Differential catabolism of muscle protein in Garden Warblers (Sylvia borin): flight and leg muscle act as a protein source during long distance migration

Samples of flight and leg muscle tissue were taken from migratory garden wa rblers at three different stages of migration: (1) pre-flight: when birds f ace an extended flight phase within the next few days, (2) postflight: when they have just completed an extended flight phase, and (3) recovery: when they are at the end of a stop-over period following an extended flight phas e. The changes in body mass are closely related to the changes in flight (P < 0.001) and leg muscle mass (P < 0.001), suggesting that the skeletal mus cles are involved in the protein metabolism associated with migratory fligh t. From pre- to post-flight, the flight and the leg muscle masses decrease by about 22%, but are restored to about 12% above the pre-flight masses dur ing the recovery period. Biochemical analyses show that following fight a s elective reduction occurred in the myofibrillar (contractile) component of the flight muscle (P < 0.01). As this selective reduction accounts only for a minor part of the muscle mass changes, sarcoplasmic (non-contractile) an d myofibrillar proteins of both the flight and leg muscle act as a protein source during long-distance migration. As a loss of leg muscle mass is addi tionally observed besides the loss in flight muscle mass, mass change seems not to be strictly associated with the mechanical power output requirement s during flight. Whereas the specific content of sarcoplasmic proteins in t he flight muscle is nearly twice as high as that in the leg muscle (P < 0.0 01), the specific content of myofibrillar proteins differs only slightly (P < 0.05), being comparably low in both muscles. The ratio of non-contractil e to contractile proteins in the flight muscle is one of the highest observ ed in muscles of a vertebrate.