Flight muscle catabolism during overnight fasting in a passerine bird, Eremophila alpestris

Brood-rearing passerine birds often have sparse lipid reserves coupled with potentially high energy demands. This may necessitate increased fasting protein catabolism; however, the largest source of protein, flight muscle, must be maintained. This problem was examined in the horned lark (Eremophila alpestris), a 28-g passerine. Overnight fasting caused significant depletion of protein in flight muscle and liver, but not in other muscle groups. Proteolytic enzyme activity of the flight muscle doubled during fasting. Biochemical and ultrastructural studies revealed that protein was depleted disproportionately from the sarcoplasm of flight muscle cells. Fasting caused a reduction in the protein-specific glycolytic capacity of flight muscle tissue. Oxidative capacity of the flight muscle, as measured by both in vivo and in vitro assays, was not significantly affected. The disproportionate catabolism of flight muscle sarcoplasmic protein may be due to a greater susceptibility to proteolysis, and not necessarily because it represents a source of redundant storage protein.