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Measuring the oxidative cost of breathing : a comparison of methods using red-eared sliders (Trachemys scripta elegans) Lee, Stella Yim Jung


There is a metabolic cost associated with the work required to overcome the elastic and resistive forces associated with breathing. These forces vary widely as a function of the anatomy of the lungs and body wall in different species. In turtles, the lungs are relatively compliant but the body wall with the carapace is very stiff. Studies designed to measure the cost of ventilation in turtles have proven to be difficult. Two different methods have been used to estimate costs and each method has produced a different result. In an attempt to resolve this controversy, the present study obtained data using three different methods in a single group of turtles; a data regression method, a unidirectional ventilation method (UDV), and a hybrid method combining data regression and unidirectional ventilation. All three methods produced highly variable results (individual variability, differences between use of hypoxia and hypercapnia, evidence of hypoxia and hypercapnia-induced metabolic suppression). Based on data plotted for individual animals versus groups of animals and using all data points versus mean values for different levels of inspired gases, oxidative costs were obtained ranging from -0.0005 to 0.022 ml O₂/ml air ventilated using the regression method, 0.014 ml O₂/ml air for the UDV method and 0.025 ml O₂/ml air for the hybrid method. Most values were high compared to a theoretical estimate of 0.0003 ml O₂/ml air obtained based on measurements of mechanical work of breathing and an assumed respiratory muscle efficiency of 10%. The experimental results from the two methods appeared to be different as a result of CO₂-induced metabolic rate suppression or the non-linearity of the oxygen uptake-ventilation relationship. However, assuming the theoretical estimate best represents the oxygen uptake-ventilation relationship and the true oxidative costs of ventilation, neither theory would fit. Instead, attributing the discrepancy in results to the differences in the breathing patterns and their irregularity provided the most logical explanation and resulted in the conclusion that the oxidative cost of ventilation in turtles to be similar to that of mammals.

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