Substrate concentrations and turnover of plasma glucose during cold exposure in seasonally acclimatized house finches,Carpodacus mexicanus

Abstract

Concentration ([Glc]) and turnover (Ro) of plasma glucose, concentration of free fatty acids in plasma ([FFA]), and concentration of glycogen in muscle and liver were measured in summer- and winter-acclimatized house finches (Carpodacus mexicanus) freshly captured in Boulder, Boulder County, Colorado. The birds were exposed acutely to one of three thermal regimes: (1) ‘thermoneutral’, 30°C in air; (2) ‘moderate cold’, −12°C in air; or (3) ‘severe cold’, 1°C in 79% He and 21% O2. Plasma glucose turnover in winter house finches increases directly with severity of cold stress, rising 67% between thermoneutral and severe cold conditions. Similar values of Ro were observed for cold stressed birds in both winter and summer. However, the turnover of plasma glucose at 30°C is elevated during the latter season. The amount of glycogen remaining in the muscle after 2 h of exposure to any of the three thermal regimes did not vary seasonally. Combining data for the summer and winter yielded the following values for glycogen (mean ±SE in mg/g fresh muscle mass): 7.84±0.70, 4.62±0.39, and 1.56±0.32 at 30°C, −12°C and 1°C in He−O2, respectively. Liver glycogen was relatively low (<17 mg/g fresh liver mass) under all test conditions. In winter birds exposed to −12°C, liver glycogen was significantly higher than in control individuals, suggesting a rate of gluconeogenesis exceeding glucose requirements. Plasma glucose concentrations and [FFA] did not vary in house finches between winter and summer, except for an elevated [FFA] in winter individuals at 30°C. Plasma [FFA] was relatively high (ca. 2 meq/l) under severe cold stress, but was not elevated during exposure of the birds to more moderate cold. We conclude that, with the possible exception of an increased rate of gluconeogenesis in winter house finches, carbohydrate metabolism during exposure to cold does not change between summer and winter. This is consistent with predictions based on the limited seasonal acclimatization observed in the cold resistance of house finches.