The combined effect of hypoxia and nutritional status on metabolic and ionoregulatory responses of common carp (**Cyprinus carpio**)
Faculty of Sciences. Biology
Comparative biochemistry and physiology : A : molecular & integrative physiology. - London
, p. 133-143
University of Antwerp
Since hypoxia is a common event in aquatic environments, oxygen has been a major driving force in the evolution of fish. When fish are simultaneously faced with food deprivation in their natural habitat, this poses an even greater challenge to energy and ion homeostasis. In the present study, the combined effects of hypoxia and nutritional status (fed versus fasted) were examined in common carp (Cyprinus carpio), a relatively hypoxia tolerant cyprinid. Fish were either fed or fasted and were exposed to hypoxia (1.51.8 mg O2 L− 1, 1619% saturation at 18 °C) at or slightly above their critical oxygen concentration (1.4 mg O2 L− 1, 16.5% saturation at 20 °C) during 1, 3 or 7 days followed by a 7 day recovery period. Fasted fish had lower ventilation frequencies than fed fish but under both feeding regimes, ventilation initially increased during hypoxia. In fed fish, ventilation returned to control levels during hypoxia, while in fasted fish recovery only occurred after reoxygenation. Due to this, C. carpio managed, at least in part, to maintain aerobic metabolism during hypoxia: muscle and plasma lactate levels remained relatively stable although they tended to be higher in fed fish (despite its higher ventilation rates). However, during the recovery phase compensatory responses differed greatly between both feeding regimes: plasma lactate in fed fish increased with a simultaneous breakdown of liver glycogen indicating increased energy use, while fasting fish seemed to economize energy and recycle decreasing plasma lactate levels into increasing liver glycogen levels (Cori cycle). Protein was used under both feeding regimes during hypoxia and subsequent recovery: protein levels reduced mainly in liver for fed fish and in muscle for fasting fish. Overall, nutritional status had a greater impact on energy reserves than the lack of oxygen with a lower hepatosomatic index and lower glycogen stores in fasted fish. Fasting fish transiently increased Na+/K+-ATPase activity under hypoxia, but in general ionoregulatory balance proved to be only slightly disturbed, showing that sufficient energy was left for ion regulation.