Gas exchange, storage and transport in voluntarily diving Xenopus laevis

Abstract

A closed, extracorporeal loop, containing oxygen and carbon dioxide electrodes, was developed in order to make continuous measurements of lung gas PO2 (PAO2) and PCO2 (PACO2) in undisturbed Xenopus laevis. Pulmonary R values (delta PACO2/delta PAO2) are about 0.8 during periods of lung ventilation in resting animals, but they fall very rapidly as a voluntary dive proceeds. In fact, the instantaneous R values for lung eventually fall to zero during a voluntary dive, since PAO2 continues to decline whilst PACO2 (after an initial increase) comes into a steady state as transcutaneous CO2 losses balance metabolic production. These relationships change during spontaneous underwater activity, aquatic hypercapnia or enforced diving, with significantly higher PACO2 levels being found at any PAO2 value than in resting animals. Emergence from such dives is marked by a considerable hyperventilation, leading to lung R values which are 2–3 times higher than those seen during lung ventilation of animals at rest. The lungs of Xenopus are therefore important in eliminating the CO2 stored during a period of breath-holding but not of major importance in forming part of that store themselves. The lungs of Xenopus are, however, important sources of stored oxygen during voluntary dives, the rate of use being clearly related to activity levels and dive durations. There could be sudden changes in the rate of PAO2 depletion during a dive, suggesting that factors additional to the metabolic rate of the cells may be important in determining the way in which the lung store is used. In a parallel series of experiments, O2 and CO2 partial pressures were determined in lung gas samples and in simultaneously drawn samples of blood from the femoral artery (systemic arterial) and left auricle (pulmonary venous) of animals making voluntary dives. These blood/gas data, together with results of previous experiments on Xenopus, have been used to develop an idealized model of O2 exchange, storage and transport during a 30-min voluntary dive. The volume of the O2 stores held in the lungs and various subdivisions of the circulation are shown in the model by plotting the PO2 of the store against its respective O2 capacitance. The model illustrates the overall importance of the lung as a source of oxygen during breath-holding and that early use of a large systemic venous O2 store may be an important basic function for cardiovascular adjustments seen in a dive.