In vivo MR spectroscopy and MR imaging on non-anaesthetized marine fish: techniques and first results.
In various phyla of marine invertebrates limited capacities of both ventilatory and circulatory performance were found to set<lb>the borders of the thermal tolerance window with limitations in aerobic scope and onset of hypoxia as a first line of sensitivity to<lb>both cold and warm temperature extremes. The hypothesis of oxygen limited thermal tolerance has recently been investigated in<lb>fish using a combination of non-invasive nuclear magnetic resonance (NMR) methodology with invasive techniques. In contrast<lb>to observations in marine invertebrates arterial oxygen tensions in fish were independent of temperature, while venous oxygen<lb>tensions displayed a thermal optimum. As the fish heart relies on venous oxygen supply, limited cardio-circulatory capacity is<lb>concluded to set the first level of thermal intolerance in fish. Nonetheless, maximized ventilatory capacity is seen to support<lb>circulation in maintaining the width of thermal tolerance windows. The interdependent setting of low and high tolerance limits<lb>is interpreted to result from trade-offs between optimized tissue functional capacity and baseline oxygen demand and energy<lb>turnover co-determined by the adjustment of mitochondrial densities and functional properties to a species-specific temperature<lb>range. At temperature extremes, systemic hypoxia will elicit metabolic depression, thereby widening the thermal window<lb>transiently sustained especially in those species preadapted to hypoxic environments.<lb>© 2004 Elsevier B.V. All rights reserved.