Cardiorespiratory responses to hypoxia in the African catfish, <Emphasis Type="Italic">Clarias gariepinus</Emphasis> (Burchell 1822), an air-breathing fish |
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Authors: | T C Belão C A C Leite L H Florindo A L Kalinin F T Rantin |
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Institution: | 1.Department of Physiological Sciences,Federal University of S?o Carlos (UFSCar),S?o Carlos,Brazil;2.Department of Zoology,S?o Paulo State University (UNESP),Rio Claro,Brazil;3.Department of Zoology and Botany,S?o Paulo State University (UNESP),S?o José do Rio Preto,Brazil;4.National Institute of Science and Technology, Comparative Physiology (FAPESP/CNPq),S?o Paulo,Brazil |
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Abstract: | The African catfish, Clarias gariepinus, possesses a pair of suprabranchial chambers located in the dorsal-posterior part of the branchial cavity having extensions
from the upper parts of the second and fourth gill arches, forming the arborescent organs. This structure is an air-breathing
organ (ABO) and allows aerial breathing (AB). We evaluated its cardiorespiratory responses to aquatic hypoxia. To determine
the mode of air-breathing (obligate or accessory), fish had the respiratory frequency (f
R) monitored and were subjected to normoxic water (PwO2 = 140 mmHg) without becoming hyperactive for 30 h. During this period, all fish survived without displaying evidences of
hyperactivity and maintained unchanged f
R, confirming that this species is a facultative air-breather. Its aquatic O2 uptake (
(V)\dot]\textO2 \dot{V}{\text{O}}_{2} ) was maintained constant down to a critical PO2 (PcO2) of 60 mmHg, below which
(V)\dot]\textO2 \dot{V}{\text{O}}_{2} declined linearly with further reductions of inspired O2 tension (PiO2). Just above the PcO2 the ventilatory tidal volume (V
T) increased significantly along with gill ventilation (
(V)\dot]\textG \dot{V}_{\text{G}} ), while f
R changed little. Consequently, the water convection requirement
( (V)\dot]\textG /(V)\dot]\textO2 ) \left( {\dot{V}_{\text{G}} /\dot{V}{\text{O}}_{2} } \right) increased steeply. This threshold applied to a cardiac response that included reflex bradycardia. AB was initiated at PiO2 = 140 mmHg (normoxia) and air-breathing episodes increased linearly with more severe hypoxia, being significantly higher
at PiO2 tensions below the PcO2. Air-breathing episodes were accompanied by bradycardia pre air-breath, to tachycardia post air-breath. |
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