In nature, under adverse conditions like low oxygen or low environmental temperatures, some fish species can slow down their metabolism ('metabolic depression') in a manner comparable with hibernation and/or aestivation. This strategy may be applied to slow down ATP flux, save energy stores and diminish end product (e.g. lactic acid) accumulation. It has been suggested that metabolic rate and metabolic depression are controlled by a combination of blood perfusing the organs and the factors carried in that blood, e.g. oxygen and substrates.

In this study, two techniques were combined to investigate these issues: direct heat measurements of cold-blooded animals via calorimetry, to measure overall metabolic rate; and radio-telemetry, using small implanted transmitters (weighing 3 g: TA10ETA-F20-L20, Data Science International, St. Paul, MN, U.S.A.) to gather information about cardiovascular properties (heart frequency).

At hypoxic levels of 40%, 20%, 10% and 3%, the metabolic rate of goldfish (Carassius auratus) dropped to 94%, 84%, 61% and 55% of its normoxic level, respectively. This implies that metabolic depression is flexible, depending on the supply of oxygen. Heart frequency at normoxia (100% oxygen) was 34 beats per minute, but this too fell under the conditions of hypoxia listed above, to 26, 22, 14 and 9 beats per minute, respectively.

To explain the close coupling between the metabolic demand of the tissues, respiratory gas exchange and cardiac activity, nonlinear regression techniques were performed. This statistical analysis indicated that the level of metabolic depression was determined more by heart frequency than by environmental oxygen concentration or oxygen consumption. This suggests that, to a large extent, these animals determine their level of metabolic depression by regulating their heart frequency.

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