Respiratory responses to progressive hypoxia in the Amazonian Oscar, **Astronotus ocellatus**
Faculty of Sciences. Biology
Respiratory physiology and neurobiology. - Amsterdam, 2002, currens
, p. 109-116
University of Antwerp
This study determined the respiratory responses to progressive hypoxia in oscar, an extremely hypoxia-tolerant Amazonian cichlid. Oscar depressed oxygen consumption rates (), beginning at a critical O2 tension (Pcrit) of 46 Torr, to only 14% of normoxic rates at 10 Torr. Total ventilation () increased up to 4-fold, entirely due to a rise in ventilatory stroke volume (no change in ventilatory frequency), and water convection requirement () increased substantially (up to 15-fold). Gill O2 extraction fell steadily, from 60% down to 40%. Although O2 transfer factor (an index of gill O2 diffusion capacity) increased transiently in moderate hypoxia, it decreased at 10 Torr, which may have caused the increased expired¨Carterial PO2 difference. Venous PO2 was always very low (¡Ü7 Torr). Anaerobic metabolism made a significant contribution to ATP supply, indicated by a 3-fold increase in plasma lactate that resulted in an uncompensated metabolic acidosis. Respiration of isolated gill cells was not inhibited until below 5 Torr; because gill water PO2 always exceeded this value, hypoxic ion flux arrest in oscars [Wood et al., Am. J. Physiol. Reg. Integr. Comp. Physiol. 292, R2048-R2058, 2007] is probably not caused by O2 limitation in ionocytes. We conclude that metabolic depression and tolerance of anaerobic bi-products, rather than a superior capacity for O2 supply, allow oscar to thrive in extreme hypoxia in the Amazon.