Temporal analysis of mechanisms leading to stimulation of glucose uptake in skeletal muscle cells by an adipokine mixture derived from primary rat adipocytes.

OBJECTIVE: The direct effects of adipokines on skeletal muscle metabolism have been well established. As the combinatorial effects of adipokine mixtures are likely to be of more physiological relevance, we used a coculture system of primary rat adipocytes and L6 skeletal muscle cells to examine the effects of adiponectin derived from primary rat adipocytes on rat skeletal muscle cells. RESULTS: We showed that coculture with adipocytes stimulated glucose uptake in L6 cells within 30 min and this correlated with an increase of glucose transporter isoform 4 (GLUT4) localization to the plasma membrane. These effects were dependent on the reorganization of the actin cytoskeleton, demonstrated by rhodamine-labeled phalloidin immunofluorescence, as cytochalasin D attenuated the glucose uptake induced by adipocyte-conditioned media. Temporal analysis revealed that enhanced glucose uptake was maintained after 24 h of coculture, and this was attributed to an increase in both GLUT1 expression and the cell surface content of GLUT4. We established a role for adiponectin in mediating these effects as antibody-mediated neutralization attenuated the metabolic effects of adipocyte-conditioned media. Furthermore, compound C blocked these effects, suggesting an important role for AMPK. Importantly, when we compared the effects of full-length recombinant adiponectin with adipocyte-conditioned media, we confirmed that recombinant adiponectin was unable to stimulate glucose uptake in L6 cells despite having an important role in adipocyte-conditioned media. CONCLUSIONS: Our results demonstrate the importance of examining the effects of adipokines in the context of physiologically relevant mixtures to accurately determine their metabolic effects on skeletal muscle.

Regulation of glucose transporter 4 traffic by energy deprivation from mitochondrial compromise.

Skeletal muscle is the major store and consumer of fatty acids and glucose. Glucose enters muscle through glucose transporter 4 (GLUT4). Upon insufficient oxygen availability or energy compromise, aerobic metabolism of glucose and fatty aids cannot proceed, and muscle cells rely on anaerobic metabolism of glucose to restore cellular energy status. An increase in glucose uptake into muscle is a key response to stimuli requiring rapid energy supply. This chapter analyses the mechanisms of the adaptive regulation of glucose transport that rescue muscle cells from mitochondrial uncoupling. Under these conditions, the initial drop in ATP recovers rapidly, through a compensatory increase in glucose uptake. This adaptive response involves AMPK activation by the initial ATP drop, which elevates cell surface GLUT4 and glucose uptake. The gain in surface GLUT4 involves different signals and routes of intracellular traffic compared with those engaged by insulin. The hormone increases GLUT4 exocytosis through phosphatidylinositol 3-kinase and Akt, whereas energy stress retards GLUT4 endocytosis through AMPK and calcium inputs. Given that energy stress is a component of muscle contraction, and that contraction activates AMPK and raises cytosolic calcium, we hypothesize that the increase in glucose uptake during contraction may also involve a reduction in GLUT4 endocytosis.

Plasma leptin in moderately obese men: independent effects of weight loss and aerobic exercise.

The independent effects of weight loss and exercise on plasma leptin and total (AT), subcutaneous (SAT), and visceral (VAT) adipose tissue were investigated in 52 obese men. Subjects were randomly assigned to four 12-wk protocols: 1) control (C, n = 8), 2) diet-induced weight loss (DWL, n = 14), 3) exercise-induced weight loss (EWL, n = 14), and 4) exercise with weight maintenance (EWS, n = 16). Plasma leptin was unchanged in C (from 7.8 +/- 1.3 to 7.7 +/- 1.0 ng/ml). Equivalent weight loss (7.5 kg) decreased leptin significantly but similarly (DWL, from 8.5 +/- 1.0 to 4.8 +/- 0.6 ng/ml; EWL, from 10.1 +/- 1.0 to 5.0 +/- 0.6 ng/ml). Exercise in the absence of weight loss did not alter leptin levels (from 10.1 +/- 1. 3 to 9.2 +/- 1.2 ng/ml). Changes in leptin correlated with changes in AT and SAT (both P < 0.05) but not in VAT. We conclude that reduction in adipose tissue after weight loss results in a collateral decrease in circulating leptin, and exercise, independent of its effects on weight loss, has no profound influence on leptin secretion.

Plasma leptin in female athletes: relationship with body fat, reproductive, nutritional, and endocrine factors.

The relationship of leptin to thyroid and sex hormones, insulin, energy intake, exercise energy expenditure, and reproductive function was assessed in 39 female athletes. They comprised elite athletes who were either amenorrheic (EAA; n = 5) or cyclic (ECA; n = 8) and recreationally active women who were either cyclic (RCA; n = 13) or taking oral contraceptives (ROC; n = 13). Leptin was significantly lower in EAA (1.7 +/- 0.2 ng/ml) than in ECA (2.9 +/- 0.3 ng/ml), RCA (5.8 +/- 0.9 ng/ml), and ROC (7.4 +/- 1.3 ng/ml). Hypoleptinemia in EAA was paralleled by reductions (P < 0.05) in caloric intake, insulin, estradiol, and thyroid hormones. Leptin increased by 40-46% (P < 0.05) in the luteal phase of the menstrual cycle in RCA and ECA. Plasma leptin was similar in the placebo and active pill phases in ROC despite a significant increase in ethinylestradiol. Leptin correlated (P < 0.05) with triiodothyronine and insulin but not with estrogen, energy intake, or exercise energy expenditure. These data suggest that in female athletes 1) leptin may be a metabolic signal that provides a link between adipose tissue, energy availability, and the reproductive axis and 2) sex hormones do not directly regulate leptin secretion.

Leptin and reproduction: is it a critical link between adipose tissue, nutrition, and reproduction?

Exercise-associated reproductive disorders are frequently reported among recreationally active and elite female athletes. Although an association between exercise and menstrual disorders has been established, the mechanism by which exercise disrupts reproductive function remains unknown. Recent findings suggest that low energy availability rather than inadequate body fatness or exercise stress is likely the mechanism by which exercise impinges negatively on the hypothalamic-pituitary-ovarian axis in female athletes. The peripheral signal that informs the neural network of energy availability remains unknown. The identification of the adipocyte-derived ob gene product, leptin, and subsequent findings of its association with reproduction in both rodents and humans, led to speculations that it may be involved in the interactions between nutrition and reproduction. This review article focuses on leptin's role in modulating reproduction, and in particular, as a peripheral signal of nutritional status that integrates adipose tissue, nutrition, and reproduction in female athletes.