Pancreatic beta-cells are energy-sensing cells because they sense the ambient plasma-glucose concentration and secrete insulin to signal other tissues to take up glucose. An increase in plasma-glucose levels leads to an increase in the fluxes through the glycolytic and mitochondrial metabolism pathways and an increase in the cellular ATP-to-ADP ratio. This causes a series of events to occur, in which ATP-sensitive potassium channels close, voltage-gated calcium channels open, and insulin secretion is triggered. Environmental factors, such as low physical activity and hypercaloric lipid-rich diets, can lead to decreased glucose-induced insulin secretion in the long term, and there is evidence to suggest that ROS and the activity of uncoupling proteins play a key role in determining beta-cell dysfunction under these conditions .? ? Model simulations show that, under low plasma-glucose conditions, beta-cells in the presence of fatty acids have a higher ATP-to-ADP ratio, and therefore higher basal insulin secretion, than cells in the absence of fatty acids. Increasing glucose levels increases ROS levels and the likelihood of tissue damage. This increase is more pronounced in the presence of fatty acids, assuming total UCP concentrations remain fixed, as nearly all the available UCP are activated. It has been shown, however, that in response to this challenge, fatty acid-treated beta-cells increase their UCP content to about two and a half times that of untreated beta-cells. Under such conditions, the model shows increased ROS protection, but inhibited glucose-stimulated insulin secretion, as indicated by a decreased response in the ATP-to-ADP ratio to increased glucose levels. These predictions are consistent with experimental observations, and may have important implications in beta-cell function and insulin secretion in response to diet and obesity.? ? Defense against oxidative damage is a delicate balance. Decreasing UCP concentration or activity can improve insulin secretion by preventing energy diversion from ATP production. However, accompanying this benefit is an increase in ROS levels. Therefore, therapeutic strategies that focus specifically on deactivating uncoupling proteins to treat impaired glucose-stimulated insulin secretion would need to be accompanied by an antioxidant treatment or a strategy that prevents ROS overproduction. Debates are ongoing about the effectiveness of antioxidant treatments citeRB05. An alternative treatment strategy is to increase the cellular mitochondrial-density (with exercise, for example). By dividing our model parameters p_1 , p_2 , p_4 , p_6 , and p_31 by a mitochondrial-density parameter, p_0 , we can investigate the effects of changes in the mitochondrial content of a cell. p_0 = 1 represents the typical mitochondrial content, p_0 > 1 represents a proportional increase in mitochondrial content, and p_0 < 1 represents a proportional decrease. Decreasing the UCP concentration while increasing the mitochondrial density can increase both the ATP-to-ADP ratio and the amount of mitochondrial ATP per ROS.? ? There is an upper limit, dependent on the availability of nutrients, to how far the mitochondrial density of a cell can be increased (simulations not shown). The nutrients being used to maintain DeltaPsi must be shared among the mitochondria. As nutrient levels decrease or mitochondrial content increases, it becomes more difficult for the mitochondria to maintain DeltaPsi . The model predicts that beyond a certain value, the mitochondria would depolarize and ATP production would cease. At this point, it is expected that some other compensating mechanism, such as the reversal of the F _1 F _0 -ATP synthase to hydrolyze ATP and pump protons out of the matrix, would work to maintain DeltaPsi , or mitochondria would be removed through autophagy. However, these mechanisms are not included in our model. Depolarization can also occur as the result of inactivation of scavenging enzymes, in which case inadequate ROS removal promotes continual activation of uncoupling proteins, consistent with experimental observations that total and partial inactivation of scavenging enzymes (such as MnSOD) result in neonatal lethality and accelerated aging in mice, respectively.? ? To investigate the effects of diets on mitochondrial responses, experimentally measured plasma-glucose and fatty acid profiles can be applied as inputs to our model. We demonstrate this using the glucose and fatty acid plasma-concentration profiles from eight overweight subjects (BMI = 30 pm 1 kg/m 2 ; 5 men, 3 women), as measured by Knuth it et al.. Nine subjects were included in the original study; however, we exclude one subject who showed significant weight gain during the course of the study. On three separate occasions, each of the eight subjects consumed three control meals ( sim 800 kcals; 50% carbohydrate, 35% fat, and 15% protein), three low-fat meals ( sim 530 kcals; 75% CHO, 2% fat, 23% protein), or three low-fat meals with lipid infusion (same as low-fat meal but with the missing fat sim 30 g, provided via an intravenous lipid infusion), over a fifteen-hour period after a fourteen-hour overnight fast. Meals were given at times zero, five, and ten hours from the start of each trial and blood samples were collected throughout each trial.? ? Postulating a it standard beta-cell is placed in each subject and exposed to the individual glucose and fatty acid profiles, we use the model to simulate the ATP-to-ADP ratio and ROS response profiles for that beta-cell under the different diet conditions. The resulting profiles are shown in Figures refcontrol-refinfusion along with the measured plasma-insulin concentrations. For visualization purposes, the profiles in Figures refcontrol-refinfusion have all been normalized.footnoteEach variable, y , was normalized to yield y_normalized = left(y-aright)/b , where a=10 and b=8 for the insulin profiles, a=0.2557 and b=0.0256 for the ATP-to-ADP ratio profiles, and a=0.0205 and b=0.005 for the ROS profiles. Although the ROS profiles closely follow the ATP-to-ADP ratio profiles, there are instances where the two curves separate. This separation is most evident after meals in subjects one, five, and seven, who, we note, are the most insulin resistant subjects (as indicated by low ISIs).? ? In comparing the total mitochondrial ROS-per-ATP (calculated as the area under the curve during the first five hours of the trial) from our simulations to the individual ISIs, we find a strong (i.e., statistically significant) negative correlation between these quantities for each diet. This indicates that insulin-resistant subjects produce more ROS per ATP than insulin-sensitive subjects, and that these quantities can be indicative of each other. Such consequences may have important long-term implications for physiology as well as tissue-specific cellular function, just as fat accumulation is a long-term effect that gradually overwhelms homeostatic mechanisms. The model does not show a significant difference between the means of total ROS-per-ATP for control and low-fat diets (notwithstanding the fact that the low-fat result was approximately 8% less than that for control); however, it does show a significant increase of approximately 40% in the mean of total ROS-per-ATP for low-fat with lipid infusion compared to the control and low-fat diets (one-way ANOVA for correlated samples and Tukey HSD tests; p-value < 0.01).? ? The model we developed goes beyond the models upon which it was based by combining both glucose and fatty acid inputs and incorporating ROS production and the activity of scavenging enzymes and uncoupling proteins.
