Effect of exercise on lipid levels in the pigeon

Fuel use and metabolic response to endurance exercise: a wind tunnel study of a long-distance migrant shorebird

This study examines fuel use and metabolism in a group of long-distance migrating birds, red knots Calidris canutus (Scolopacidae), flying under controlled conditions in a wind tunnel for up to 10 h. Data are compared with values for resting birds fasting for the same time. Plasma levels of free fatty acids, glycerol and uric acid were elevated during flight, irrespective of flight duration (1-10 h). Triglyceride levels, the estimated concentration of very-low-density lipoproteins (VLDLs) and β-hydroxybutyrate levels were lower during flight, while glucose levels did not change. In flying birds, plasma levels of uric acid and lipid catabolites were positively correlated with the residual variation in body mass loss, and lipid catabolites with energy expenditure (as measured using the doubly labelled water method), after removing the effect of initial body mass. The plasma metabolite levels indicate: (i) that the rates of catabolism of lipids from adipose tissue and of protein are higher during flight; (ii) that low ketone body concentrations probably facilitate fatty acid release from adipose tissue; (iii) that low triglyceride and VLDL levels do not indicate the use of an additional pathway of fatty acid delivery, as found in small birds; and(iv) that the relationships between energy expenditure, body mass loss and metabolic pattern suggest that a higher individual energy expenditure entails a higher rate of catabolism of both lipids and protein and not a shift in fuel substrate.

The hormonal regulation of premigratory fat deposition and winter fattening in red-winged blackbirds

Glucagon is a highly potent lipolytic agent in birds and a candidate for regulating premigratory and winter fattening. The seasonal role of glucagon in fat metabolism was determined by monitoring plasma glucagon, fatty acids and glucose in two groups of red-winged blackbirds; one group exposed to outside environmental conditions (September to May) and a second group maintained at summer conditions with respect to day length and temperature. The results of this investigation demonstrate significantly lower plasma glucagon (480.1 pg/ml) in birds exposed to outdoor conditions than in birds maintained at summer conditions (734.6 pg/ml) during September/October. The data are consistent with the view that low plasma glucagon in outdoor birds ensures the preservation of fat stores for autumn migration. Lower plasma free fatty acid (FFA) levels (0.35 mEq/l) in outdoor birds (vs. 0.54 mEq/l in indoor birds) in autumn may reflect the rapid transport of FFA to adipose tissue for lipogenesis resulting in a steady increase in body weight from September to January. The sharp decline in plasma FFA in indoor birds from 0.54 mEq/l in September/October to 0.28 mEq/l in January/February may be attributed to a marked decrease in food consumption, rather than a dramatic change in the rate of lipid transport from blood to muscle or adipose tissue. Glucagon injections caused a 600% increase in plasma FFA and a more modest (50%) increase in plasma glucose. This confirms the major role of glucagon in fat mobilization. Its lipolytic effects, however, can vary seasonally by way of down regulation of glucagon receptors. Down regulation of glucagon receptors in adipose tissue and the associated reduced sensitivity of adipocytes to the lipolytic action of glucagon would account for the progressive increase in weight of the birds throughout November/December when plasma glucagon levels were significantly higher (578.9 pg/ml) in outdoor birds as compared to indoor birds (436.9 pg/ml). Lower plasma glucagon levels (405.5 pg/ml) in outdoor birds in January/February (vs. 638.6 pg/ml in indoor birds) may reflect the same physiological conditions prevailing in September/October favoring the preservation of fat stores.