Regulation of protein breakdown and adrenocortical response to stress in birds during migratory flight

During long-term fasting at rest, protein utilization is maintained at low levels until it increases at a threshold adiposity. This study examines 1) whether such a shift in energy substrate use also occurs during endurance e xercise while fasting, 2) the role of corticosterone, and 3) the adrenocort ical response to an acute stressor. Ten species of migrating birds caught a fter an endurance flight over at least 500 km were examined. Plasma uric ac id and corticosterone levels were low in birds with fat stores >5% of body mass and high in birds with smaller fat stores. Corticosterone levels were very high in birds with no visible fat stores and emaciated breast muscles. Corticosterone levels increased with handling time only in birds with larg e fat stores. These findings suggest that 1) migrating birds with appreciab le fat stores are not stressed by endurance flight, 2) a metabolic shift (i ncreased protein breakdown), regulated by an endocrine shift (medium cortic osterone levels), occurs at a threshold adiposity, as observed in birds at rest, 3) adrenocortical response to an acute stressor is inhibited after th is shift, and 4) an adrenocortical response typical for an emergency situat ion (high corticosterone levels) is only reached when muscle protein is dan gerously low.