Meeting Abstract
Previous work on maximal thermogenic capacity (Msum) in wild black-capped chickadees has revealed that phenotypic adjustments seem slow and begin to take place in early fall, well before the peak of winter cold. However, when mean minimal Ta reaches -10°C, the birds’ phenotype provide enough reserve capacity in cold endurance to buffer days with Ta of -20°C or below. Birds and mammals have muscle fiber diameters that typically range from 10-100 µm. Minimal fiber diameters are limited by diffusion constraints for O2 and ATP while maximal fiber diameters seem to be dictated by the cost of the Na+-K+-ATPase on the sarcolemmal membrane. Although muscles mass is flexible in size, often being larger in winter; whether cold acclimated birds change muscle ultrastructure in response to sudden environmental challenges is unknown. Here, we investigated whether cold (-5°C) acclimated chickadees challenged with an experimental decline in temperature alter their muscle ultrastructure at the cell level in the first 3 h after a 15°C temperature drop. We compared muscle ultrastructure in birds that experienced the temperature decline (treatment ; -5°C to -20°C) to that of individuals remaining at -5°C (control). We found that treatment birds had a significantly higher total capillary density (0.005 ± 0.0003 capillaries/um2) compared with control birds (0.004 ± 0.0002 capillaries/um2). Treatment birds also had more nuclei per fiber (113.84 ± 3.72 nuclei/fiber) compared to controls (95.23 ± 2.97 nuclei/fiber) and had significantly smaller myonuclear domain (7432.33 ± 421.19 µm3) compared with control birds (8853.65 ± 369.01 µm3). Our data, therefore, suggest that cold acclimated birds can adjust their muscle cell phenotype within hours during cold spells.
