Substrate Metabolism under Cold Stress in Seasonally Acclimatized Dark-Eyed Juncos

Abstract

Seasonally acclimatized dark-eyed juncos were exposed to one of three temperature regimes: 30° C (thermoneutrality), - 12° C (moderate cold), or 2° C in a 20.9% oxygen/79.1% helium gas mixture (severe cold), for 2 h or until they became hypothermic. Plasma glucose and free fatty acids (FFA), pectoralis muscle and liver glycogen, and pectoralis lactate were measured after exposure. Pectoralis muscle mass increased by 28% in winter. This increased muscle mass may assist in improving cold tolerance by increasing capacity for shivering thermogenesis in winter birds. Plasma glucose was signiicantly reduced and plasma FFA were significantly elevated under severe cold in winter No differences in plasma metabolites with temperature were detected in summer. Pectoralis muscle glycogen decreased with increasing severity of cold stress at both seasons, but winter levels were significantly greater than summer levels under severe cold. No temperature-induced differences in liver glycogen were apparent, but pooled winter values significantly exceeded summer values. Pectoralis lactate was significantly lower under severe cold at both seasons. In addition, mean lactate values for birds becoming hypothermic in both summer (n = 7) and winter (n = 1) were markedly lower than for normothermic birds at the same test temperatures. These data are consistent with a pattern of augmentedpreferential use of FFA to support shivering, coupled with sparing of muscle glycogen, in winter birds relative to summer birds. Furthermore, the decrement of pectoralis lactate with hypothermia may suggest a link between a reduced ability to mobilize muscle glycogen and the onset of hypothermia.