Gas exchange in the amphibious fish,Amphipnous cuchia

Abstract

1. Gas exchange and blood gas transport has been studied in the amphibious teleost,Amphipnous cuchia. A. cuchia is a bimodal breather. Respiratory gas exchange takes place in a pair of specialized air sacs extending from the pharyngeal cavity. Aquatic and aerial gas exchange also takes place in vestigial gills, across buccopharyngeal surfaces and in the skin. All blood draining the air sacs is returned via systemic veins to the heart before systemic distribution. 2. Oxygen uptake in fish kept in water with access to air was 33.3±8.0 ml O2STP·kg−1·h−1. About 65% of this uptake resulted from air breathing. Upon removal from water the O2 uptake rose to 44.6±15.7 ml O2· kg−1·h−1, while confinement to water breathing reduced the O2 uptake to 16.4±2.7 ml O2·kg−1·h−1. The latter value was 50% higher than aquatic O2 uptake when air breathing was available. 3. Amphipnous practices periodic breathing and normal breathhold periods last 8–10 min. In the early phase of breathholding the gas exchange ratio (RE) was close to 0.7 but declined to low levels with breathholding. Mean RE for an average breathhold was 0.2. The low RE of the air sacs results from a high cutaneous CO2 elimination in water as well as in moist air. Estimated blood flows to the air sacs indicate flow of about 20 ml min−1 shortly after an air breath declining to 5 ml·min−1 late in a breath-hold period. 4. Due to the shunting of air sac blood to systemic venous (jugular vein) blood, the jugular vein\(P_{{\text{O}}_2 } \) carried the most oxygenated blood averaging 35.2 mm Hg, the dorsal aorta 23.4 mm Hg and the hepatic vein 18.6 mm Hg. 5. A. cuchia blood has a very high Hb concentration and O2 capacity reaching 15.5 gram % and 22 vol%, respectively. TheP50 value was 7.9 mm Hg at pH 7.6. The Bohr factor, Φ was −0.57, then-value 2.05 and the temperature sensitivity of the O2-Hb binding expressed by ΔH=−13.1 Kcal·mole Hb−1. Buffering capacity was high: 34.1 mM HCO3−·1−1. 6. The vascular configuration inA. cuchia suggests a low efficiency of gas transport. A high blood O2 capacity and O2 affinity and a high cardiac output reduce the efficiency loss and permit the fish to suspend with air breathing for up to 30 min with a modest reduction in arterial O2 saturation from near 90% to 60%. The high blood O2 affinity allows breathholding to occur at reduced rates of systemic blood flow due to the large O2 stores available in venous blood during normal breathing. 7. Ventral aortic blood pressure fell from about 60 mm Hg systolic value to 40 mm Hg in the dorsal aorta indicating considerable vascular resistance in the shunt connecting these vessels. The pressure gradient across the shunt remained unchanged with the breathhold cycle and is thus not part of the vasomotor activity controlling blood flow to the aerial gas exchanger. 8. The data are discussed in relation to other air breathing fishes, notably the electric eel,Electrophorus electricus, and the African lungfish,Protopterus aethiopicus.