Respiratory responses to progressive hypoxia in the Amazonian oscar, Astronotus ocellatus.
This study determined the respiratory responses to progressive hypoxia in oscar, an extremely hypoxia-tolerant Amazonian cichlid. Oscar depressed oxygen consumption rates (MO2), beginning at a critical O2 tension (Pcrit) of 46Torr, to only 14% of normoxic rates at 10Torr. Total ventilation (Vw) increased up to 4-fold, entirely due to a rise in ventilatory stroke volume (no change in ventilatory frequency), and water convection requirement (Vw/MO2) 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 10Torr, which may have caused the increased expired-arterial PO2 difference. Venous PO2 was always very low (< or =7Torr). 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 5Torr; 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.
Department of Zoology, University of British Columbia, Vancouver V6T 1Z4, Canada. scott@zoology.ubc.ca
Graham R
GR
Scott
Chris M
CM
Wood
Katherine A
KA
Sloman
Fathima I
FI
Iftikar
Gudrun
G
De Boeck
Vera M F
VM
Almeida-Val
Adalberto L
AL
Val
D000222:Adaptation, Physiological; D015308:Anaerobic Threshold / Q000502:physiology*; D000818:Animals; D019069:Cell Respiration / Q000502:physiology; D023681:Cichlids / Q000502:physiology*; D004734:Energy Metabolism / Q000502:physiology; D005880:Gills / Q000166:cytology / Q000502:physiology; D000860:Hypoxia / Q000378:metabolism*; D019344:Lactic Acid / Q000097:blood; D010101:Oxygen Consumption / Q000502:physiology*; D015656:Respiratory Mechanics / Q000502:physiology*
D019344:Lactic Acid