The effects of hypoxia on energy economy of cardiac muscle were compared between the hypoxia-tolerant freshwater turtle at 20°C and the hypoxia-sensitive rainbow trout at 15°C. Isolated ventricular preparations

Freshwater turtles possess high hypoxia tolerance relating to their diving behaviour and ability to hibernate for several months without access to air (Jackson, 2000, 2002). While sustained cardiac function during hypoxia is essential for survival (Belkin, 1968), cardiac performance is markedly reduced and may depend on the high circulating levels of catecholamines during anoxia (White and Ross, 1966; Jackson, 1987; Keiver et al., 1992; Wasser and Jackson, 1991; Hicks and Wang, 1998). The ability of cardiac muscle from turtles to tolerate anoxia is associated with a superior maintenance of cardiac cellular energy state relative to most other species of ectothermic vertebrates (Wasser et al., 1990; Jackson et al., 1995; Hartmund and Gesser, 1996). Furthermore, it seems that the myocardium preserves most contractile activity during severe hypoxia/anoxia at low workload (Wasser et al., 1990; Jackson et al., 1995; Hartmund and Gesser, 1996). This ability has been related to a high glycolytic capacity (Bing et al., 1972) and is reflected in a high anaerobic metabolic capacity relative to aerobic capacity (Christensen et al., 1994). Other studies, however, indicate that neither the glycolytic nor the mechanical capacities of turtle hearts are exceptional, and it has been suggested that their anoxia tolerance relates primarily to downregulation of metabolic requirements so that cellular energy state is protected (Arthur et al., 1997; Farrell et al., 1994). Studies of perfused hearts from turtle and trout indicate that work output is achieved at similar rates of ATP consumption during full oxygenation and severe hypoxia (Reeves, 1963b; Arthur et al., 1992, 1997). However, recent studies on rats, guinea pigs and frogs raised the possibility that the energetic cost of developing tension during hypoxia is lower in hypoxiatolerant vertebrates (Joseph et al., 2000). In addition to inherent differences in the efficiency of the contractile system, a low cost of contraction (i.e. overall cellular ATP consumption/ force developed) would also occur if the metabolic demands The Journal of Experimental Biology 207, 1915-1924 Published by The Company of Biologists 2004 doi:10.1242/jeb.00975