Acid-base regulation in the air-breathing swamp eel (Monopterus albus) at different temperatures

Vertebrates reduce arterial blood pH (pHa) when body temperature increases. In water breathers, this response occurs primarily by reducing plasma HCO₃^- levels with small changes in the partial pressure of CO₂ (P_CO2 ). In contrast, air breathers mediate the decrease in pHa by increasing arterial P_CO2 (Pa_CO2 ) at constant plasma HCO₃^- by reducing lung ventilation relative to metabolic CO₂ production. Much less is known about bimodal breathers, which utilize both water and air. Here, we characterized the influence of temperature on arterial acid-base balance and intracellular pH (pH_i) in the bimodal-breathing swamp eel, Monopterus albus This teleost uses the buccopharyngeal cavity for gas exchange and has very reduced gills. When exposed to ecologically relevant temperatures (20, 25, 30 and 35°C) for 24 and 48 h, pHa decreased by -0.025 pH units (U) °C^-1 in association with an increase in Pa_CO2 , but without changes in plasma [HCO₃^-]. pH_i was also reduced with increased temperature. The slope of pH_i of liver and muscle was -0.014 and -0.019 U °C^-1, while the heart muscle showed a smaller reduction (-0.008 U °C^-1). When exposed to hypercapnia (7 or 14 mmHg) at either 25 or 35°C, M. albus elevated plasma [HCO₃^-] and therefore seemed to defend the new pHa set-point, demonstrating an adjusted control of acid-base balance with temperature. Overall, the effects of temperature on acid-base balance in M. albus resemble those in air-breathing amniotes, and we discuss the possibility that this pattern of acid-base balance results from a progressive transition in CO₂ excretion from water to air as temperature rises.