Flight muscle and heart phenotypes in the high-flying ruddy shelduck.

Ruddy shelduck migrate from wintering grounds in lowland India and Myanmar to breeding grounds in central China and Mongolia, sustaining flight over the Himalayas, where oxygen availability is greatly reduced. We compared phenotypes of the pectoralis muscle and the ventricle of the heart from ruddy shelduck and common shelduck (a closely related low-altitude congener) that were raised in common conditions at sea level, predicting that oxidative capacity would be greater in ruddy shelduck to support high-altitude migration. Fibre-type composition of the pectoralis and the maximal activity of eight enzymes involved in mitochondrial energy metabolism in the pectoralis and heart, were compared between species. Few differences distinguished ruddy shelduck from common shelduck in the flight muscle, with the exception that ruddy shelduck had higher activities of complex II and higher ratios of complex IV (cytochrome c oxidase) and complex II when expressed relative to citrate synthase activity. There were no species differences in fibre-type composition, so these changes in enzyme activity may reflect an evolved modification in the functional properties of muscle mitochondria, potentially influencing mitochondrial respiratory capacity and/or oxygen affinity. Ruddy shelduck also had higher lactate dehydrogenase activity concurrent with lower pyruvate kinase and hexokinase activity in the left ventricle, which likely reflects an increased capacity for lactate oxidation by the heart. We conclude that changes in pathways of mitochondrial energy metabolism in the muscle and heart may contribute to the ability of ruddy shelduck to fly at high altitude.

Control of breathing and ventilatory acclimatization to hypoxia in deer mice native to high altitudes.

AIM: We compared the control of breathing and heart rate by hypoxia between high- and low-altitude populations of Peromyscus mice, to help elucidate the physiological specializations that help high-altitude natives cope with O2 limitation. METHODS: Deer mice (Peromyscus maniculatus) native to high altitude and congeneric mice native to low altitude (Peromyscus leucopus) were bred in captivity at sea level. The F1 progeny of each population were raised to adulthood and then acclimated to normoxia or hypobaric hypoxia (12 kPa, simulating hypoxia at ~4300 m) for 5 months. Responses to acute hypoxia were then measured during stepwise reductions in inspired O2 fraction. RESULTS: Lowlanders exhibited ventilatory acclimatization to hypoxia (VAH), in which hypoxia acclimation enhanced the hypoxic ventilatory response, made breathing pattern more effective (higher tidal volumes and lower breathing frequencies at a given total ventilation), increased arterial O2 saturation and heart rate during acute hypoxia, augmented respiratory water loss and led to significant growth of the carotid body. In contrast, highlanders did not exhibit VAH - exhibiting a fixed increase in breathing that was similar to hypoxia-acclimated lowlanders - and they maintained even higher arterial O2 saturations in hypoxia. However, the carotid bodies of highlanders were not enlarged by hypoxia acclimation and were similar in size to those of normoxic lowlanders. Highlanders also maintained consistently higher heart rates than lowlanders during acute hypoxia. CONCLUSIONS: Our results suggest that highland deer mice have evolved high rates of alveolar ventilation and respiratory O2 uptake without the significant enlargement of the carotid bodies that is typical of VAH in lowlanders, possibly to adjust the hypoxic chemoreflex for life in high-altitude hypoxia.