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

Avian Adjustments to Cold

“…The data accumulated thus far also leave in doubt the factors that cause thermoregulatory failure and subsequent unregulated hypothermia in naturally acclimatized finches. Earlier studies revealed an overall correlation between depletion of muscle glycogen and hypothermia (Marsh and Dawson 1982;Marsh et al 1984). However, more extensive sampling has invalidated this apparent correlation ).…”

“…But this deduction must remain tentative because it is based on static measures of plasma free fatty acid concentrations. Liver glycogen reserves are small and do not provide significant mobilizable substrates (Carey et al 1978;Marsh and Dawson 1982;Marsh et al 1984). The gluconeogenic capacity ofthe liver is possibly augmented in winter birds (Marsh and Dawson 1982;Marsh et al 1984), but the extent and role of this process has yet to be delineated.…”

“…This lack of depletion of fat reserves could be due to insufficient mobilization from adipose tissue, or insufficient uptake and oxidation of plasma fatty acids. Concentrations of plasma free fatty acids are similar in birds subjected to cold stress in summer and winter (Marsh and Dawson 1982;Marsh et al 1984), suggesting that mobilization is not limiting. But this deduction must remain tentative because it is based on static measures of plasma free fatty acid concentrations.…”

“…Respiratory exchange ratios of fasted birds in the cold are approximately 0.70-0.75, also indicating the quantitative importance of fat oxidation (Wallgren 1954;Dawson and Tordoff 1959;West 1972b;Mugaas and Templeton 1970). Additionally, rates of glucose turnover in cold-exposed birds indicate that this substrate can only account for a small percentage of the energy required (Riesenfeld et al 1979;Marsh and Dawson 1982;Marsh et al 1984). In replete birds the presence of dietary carbohydrate might alter this primary reliance on fatty acids, but data on this point are lacking.…”

“…In mammals, including cold-acclimated rats, and in the domestic fowl, glucose turnover increases with increasing metabolic rate during cold exposure (e.g., Depocas 1962;Minaire et al 1973;Riesenfeld et al 1979). Compared to American goldfinches, house finches show a rather small change in thermogenic endurance between summer and winter, and interestingly do not show significant seasonal changes in glucose turnover or glycogen depletion during cold exposure (Marsh et al 1984).…”