Norepinephrine thermogenesis in seasonally acclimatized and cold acclimated red-backed voles in Alaska

Abstract

The calorigenic response (millilitres O₂ per gram per hour) to injected norepinephrine (NE) was compared as an index of nonshivering thermogenesis (NST) in the following groups of the Alaskan red-backed vole (Clethrionomys rutilus): (1) summer, (2) fall acclimatized, (3) winter acclimatized, (4) 20 °C acclimated and (5) 5 °C acclimated. The metabolic response was tested at thermoneutrality (25 °C) and during cold exposure (5 °C).Winter acclimatized voles showed a significantly greater metabolic response to NE than summer voles at both 25 °C and 5 °C. In summer or winter voles the total metabolic rate after NE (M_NE) was similar at 25 °C and 5 °C but the fraction of the total caused by exogenous NE was lower at 5 °C. Thus, thermogenesis during cold exposure and resulting from exogenous NE appear to be based on the same mechanism, and NE has thermoregulatory significance in these voles. The magnitude of the NE response in winter voles was comparable to the highest values reported for bats and exceeded levels reported for other adult small mammal species. Summer acclimatized voles and those acclimated to 20 °C in the laboratory were comparable in their response to NE but winter acclimatized voles were significantly more sensitive to NE than voles acclimated to 5 °C. The seasonal winter peak in M_NE coincided with peaks previously found for maximum metabolic capacity (M_max), maximum brown fat, and the period of coldest temperatures in December–January. The ratio of M_NE to M_max was similar throughout the year. The results suggest that small arctic–subarctic rodents have a greater capacity for NE stimulated NST than rodents from temperate latitudes probably because they are acclimatized to colder seasonal conditions.