Respiration and hemoglobin function in the giant African bullfrog Pyxicephalus adspersus Tschudi (Anura: Pyxicephalidae) during rest, exercise and dormancy

Abstract

The survival strategies of frogs exposed to harsh conditions include various forms of dormancy. Giant bullfrogs Pyxicephalus adspersus subjected to slow desiccation without access to moist substrata over winter in cocoons during dormancy. We found that Pyxicephalus can survive long periods of dormancy in soil without forming cocoons. With virtually no information available on the associated respiratory adaptations, we measured oxygen consumption rates of resting and exercising bullfrogs together with haematological and gas-binding properties of blood and haemoglobin (Hb) solutions from non-dormant frogs and cocoon-less dormant frogs. Treadmill (0.054 ms−1) exercising frogs consumed nearly seven times more oxygen than resting individuals (8.45 and 1.25 mmol O2 kg−1h−1, respectively). During a six-month dormancy period Pyxicephalus lost an average of 13.7% of its body mass, while plasma osmotic pressure increased from 231 to 320 mOsm l −1 coupled with concomitant increases in plasma sodium level, haematocrit, as well as in total Hb and metHb concentrations. Whole blood oxygen affinity was markedly higher in dormant frogs than in non-dormant resting frogs [half-saturation oxygen tensions, P50=33.0 mm Hg (pH: 7.63) and 44.9 mm Hg (pH 7.76), respectively, at 25 °C], which predictably favours pulmonary oxygen loading during dormancy in hypoxic substrata. Although hibernation resulted in sharp decreases in blood ATP concentration and the ATP: tetrameric Hb molar ratio that would increase blood oxygen affinity, the affinity of ‘stripped’ (cofactor-free) Pyxicephalus Hb shows low sensitivity to this effector. Hb-oxygen affinity is moreover insensitive to Cl− ions (that depress O2 affinity of most vertebrate haemoglobins) but oxygen affinity is strongly increased by urea and metHb formation that increase during dormancy. Isoelectric focusing reveals a single major isoHb (<95% of the total) and two trace components. The adaptive significance of the data is discussed in relation to the physiology of dormancy in Pyxicephalus adspersus, particularly the factors that secure oxygen uptake.