Submitted to: Comparative Biochemistry and Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 19, 2006
Publication Date: May 9, 2006
Citation: Sweazea, K.L., McMurtry, J.P., Braun, E.J. 2006. Inhibition of fatty acid lipolysis does not affect insulin sensitivity to glucose uptake in the mourning dove. Comparative Biochemistry and Physiology, Part B. 144:387-394.
Interpretive Summary: Domestic poultry and other avian species utilize carbohydrate depots in muscle and liver as energy reserves. Free fatty acid metabolism supplies much of the energy utilized by birds. Despite very elevated circulating levels of free fatty acids, birds exhibit very high blood glucose levels, and circulating glucose is resistant to the glucose lowering effects of insulin, the insulin-resistance dogma characteristic in avian species. This is opposite to the situation in mammals. Insulin resistance (diabetes) is a serious health issue in humans. One of the theories for the etiology of insulin-resistance in humans is that it stems from elevated plasma free fatty acids. The bird serves as an excellent model to evaluate the relationship between free fatty acid metabolism and insulin-resistance. In this study birds were treated with a compound that inhibits lipolysis, thereby lowering circulating free fatty acid levels. Glucose uptake in the presence or absence of added insulin by various tissues was monitored. The results of this study demonstrated that free fatty acids alone may not be contributing to insulin resistance in birds. The results of this study will be of interest to other scientists.
Birds have much higher plasma glucose and fatty acid levels in comparison to mammals. In addition, they are resistant to the blood glucose lowering effects of insulin. Recent studies have demonstrated that decreasing fatty acid utilization alleviates insulin resistance in mammals, thereby decreasing glucose levels. This has yet to be examined in birds. In the present study, the levels of glucose and ketone bodies, an indicator of fatty acid utilization, were measured after the administration of insulin, acipimox (an agent used to decrease fatty acid utilization), or insulin and acipimox. Insulin significantly decreased whole blood glucose levels (19%), but had no effect on ketone body concentrations. In contrast, acipimox decreased whole blood ketone body levels as expected, but had no effect on whole blood glucose. In addition to changes in blood composition, levels of glucose uptake by various tissues were measured after the individual and combined administration of insulin and acipimox. Under basal conditions, the uptake of glucose appeared to be greatest in the kidney followed by the brain, skeletal muscle, heart and fat. Acipimox significantly decreased glucose uptake by brain and there was a tendency for acipimox to decrease glucose uptake by the kidney, pectoralis and latissimus dorsi, but these were not significant. No effect of acipimox was observed in other tissues. In summary, the acute inhibition of fatty acid lipolysis had no effect on glucose uptake in the presence or absence of insulin. This suggests that free fatty acids alone may not be contributing to insulin resistance in birds.