Hypoxic acclimation
Low water oxygen content (hypoxia) is a feature common to shallow waters that are highly eutrophic or ice-covered for prolonged periods in the winter, to continental slopes, and to deep basins found in the Baltic, the North Sea, and the East and West Coasts of North America. Surprisingly, however, we are aware of only three studies that have directly examined the effects of hypoxic acclimation on the fish heart, and all have focused on species that routinely experience hypoxic conditions. Paajanen and Vornanen (2003
) acclimated crucian carp Carassius carassius to hypoxia (PO2for 3 weeks, and reported that the Na+/K+ ATPase activity of cardiac homogenates was reduced by 33%. Lennard and Huddart (1992) found that 3 weeks of exposure to hypoxia (PO2 ~40 mmHg) caused numerous morphological changes in flounder Platichthys flesus cardiac mitochondria (decreased size, increased budding and cristae density) that would increase the area/volume ratio for oxygen diffusion, and may have led to changes in the concentration of oxidative enzymes. Finally, Driedzic et al. (1985) exposed the pout Zoarces viviparous to 4–6 weeks of hypoxia (PO2 approx. 75 mmHg) and found that ventricular strips from hypoxia-acclimated animals were better able to sustain tension development during anoxia in the presence of high levels of external Ca2+, even though no alterations in key enzymes of energy metabolism were detected. Thus, these studies suggest that hearts of species that normally experience aquatic hypoxia, undergo morphological and physiological adjustments that enhance function when exposed to environmental hypoxia.
Whether such adaptations are found in more active, hypoxia-sensitive, species is unclear. Bushnell et al. (1984) reported that 3 weeks of hypoxic acclimation (PO2 ~40 mmHg) failed to enhance the swimming performance or oxygen consumption of rainbow trout when swum at this O2 level, which argues against significant hypoxia-induced compensation in trout heart function. In contrast, recent experiments (Faust et al., 2004; vs Gamperl et al., 2001; Gesser, 1977; Fig. 3) show cardiac differences among rainbow trout obtained from different hatcheries, and report an unusual degree of myocardial hypoxia tolerance for fish reared at a facility where oxygen and other water quality parameters are sub-optimal. Clearly, further experiments are required to determine whether these differences in myocardial hypoxia tolerance are a result of acclimation to poor water quality (e.g. low O2 saturation) or of genetic selection by hatchery operators.
Preconditioning
So far, this review has focused on cardiac alterations following long-term environmental change. However, recent research shows that fish can also respond rapidly to acute hypoxic exposure. Zebrafish Danio rerio exposed to just 48 h of non-lethal hypoxia (PO2=15 mmHg) have a significantly increased survival time (by 9x in males and 3x in females) when subsequently exposed to more severe hypoxia (PO2=8 mm Hg) (Rees et al., 2001). Further, Gamperl et al. (2001) demonstrated a cardioprotective response, in that pre-exposure to only 5 min of hypoxia (PO2=5–10 mmHg) completely eliminated the loss of in situ maximum cardiac function that normally follows 15 min of exposure to hypoxia in rainbow trout (Fig. 4A). This cardioprotective response, termed preconditioning, is broadly defined as the ability of brief periods of stress (e.g. hypoxia, ischaemia, stretch, heat shock) or biochemical/pharmacological substances to make tissues resistant to damage caused by a subsequent period of ischaemia or hypoxia. Gamperl et al. (2001) provided the first evidence (using hypoxia-sensitive trout) that preconditioning exists in fishes, and thus that preconditioning is a mechanism of cardioprotection that appeared early in the evolution of vertebrates. In mammals, numerous cellular pathways and end-effectors are involved in preconditioning (Okubo et al., 1999; Nakano et al., 2000; Yellon and Downey, 2003). No experiments have directly investigated the cellular mechanisms that mediate myocardial preconditioning in fishes, although recent studies suggest that sarcolemmal (Cameron et al., 2003) and mitochondrial (MacCormack and Driedzic, 2002) ATP-sensitive K+ channels, and MAPK signaling pathways (ERK, JNKs and p38-MAPK; Gaitanaki et al., 2003) may be involved.
The importance and indeed existence of preconditioning in hypoxia-tolerant vertebrate hearts has been questioned in recent years. For example, ischaemic preconditioning failed to improve contractile function following 40 min of global ischemia in hypoxia-tolerant neonatal rat hearts (1 or 4 days post partum), only slightly (by 7%) improved contractile function in relatively hypoxia-sensitive rat hearts tested 7 days post partum (Ostadalova et al., 1998), and Baker et al. (1999) showed that hearts from 7–10 day old rats that were reared in a hypoxic environment (12% oxygen) no longer experienced increased functional recovery in response to preconditioning. In contrast, both Tajima et al. (1994) and Nechá
et al. (2002) demonstrated that although hearts from chronically hypoxic adult rats had increased resistance to ischaemia-related damage, preconditioning conferred an additional amount of protection. Thus, to examine whether hearts from hypoxia-tolerant trout can be preconditioned, we recently conducted in situ studies on two different populations of rainbow trout that display an unusual degree of myocardial hypoxia tolerance. Gamperl et al. (2004) performed in situ experiments using trout with hearts that Faust et al. (2004; Fig. 3) previously identified as hypoxia-tolerant (again using 5 min of hypoxia as the preconditioning stimulus), while Overgaard et al. (2004) used a population of trout from British Columbia (Canada) and 2x 5 min cycles of hypoxia or exposure to high adrenaline (250 nmol l–1) as preconditioning stimuli. Both studies (e.g. Fig. 4B) showed that hypoxia-tolerant trout hearts could not be preconditioned, and thus that the protection afforded by inherent myocardial hypoxia tolerance and preconditioning was not additive. These data suggest that the relationship between hypoxic adaptation and preconditioning in the trout heart resembles that of the neonatal/immature, not adult, mammalian heart.
It is tempting to associate myocardial preconditioning with myocardium that is supplied with blood from the coronary circulation because the rat heart becomes increasingly dependent on its coronary circulation as it ages, and rainbow trout possess a coronary circulation (Tota et al., 1983) that supplies blood to the compact myocardium, which comprises the outer one-third of the heart (Fig. 5). However, the hypoxia-sensitive heart of Atlantic cod Gadus morhua, which lacks a coronary circulation and is composed entirely of spongy myocardium, can be preconditioned (A. G. Genge and A. K. Gamperl, unpublished; Fig. 6) in much the same way as rainbow trout (Gamperl et al., 2001). Why cod hearts that have only spongy myocardium and display a moderate degree of hypoxia tolerance, but not trout hearts that have developed a high degree of hypoxia tolerance (Gamperl et al., 2004; Overgaard et al., 2004), can be preconditioned is not known. However, investigations into the cellular mechanisms that mediate these differences are likely to provide valuable information on how the hearts of fish and other lower vertebrates deal with oxygen deprivation.
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