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

Abstract

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.