Effects of acute exercise on insulin sensitivity, glucose effectiveness and disposition index in type 2 diabetic patients.

AIM: The aim of this work was to quantify the magnitude of changes in insulin sensitivity (S(I)) and glucose effectiveness (S(G)) in response to acute exercise in type 2 diabetic (T2D) patients, as previously studied in non-diabetic subjects. METHODS: Seven T2D patients and seven non-diabetic controls participated in the study. Two intravenous glucose tolerance tests (0.5 g/kg) with frequent blood sampling over 180 minutes and mathematical modelling were carried out in a randomized fashion, one at rest and the other immediately following 15 minutes of exercise at 50% of the maximum theoretical heart rate (HR(max)) followed by five minutes at 85% of the HR(max). S(I) and S(G) were calculated using Bergman's minimal model. RESULTS: After exercise, S(I) was increased by 773% (from 0.62+/-0.16 to 5.41+/-1.59 min(-1) x 10(-4)/(microU/mL) and even reached the zone of control values at rest (5.52+/-2.28), whereas S(G) remained unchanged. The disposition index acute insulin response (AIR(G)) x S(I) and the product of fasting insulin (I(B)) x S(I) also increased after exercise. CONCLUSION: A single bout of exercise at moderate intensity in type 2 diabetics did not improve S(G), but markedly improved the low S(I) values found in these patients, indicating that the acute effects of exercise on S(I) are quantitatively important in the interpretation of training-related S(I) changes and may even be therapeutically useful on their own. Surrogates such as homoeostasis model assessment (HOMA) and quantitative insulin-sensitivity check index (QUICKI) were not sensitive enough to detect this increase in S(I) and should probably be used with caution in the follow-up of exercise protocols in diabetic patients.

The intensity level of physical exercise and the bone metabolism response.

This study investigated the short-term effects of the intensity level of physical exercise on bone metabolism and related hormones. The responses of calciotropic hormones and bone biochemical markers were evaluated in seven male cyclists (mean age 24.4 years, range 20-39) during two 50-min cycling tests performed 15% below (-VT) and 15% above (+VT) the ventilatory threshold. In each test, venous blood samples were drawn at rest, at the 30th and 50th min of exercise, and after 15 min of recovery. For both intensity levels, no significant variation in calcium, 25-hydroxyvitamin D, 1.25-dihydroxyvitamin D, or cortisol level was observed. Intact parathyroid hormone (iPTH) level increased significantly after the last minute of the test (41%, p < 0.05) and peaked during the recovery (80%, p < 0.05) only in response to exercise performed at +VT. Serum phosphorus concentration rose during both tests, while albumin levels increased only at +VT. Concerning bone cell activity, osteocalcin, and type I-C telopeptide breakdown products transiently increased only in response to exercise performed at +VT (11% and 16.8%, respectively; p < 0.05). Bone alkaline phosphatase increased similarly for both intensity levels after 30 min (12%, p < 0.05) and 50 min (12% for -VT vs. 14% for +VT, p < 0.05). All markers of bone turnover returned to initial values during the recovery. In conclusion, a no-impact but intense and sustained exercise performed at +VT transiently stimulated bone turnover and iPTH secretion, suggesting the existence of a bone stimulation threshold. In addition to the well known effect of mechanical constraints, both the duration and intensity of exercise may induce changes in bone turnover.

Effects of physical activities that induce moderate external loading on bone metabolism in male athletes.

Sports characterized by little or moderate weight bearing or impact have a low osteogenic effect. However, the action of such sports on bone turnover remains unclear. The objective of this study was to determine the effect on bone remodelling of physical activities that induce moderate external loading on the skeleton. Thirty-eight male athletes aged 18-39 years (cyclists, n = 11; swimmers, n = 13; triathletes, n = 14) and 10 age-matched sedentary controls aged 22-35 years participated in the study. The study combined measurement of bone mineral density by dual-energy X-ray absorptiometry and bone turnover assessment from specific biochemical markers: serum bone-specific alkaline phosphatase, osteocalcin, urinary type I collagen C-telopeptide and calcium. Compared with the controls and swimmers, adjusted bone mineral density was higher (P < 0.05) in triathletes at the total proximal femur and lower limbs. No differences in bone mineral density were found between cyclists, swimmers and controls. Compared with controls, osteocalcin was higher (P < 0.05) in triathletes and swimmers and urinary type I collagen C-telopeptide was higher in swimmers only. Serum bone-specific alkaline phosphatase was lower (P < 0.05) in cyclists than in all other groups. In conclusion, an osteogenic effect was found only in triathletes, mainly at bone sites under high mechanical stress. Bone turnover differed in athletes compared with controls, suggesting that bone turnover may be sport-practice dependent. Despite some encouraging observations, it was not possible to show that changes in the bone remodelling process were sport-discipline dependent.

Competitive season of triathlon does not alter bone metabolism and bone mineral status in male triathletes.

This longitudinal study evaluated the effects of a triathlon season on bone metabolism and hormonal status. Seven male competitive triathletes (mean age 19.3 years, range 18 - 20) with 5.0 +/- 0.3 years of competition experience were tested twice during the season: at the beginning of training and 32 weeks later. Total and regional bone mineral density (BMD) was determined by dual-energy X-ray absorptiometry, while bone turnover was evaluated by specific biochemical markers: bone-specific alkaline phosphatase (B-ALP), osteocalcin, and urinary type I collagen C-telopeptide. In addition, sexual, calciotropic and somatotropic hormones were also analyzed. After 32 weeks, a BMD increase was found at the lumbar spine (1.9 %; p = 0.031) and skull (3.1 %; p = 0.048), while no variation was observed for total body or at the proximal femur. The B-ALP level decreased (-23.2 %; p = 0.031), but no variation was found for the other bone markers. 1.25 (OH) (2)D3, IGF-1 and the bioavailability IGF-1 index (IGF-1/IGFBP-3) increased by 18.3 % (p = 0.047), 29 % (p = 0.048), 33 % (p = 0.011), respectively, while PTH, testosterone, IGFBP-3 and cortisol concentrations were unchanged. In conclusion, the triathlon season had a moderately favourable effect on BMD, although a slowing down of bone formation activity was observed. No variation in hormonal levels was observed that could have limited the effects of exercise on bone tissue.

Serum levels of insulin-like growth factor-I (IGF-I), and IGF-binding proteins-1 and -3 in middle-aged and young athletes versus sedentary men: relationship with glucose disposal.

The goal of this study was to characterize the respective effects of aging and endurance training on serum insulin-like growth factor I (IGF-I), as well as IGF-binding proteins (IGFBP)-1 and -3 in relationship with glucose disposal. Thirty-two subjects (16 middle-aged men: 8 cyclists and 8 sedentary men; and 16 young men: 8 cyclists and 8 sedentary men) were compared in this study. Insulin sensitivity (SI) and glucose effectiveness (Sg) were assessed by the minimal model. Endurance training increased SI, Sg, and IGFBP-1 and -3 in both age groups (P<.05), but the older group showed a greater increase in SI and IGFBP-1 than the younger group (P<.05). IGF-I was increased only in the middle-aged trained men (P<.05). An effect of aging was found in the sedentary subjects, who presented lower IGF-I and SI (P<.05) when older. This effect disappeared with training since IGF-I and SI were nearly identical in young and middle-aged trained subjects. SI was correlated with IGFBP-1 (P<.01). These data suggest that (1) endurance training increases SI, Sg, and IGFBP-1 and -3 in men and, for SI and IGFBP-1, this increase becomes more pronounced with age; (2) endurance training may attenuate the aged-related decline in SI and IGF-I; and (3) IGFBP-1 may protect against the risk of hypoglycemia by counteracting the hypoglycemic effect of IGF-I in such situations of high SI.

Testosterone is significantly reduced in endurance athletes without impact on bone mineral density.

AIMS: To compare the basal plasma reproductive hormonal profile in three groups of athletes involved in different training programs, and to define the relationship between androgen level and bone mineral density (BMD) in male athletes. METHODS: Basal serum total testosterone (TT), free androgen index (FAI), sex hormone-binding globulin (SHBG), cortisol, cortisol to TT ratio, luteinizing hormone (LH), estrogen and BMD were evaluated in cyclists (CY; n = 11), triathletes (TR; n = 14) and swimmers (SW; n = 13) and compared with less active controls (n = 10). RESULTS: TT and FAI levels were lower (p < 0.05) in CY and TR, whereas the ratio of cortisol to TT was increased in CY only (p < 0.05). No alteration in serum LH, SHBG, estrogen or cortisol concentration was observed. BMD was higher in the proximal femur in TR (p < 0.05). No BMD or hormonal differences were found in SW. CONCLUSION: Only the endurance training of CY and TR induced androgen deficiency without apparent alteration of BMD.

Maximal oxygen uptake and power of lower limbs during a competitive season in triathletes.

BACKGROUND: In order to study the effect of a competitive triathlon season on maximal oxygen uptake (VO2max), aerobic power (AeP) and anaerobic performance (AnP) of the lower limbs, eight triathletes performed exercise tests after: (1) a pre-competition period (Pre-COMP) (2) a competitive period (COMP), and (3) a low (volume and intensity) training period (Post-COMP). The tests were a vertical jump-and-reach test and an incremental exercise test on a cycle ergometer. Ventilatory data were collected every minute during the incremental test with an automated breath-by-breath system and the heart-rate was monitored using a telemetric system. RESULTS: No changes in VO2max were observed, whereas AeP decreased after Post-COMP compared to Pre-COMP and COMP and AnP decreased during COMP compared to Pre-COMP and Post-COMP. In addition, second ventilatory threshold (VT2) and power output at first ventilatory threshold (VT1) and VT2 decreased after Post-COMP. CONCLUSION: This study showed that six weeks of low volume and intensity of training is too long a period to preserve adaptations to training, although a stable maximal oxygen uptake throughout the triathlon season was observed. Moreover, the AnP decrease during COMP was probably in relation with the repetitive nature of the training mode and/or triathlon competitions.

The effects of intensive training on insulin-like growth factor I (IGF-I) and IGF binding proteins 1 and 3 in competitive cyclists: relationships with glucose disposal.

The aim of the present study was to determine whether 4 months of intensified training would result in modified plasma insulin-like growth factor I (IGF-I), insulin-like growth factor binding protein 1 (IGFBP-1) or IGFBP-3 in eight competitive cyclists and eight sedentary individuals and to define the relationships of these factors with glucose disposal. Insulin sensitivity and glucose effectiveness--that is, the fractional disappearance of glucose independent of any change in insulinaemia--were measured with the minimal model (mathematical analysis of frequently sampled intravenous glucose tolerance test). Both glucose effectiveness and insulin sensitivity were higher in the cyclists than in the sedentary individuals, but did not increase further with training. IGF-I was higher in the cyclists than in the sedentary group only after raining (P < 0.05). Plasma IGFBP-1 and IGFBP-3 increased after training (38 and 20%, respectively; P < 0.05) in the cyclists and were higher than in the sedentary individuals (P < 0.05). IGF-I was negatively correlated with insulin sensitivity before and after training (r = -0.66 and -0.67, respectively; P < 0.05) and IGFBP-1 was negatively correlated with glucose effectiveness before andafter training (r = -0.68 and -0.77, respectively; P < 0.05). Our results show that strenuous endurance training improves the somatotrope axis (growth hormone-IGF) and that IGFBP-1 may be involved in glucose homeostasis, possibly by limiting the exercise-induced increase in glucose disposal, in competitive cyclists.

Carbohydrate dependence during hard-intensity exercise in trained cyclists in the competitive season: importance of training status.

To test the hypothesis that intensive endurance training increases CHO utilisation during hard-intensity exercise, seven competitive road cyclists (Cy) performed three 50-min steady-state exercise tests on a cycle ergometer above their ventilatory threshold (+ 15 %) over the course of a cycling season (January [ET1], May [ET2] and September [ET3]). We compared the data with the baseline values of seven sedentary controls (Sed). CHO oxidation in Cy was higher in ET2 and ET3 than in ET1 (p < 0.05), was lower in ET3 than in ET2 (p < 0.05) and was higher in Cy than in Sed only in ET2 (p < 0.05). Lactate kinematics were lower in Cy than in Sed in all conditions (p < 0.05), but in Cy they were lower in ET2 than in ET1 and higher in ET3 than in ET2 (p < 0.05). Race performance was impaired and the overtraining score was increased at ET3 in comparison with ET2 (p < 0.05). We conclude that competitive cyclists increase CHO oxidation during hard-intensity exercise over the course of a season, but show a decline by the end of the season in association with the appearance of an overtraining state. Thus, well-trained cyclists develop a CHO dependence, which is modified with training status.

Insulin and non-insulin-dependent glucose disposal in middle-aged and young athletes versus sedentary men.

The purpose of this study was to delineate the respective roles of aging and endurance training on glucose disposal. Thirty-two subjects (16 middle-aged men: 8 cyclists [MAcy], and 8 sedentary men [MAsed] and 16 young men: 8 cyclists [Ycy] and 8 sedentary men [Ysed]) were compared in this study. After overnight fasting, glucose was administered intravenously (0.5 g. kg(-1), 30% solution) and insulin-glucose interactions were assessed by measuring indices of insulin sensitivity (SI) and glucose effectiveness (Sg) using Bergman's minimal model. Sg includes basal insulin effectiveness (BIE) and glucose effectiveness at zero insulin (GEZI). Endurance training improved SI and Sg in all subjects, regardless of age (P <.05), but an increase in GEZI was found only in young men (P <.05). An effect of aging was found in sedentary subjects, who exhibited a lower SI (P <.05) when older. However, this effect disappeared with training, in which SI was nearly identical in young and middle-aged subjects. There was a correlation between SI and &Vdot;omicron(2max) in middle-aged men (r =.76, P <.01). These data suggest that the higher glucose uptake in endurance-trained male cyclists was mostly attributable to an increase in non-insulin-dependent glucose uptake in the young men and to an increase in its insulin-dependent component in the middle-aged men.

The effects of exercise training intensification on glucose disposal in elite cyclists.

To assess the effect of training on glucose disposal, we performed a longitudinal study of 11 elite cyclists before and after 4 months of intensive training compared to 11 sedentary subjects. Insulin sensitivity (SI) and glucose effectiveness (Sg) were measured using Bergman's minimal model. Sg includes basal insulin effectiveness (BIE) and a parameter termed glucose effectiveness at zero insulin (GEZI). After overnight fasting glucose was administered intravenously (0.5 g x kg(-1), 30% solution given over 3 min), and insulin (0.02 U x kg(-1), 1 -2U) was injected immediately after 19 min. Sg, SI and BIE, were significantly higher in elite cyclists both before and after training than in sedentary subjects (P < 0.01). However, the non-insulin-dependent component of Sg (GEZI) was higher only after the intensive training in the cyclists (3.31 +/- 0.67% x min(-1)) than in sedentary subjects (1.7 +/- 0.2% x min(-1), P < 0.02). We conclude that insulin sensitivity (SI) and glucose effectiveness (Sg) are higher in elite cyclists than in sedentary subjects and that these high and almost optimal values are not further improved by additional training. However, the improvement in GEZI, as reflected by the difference between post-training GEZI and sedentary control values, raises the possibility of an increase of the non-insulin-mediated mobilization of glucose transporters.

Acylation of human insulin with palmitic acid extends the time action of human insulin in diabetic dogs.

To test whether the binding of insulin to an endogenous serum protein can be used to extend the time action of insulin, human insulin was acylated at the epsilon-amino group of Lys(B29) with palmitic acid to promote binding to serum albumin. Size-exclusion chromatography was used to demonstrate specific binding of the resulting analog, [N(epsilon)-palmitoyl Lys(B29)] human insulin, to serum albumin in vitro, and the time action and activity of the analog were determined in vivo using overnight-fasted, insulin-withdrawn diabetic dogs. In the diabetic animal model, the duration of action of [N(epsilon)-palmitoyl Lys(B29)] human insulin administered intravenously was nearly twice that of unmodified human insulin, and the plasma half-life was nearly sevenfold that of the unmodified protein. Administered subcutaneously, [N(epsilon)-palmitoyl Lys(B29)] human insulin had a longer duration of action; a flatter more basal plasma insulin profile; and a lower intersubject variability of response than the intermediate-acting insulin suspension Humulin L (Lilly, Indianapolis, IN). These studies support the concept that modification of insulin to promote binding to an existing serum protein can be used to extend the time action of human insulin. In addition, the time action, pattern, and decreased variability of response to [N(epsilon)-palmitoyl Lys(B29)] human insulin support the development and further testing of this soluble insulin analog as a basal insulin to increase the safety of intensive insulin therapy.

Regulation of expression of ob mRNA and protein by glucocorticoids and cAMP.

Regulation of obese gene (ob) expression in ob/ob and db/db mice and in cultured rat adipocytes was examined. It has been demonstrated that exogenous human OB protein (leptin) treatment reduces food intake and weight gain, as well as insulin, glucose, and corticosterone levels in ob/ob mice. In the present report we show that leptin treatment down-regulates endogenous adipose ob mRNA. However, treatment of isolated rat adipocytes with 100 ng/ml human or murine leptin had no direct effect on expression of endogenous ob mRNA, suggesting that leptin may be able to down-regulate its own expression by an indirect, non-autocrine mechanism. Glucocorticoids increased both ob mRNA levels and secreted leptin levels in vitro. Conversely, agents that increase intracellular cAMP, such as beta-adrenergic agonists or Bt2cAMP itself, decreased ob mRNA expression and leptin secretion. Therefore, increased glucocorticoid levels and decreased sympathetic neural activity may contribute to the elevated ob mRNA expression observed in genetically obese, hyperglucocorticoid rodents. Furthermore, leptin might regulate its own expression through a feedback mechanism involving the hypothalamic pituitary axis.

Serine 228 is essential for catalytic activities of 85-kDa cytosolic phospholipase A2.

The Ca(2+)-sensitive cytosolic phospholipase A2 (cPLA2) displays both a phospholipase A2 and a lysophospholipase activity. Numerous hydrolases, including lipases, catalyze the hydrolysis of ester bonds by means of an active site triad of amino acids that includes a serine or a cysteine residue. We have examined whether human cPLA2 belongs to this class of enzymes by using site-directed mutagenesis. Although chemical inactivation of cPLA2 by the sulfhydryl reagent N-ethylmaleimide made it appear that cysteine(s) may be essential for catalysis, all 9 cysteine residues of cPLA2 proved dispensable, allowing near-normal enzyme activity when substituted by alanine. We noted that cPLA2 contains a 110-amino-acid region with sequence homology to phospholipase B (PLB) from Penicillium notatum. Interestingly, one of the conserved serines of cPLA2, Ser-228, within this domain aligns with the lipase consensus sequence Gly-X(Leu)-Ser(137)-X(Gly)-Gly of PLB. Replacement of Ser-228 by alanine (or threonine or cysteine) yielded catalytically inactive cPLA2, even though the native conformation was maintained as determined by CD spectroscopy. Likewise, the lysophospholipase activity was completely abolished by the Ser-228 mutations. In contrast, substitution by alanine of three different serines of cPLA2 (Ser-195, Ser-215, or Ser-577) that also aligned with the PLB sequence allowed for substantial enzymatic activity of cPLA2. Our findings provide evidence that 1) Ser-228 participates in the catalytic mechanism of cPLA2 and that 2) both the phospholipase A2 and the lysophospholipase activities of cPLA2 are catalyzed by the same active site residue(s).

Calcium-sensitive cytosolic phospholipase A2 (cPLA2) is expressed in human brain astrocytes.

Calcium-sensitive cytosolic phospholipase A2 (cPLA2) is responsible for receptor-mediated liberation of arachidonic acid, and thus plays an important role in the initiation of the inflammatory lipid-mediator cascade generating eicosanoids and platelet-activating factor. In this study we have investigated the cellular distribution of cPLA2 in brain using a monoclonal antibody raised against cPLA2 to immunostain tissue sections of human cerebral cortex. We have localized cPLA2 in astrocytes of the gray matter. Colocalization with glial fibrillary acidic protein (GFAP) confirmed that cPLA2 is associated predominantly with protoplasmic astrocytes. Astrocytes of the white matter, on the other hand, were not immunoreactive. In experiments using different human astrocytoma cell lines we found that cPLA2 can be immunochemically localized in UC-11 MG cells, but cannot be detected in U-373 MG cells. This finding is consistent with the observation that cPLA2 mRNA as well as cPLA2 enzymatic activity can be readily measured in UC-11 MG astrocytoma cells, yet cannot be detected in U-373 MG cells. Our data suggest that the astrocyte is a primary source of cPLA2 in the brain and provide further evidence for the importance of this cell type in inflammatory processes in the brain.

Thrombin-induced phosphorylation and activation of Ca(2+)-sensitive cytosolic phospholipase A2 in human platelets.

Receptor-mediated activation of human platelets by thrombin initiates a series of rapid biochemical events that include activation of phospholipase A2 to liberate arachidonic acid for further conversion to thromboxane A2. The identity of the phospholipase A2 involved has not been clear. Here we show by immunochemical analysis that human platelets contain significant amounts (60 ng/10(9) platelets) of the recently identified Ca(2+)-sensitive cytosolic phospholipase A2 (cPLA2). Metabolic labeling of human platelets with 33Pi revealed that the extent of phosphorylation of cPLA2 was greatly increased after thrombin treatment. Upon stimulation of platelets with thrombin, cPLA2 exhibits enhanced catalytic activity, as well as a change in its electrophoretic and chromatographic properties compared with cPLA2 in resting platelets. These alterations of cPLA2 are reversed by treatment with phosphatase, demonstrating that they are the consequence of thrombin-stimulated phosphorylation. Thrombin-induced phosphorylation and activation of cPLA2 is rapid (half-maximal by 1 min at 1 unit/10(9) platelets) and dose-dependent. Agonist-induced phosphorylation of cPLA2 is more sensitive to thrombin than the generation of thromboxane A2, suggesting that it may be an early event in the sequence of steps leading to the mobilization and further metabolism of arachidonic acid. By comparing the functional properties of cPLA2 from control versus thrombin-stimulated platelets, we found that while activated cPLA2 exhibits the same Ca2+ requirement and apparent substrate affinity (Km), its catalytic activity (Vmax) is increased compared with control cPLA2. We conclude that 1) cPLA2 is likely to play an important role in agonist-induced mobilization of arachidonic acid and 2) thrombin elicits rapid and full activation of cPLA2 not only by promoting a rise in cytosolic free Ca2+ but also by inducing phosphorylation of cPLA2 thereby improving its catalytic activity.

Model peptides to study the effects of P2 and P3 substitutions in statine-containing HIV proteinase inhibitors.

Through a series of synthetic model peptides, we have examined the structural requirements of the P2 and P3 residues in statine-based HIV protease (PR) inhibitors. Results agree with the general observations that, the more bulky the P3 aromatic hydrophobic side chain, the more potent is the inhibitor. At P2, an isopropyl side chain is critical in maintaining potency. Three-dimensional modeling demonstrates that the steric bulk of a leucyl residue or the unfavorable energy transfer, from water to enzyme, for a basic amino acid residue at P2 markedly compromises activity. A naphthylalaninyl-valyl P3-P2 substituted analogue inhibits PR with an IC50 value of 6 nM, and was also effective as an antiviral agent.