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Journal of Experimental Biology 105,317-338 (1983)
Published by Company of Biologists 1983

Aquatic and Aerial Respiration Rates, Muscle Capillary Supply and Mitochondrial Volume Density in the Airbreathing Catfish (Clarias Mossambigus) Acclimated to Either Aerated or Hypoxic Water

IAN A. JOHNSTON ¹, LYNNE M. BERNARD ¹, and GEOFFREY M. MALOIY ²

¹ Department of Physiology, University of St. Andrews, St. Andrews, Fife, Scotland KYI6 9TS; Department of Animal Physiology, University of Nairobi P.O. Box 30197, Nairobi, Kenya
² Department of Animal Physiology, University of Nairobi, P.O. Box 30197, Nairobi, Kenya

Specimens of the African air-breathing catfish Clarias mossambicus were acclimated to either aerated (Pw_O2 15.3 KPa) or hypoxic (Pw_O2 2.4KPa) water for up to 27 days at 20 °C. Routine respiration rate for fish acclimated to aerated water was 85.7 mlO₂ (kgbodyweight)^-1 h^-1. Gas exchange across the suprabranchial chambers accounted for 25% of the total. In aerated water the interval between air-breaths varied from 1.4 to 30.6 min. On acute exposure to hypoxia air-breathing frequency was unaltered (6.3 h^-1) although aerial respiration rate increased by 70%. This suggests that ventilation of the suprabranchial chambers is variable and that air-breathing frequency is a poor measure of air-breathing effort. Total respiration decreased by 46% on acute exposure to hypoxia (Pw_O2 2.4 KPa), reflecting a reduction in routine activity. Following acclimation to hypoxia, airbreathing frequency (8.1 h^-1) was higher and total routine respiration rate increased from 46.3 to 67.8 mlO₂ kg^-1h^-1. The increased oxygen consumption with hypoxia acclimation was largely the result of an increase in aquatic respiration from 10.4 to 27.5 mlO₂kg^-1h^-1 Measurements were made of mitochondrial volume densities [Vv(mt,f)] and capillary supply to fast and slow myotomal muscles. The fraction of fibre volume occupied by mitochondria was 15 percnt; for slow and 2.5% for fast muscles. Values for [Vv(mt,f)] obtained for fish slow fibres are much higher than for homologous muscles in birds and mammals and show a good correlation with capillary density [NA(c,f)]. Hypoxia acclimation did not result in changes in either muscle Vv(mt,f) or NA(c,f). It is suggested that increased ventilation of the suprabranchial chambers and greater oxygen extraction across the gills obviates the need for modifications in these parameters.

Key words: Skeletal muscle, capillaries, hypoxia

Submitted on November 9, 1982
Accepted on January 31, 1983

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