Torrent ducks live year-round in some of the most powerful and fast-flowing rivers, resulting from the run-off of ice-capped mountains to create waterfalls and rivers, surrounded by rugged and steep mountain slopes, in the Andes (Koepcke, 1970; Todd, 1996). Due to the harsh nature of their environment, torrent ducks are considered precocial, and have a long incubation period to ensure proper development for young ducklings to be able to swim and survive in the violent waters of their environment (Johnsgard, 1968; Johnson and Moffett, 1972; Todd, 1996). Torrent ducks are very strong swimmers and divers, relying predominantly on swimming for transportation, with little emphasis on flying. Torrent ducks are notoriously cautious, and typically swim with the majority of their bodies submerged to avoid detection (Johnsgard, 1968, 1992; Todd, 1996). Torrent ducks are territorial, vocal and very aggressive in warding off invaders, and a pair of torrent ducks will occupy a length of river with little movement in their lifetime (Baldassarre and Bolen, 1994; Todd, 1996).
Respiration of permeabilized fibers, measured in the gastrocnemius, left ventricle and pectoralis of the torrent duck (Merganetta armata). (A) Gastrocnemius; (B) left ventricle; (C) pectoralis. Respiration rates were measured in the presence of pyruvate, malate and the following: ADP; ADP and glutamate (GLUT); ADP, glutamate and succinate (SUCC); ADP, glutamate, succinate, ascorbate and TMPD (ASC+TMPD). Bars represent the respiration rates of the low-altitude (LA; white) and high-altitude (HA; black) populations. Values are given as the meanss.e.m. (N=6). *Significantly different from the corresponding low-altitude value (Student's t-test; P
ac dc black ice torrent
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Differences in enzyme activity from samples of the gastrocnemius of the torrent duck (Merganetta armata). White bars represent the low-altitude (LA) population; black bars represent the high-altitude (HA) population. Values are given as the meanss.e.m. (N=6). *Significantly different from the corresponding low-altitude value (Student's t-test; P
Differences in enzyme activity from samples of the left ventricle of the torrent duck (Merganetta armata). White bars represent the low-altitude (LA) population; black bars represent the high-altitude (HA) population. Values are given as the meanss.e.m. (N=6). *Significantly different from the corresponding low-altitude value (Student's t-test; P
Consistent with the lack of variation in mitochondrial respiratory capacities in the pectoralis, the maximal activity of only one enzyme differed significantly in the pectoralis between the high- and low-altitude populations. The activity of MDH, a major enzyme of the citric acid cycle, was 26% higher in the pectoralis in the high-altitude torrent ducks (P
Differences in malate dehydrogenase (MDH) enzyme activity from samples of left ventricle, pectoralis and gastrocnemius of the torrent duck (Merganetta armata). White bars represent the low-altitude (LA) population; black bars represent the high-altitude (HA) population. Values are given as the meanss.e.m. (N=6). *Significantly different respiration in high-altitude torrent ducks in comparison to low-altitude torrent ducks (two-way ANOVA with Bonferroni post hoc test; P
Myoglobin (Mb) concentrations of the left ventricle, pectoralis and gastrocnemius of the torrent duck (Merganetta armata). Bars represent the protein content in tissue of the low-altitude (LA; white) and high-altitude (HA; black) populations. Values are given as the meanss.e.m. (N=6). *Significantly different respiration in high-altitude torrent ducks in comparison to low-altitude torrent ducks (Student's t-test; P
The enhanced respiratory capacity in the gastrocnemius muscle of highland torrent ducks (Fig. 1A) could be important at high altitudes for increasing the thermogenic capacity for heat production in the cold, and/or increasing hypoxia tolerance. Birds meet the bulk of the demand for thermogenesis by shivering (West, 1965; Bicudo, 1996). Non-shivering thermogenesis may play a role in thermoregulation in ducklings (Teulier et al., 2010), but the relative importance of non-shivering thermogenesis in most species of birds is unclear (Barré et al., 1989; Connolly et al., 1989). Regardless of whether shivering or non-shivering thermogenesis predominates, the flight muscles (particularly the pectoralis and the supracoracoideus) are believed to be a major site of thermogenesis in adult birds (Petit and Vézina, 2014; Block, 1994; Bicudo et al., 2002). The thigh muscles are very important for thermogenesis in early development, but their relative importance decreases as the flight muscles grow and eventually reach a much larger mass (Marjoniemi and Hohtola, 2000; Sirsat et al., 2016). Therefore, although the increase in aerobic capacity in the gastrocnemius could be important for facilitating thermogenesis, it is curious that similar increases do not also occur in the pectoralis. Increases in oxidative capacity might have instead arisen to promote hypoxia resistance at high altitudes, a theory that has been suggested in other high-altitude taxa (Hochachka, 1985; Scott et al., 2009a,b; Lui et al., 2015). This theory suggests that, when the maximum attainable respiration of an individual muscle fiber is impaired from declines in intracellular O2 tension, a higher oxidative capacity should increase the total mitochondrial O2 flux of the entire muscle and thus help offset the inhibitory effects of hypoxia. This mechanism might be acting in torrent ducks to overcome intracellular hypoxia in the gastrocnemius muscle during swimming or diving at high altitude.
The enhanced respiratory capacity of highland torrent ducks was associated with greater activity of CIV, without any significant differences from lowland ducks in the activities of other electron transport chain enzymes (Table 1; Fig. 2). This was somewhat surprising in light of the common perception that CIV is generally in excess capacity, and that CIV exerts less control over pathway flux than other mitochondrial complexes (Telford et al., 2009). Although this could imply that relatively large increases in CIV activity are needed to achieve relatively small changes in mitochondrial oxygen consumption, our results suggest that this is not the case; the relative differences in the highland population for mitochondrial respiration (1.7- to 2.1-fold) were nearly as large as those for CIV activity (2.5-fold). It is also possible that the larger relative excess of CIV activity in highland ducks helps increase the O2 affinity of mitochondria, by reducing the catalytic turnover rate of each CIV enzyme (Gnaiger et al., 1998; Kudin et al., 2002), and thus helps sustain ATP synthesis in hypoxia. However, although unique specializations in the activity, structure and function of CIV have been observed in the locomotor muscles of several high-altitude taxa (Sheafor, 2003; Scott et al., 2011; Lui et al., 2015), it is not clear whether these specializations affect mitochondrial O2 affinity (Scott et al., 2009a,b).
Major regulatory enzymes of glycolysis, PK and PFK, also had higher activity in the gastrocnemius of the high-altitude population compared with those from low altitudes (Table 1; Fig. 2). PK and PFK have been suggested to exert significant metabolic control over glycolytic pathway flux when assessed using metabolic control analysis (Vogt et al., 2002a,b), and they both catalyze irreversible reactions in the glycolytic pathway and have long been discussed as sites of allosteric regulation (Scrutton and Utter, 1968). This is in line with the enhanced glycolytic enzyme activities in the locomotory muscle of many high-altitude mammals, and likely serves to increase the capacity for producing ATP from carbohydrate oxidation (Semenza et al., 1994; Firth et al., 1994; McClelland et al., 1998; Schippers et al., 2012). This could be especially beneficial in high-altitude hypoxia because of the inherent O2 savings associated with oxidizing carbohydrates instead of other metabolic fuels (McClelland et al., 1998). This advantage seems to have been favored by natural selection in highland mice from the Andes, which have a greater preference for carbohydrate oxidation than lowland mice during exercise, even when compared at similar altitudes and exercise intensities (Schippers et al., 2012). The increased glycolytic activities in torrent ducks may also be reflective of an increased capacity for using anaerobic metabolism during short diving bouts, which may supply lactate for oxidation in the heart (see below). Many diving animals show increased glycolytic capacity in the muscles to support underwater locomotion (George and Ronald, 1973; Simon et al., 1974; Castellini et al., 1981), and it is predictable that the demands for anaerobic metabolism could be higher while diving in high-altitude hypoxia. 2ff7e9595c
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