. Caumo, and C. Cobelli. Am J Physiol Endocrinol Metab 276: E285-E294, 1999), in which insulin-modified [6,6-2H2]glucose-labeled IVGTTs (0.33 g/kg glucose) were performed in 10 normal subjects. In addition, a second tracer ([U-13C]glucose) was infused in a variable rate to clamp the endogenous glucose tracer-to-tracee ratio (TTR). Our new model describing endogenous glucose kinetics was incorporated into the two-compartment hot minimal-model structure. The model gave estimates of glucose effectiveness [1.54 +/- 0.31 (SE) ml x kg(-1) x min(-1)], insulin sensitivity (37.74 +/- 5.23 10(4) dl x kg(-1) x min(-1) x microU(-1) x ml), and a new parameter describing the sensitivity of EGP to the inhibitory effect of insulin (IC50 = 0.0195 +/- 0.0046 min(-1)). The model additionally provided an estimate of the time course of EGP showing almost immediate inhibition, followed by a secondary inhibitory effect caused by infusion of insulin, and a large overshoot as EGP returns to its basal value. Our estimates show very good agreement with those obtained via deconvolution and the model-independent TTR clamp technique. These results suggest that the new integrated model can serve as a simple one-step approach to obtain metabolic indexes while also providing a parametric description of EGP.

Primacy of beta-cell dysfunction in the development of hyperglycemia: a study in the Japanese general population

To elucidate the hierarchy in the evolution of glucose intolerance in the general population, the relationship between plasma glucose (PG), beta-cell function (insulinogenic index [II] = DeltaIRI(0-30)/DeltaPG(0-30) on 75 g oral glucose tolerance test [OGTT], where IRI is immunoreactive insulin), insulin sensitivity (Si; determined by quantitative insulin sensitivity check index [QUICKI]), age, and body mass index (BMI) were analyzed in 504 Japanese health examinees (men/women: 347/157). The mean (+/-SD) age was 53 (+/-11) years, BMI 23.6 (+/-3.2) kg/m2, fasting PG (FPG) 5.61 (+/-0.97) mmol/L, 2-hour PG 7.42 (+/-3.1) mmol/L, II 74.2 (+/-169.3) [pmol/L]. [mmol/l](-1), and QUICKI 0.385 (+/-0.057) [log (microU/mL) + log (mg/100 mL)](-1). Higher FPG and 2-hour PG, respectively, were independently correlated with lower II, lower QUICKI, higher age, and higher BMI; the standardized correlation coefficient was largest for the correlation between PG and II. Based on the multiple linear regression, FPG = 8.565 - 1.201. log [II] - 5.374. QUICKI + 0.007. age + 0.030. BMI (r2 = 0.442), and 2-hour PG = 14.239 - 4.206. log [II] - 0.141. QUICKI + 0.034 - age + 0.141. BMI (r2 = 0.493). Thus, elevation of PG correlated most prominently with beta-cell dysfunction and less prominently with decreased Si, higher age, and BMI (especially so in the case of 2-hour PG). In conclusion, the primacy of beta-cell dysfunction in the process of developing glucose intolerance was strongly suggested in the Japanese general population.

Effect of moderate exercise training on peripheral glucose effectiveness, insulin sensitivity, and endogenous glucose production in healthy humans estimated by a two-compartment-labeled minimal model

For examining the effects of moderate exercise training on peripheral glucose effectiveness (S(g)(2)*), insulin sensitivity (S(i)(2)*), and endogenous glucose production (EGP), seven men and one woman (24.8 +/- 1.8 years) participated in cycle ergometer training at lactate threshold intensity for 60 min/day, 5 days/week for 12 weeks. Stable-labeled frequently sampled intravenous glucose tolerance tests were performed before and 16 h and 1 week after the last training session. S(g)(2)* (pre 0.71 +/- 0.03 x 10(-2), 16 h 0.85 +/- 0.02 x 10(-2) dl. kg(-1). min(-1)) and S(i)(2)* (pre 12.6 +/- 2.6 x 10(-4), 16 h 19.7 +/- 3.3 x 10(-4) dl. kg(-1). min(-1). [ micro U/ml](-1)), analyzed using the two-compartment minimal model, were significantly elevated 16 h after the last training session. The elevated S(g)(2)* remained higher despite the cessation of exercise training for 1 week (1.00 +/- 0.03 x 10(-2) dl. kg(-1). min(-1)). EGP was suppressed within 20 min after glucose bolus, and the suppression of EGP was followed by their overshoot. The time course of EGP during the intravenous glucose tolerance test remained similar after the training period. In conclusion, moderate exercise training at lactate threshold improves not only peripheral insulin sensitivity but also peripheral glucose effectiveness with no change in the effect of glucose and/or insulin to suppress EGP in healthy humans.

C-type natriuretic peptide induces human colonic myofibroblast relaxation

Intestinal response to injury requires coordinated regulation of the tension exerted by subepithelial myofibroblasts (SEM). However, the signals governing relaxation of intestinal SEM have not been investigated. Our aim was to test the hypothesis that signal transduction pathways initiated by C-type natriuretic peptide (CNP) induce intestinal SEM relaxation. We directly quantified the effects of CNP on isometric tension exerted by cultured human colonic SEM. We also measured the effects of CNP on cGMP content, myosin regulatory light chain (MLC) phosphorylation, and cytosolic Ca2+ concentration. CNP induced relaxation of SEM within 10 s. By 10 min, relaxation reached a plateau that was sustained for 2 h. CNP-induced relaxation was saturable, with a maximal decrease in tension (51.7 +/- 3.8 dyn) observed at 250 nM. SEM relaxation in response to CNP constituted approximately 23% of total basal tension. CNP increased intracellular cGMP content and reduced MLC phosphorylation. Effects of CNP on cGMP and MLC exhibited the same dose dependence as CNP-induced relaxation. MLC phosphorylation decreased within 2 min of CNP exposure and was sustained for at least 45 min. CNP also stimulated a large transient increase in cytosolic Ca2+ concentration that occurred within 30 s and was nearly complete by 1 min. We also observed that calyculin-A, a potent inhibitor of MLC phosphatase, completely abolished the reduction in MLC phosphorylation induced by CNP. These results suggest that CNP induces intestinal SEM relaxation through cGMP-associated reductions in MLC phosphorylation. Moreover, these findings raise the possibility that CNP plays a role in intestinal wound healing.