The effect of cold on norepinephrine turnover in tissues of seasonally acclimatized redpolls (Carduelis flammea)

Summary

• 1. The response of the avian sympathetic nervous system during cold exposure was examined by injecting³H-norepinephrine and estimating norepinephrine (NE) turnover rate in skeletal muscle, heart, liver, and brain of seasonally acclimatized redpolls (a granivorous passerine) in interior Alaska.

• 2. Thermoneutral NE turnover rate (ng NE/g·h) was greater in winter (January) than in summer (July) for muscle (12 ng/g·h, +115%) and heart (65 ng/g·h, +62%). This may be correlated to higher resting metabolism of winter redpolls.

• 3. Acute cold exposure (24 h) necessary to increase metabolism 2 x caused increased NE turnover, compared to thermoneutral NE turnover, in muscle (10 ng/g·h, +95% in summer; 18 ng/g·h, +57% in winter) and heart (88 ng/g·h, +117% in summer; 108 ng/g·h, +65% in winter).

• 4. Endogenous NE concentration declined during cold exposure of summer redpolls by a mean of 22% in muscle, liver, and brain, but not in winter birds.

• 5. Thus, sympathetic nerve activity was stimulated by cold exposure in both seasonal groups, but winter acclimatization may involve an increased capacity to maintain steady state NE levels.

• 6. A peripheral NE injection (2 mg NE/kg body weight) in winter redpolls depressed metabolism by a mean of 31% from resting metabolism of 7.2 ml O₂/g·h. This may be due to the effect of NE on thermoregulatory centers in the brain.