Effect of Temperature on pH and Electrolyte Concentration in Air-Breathing Ectotherms

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The aim of this study was to determine the effects of temperature upon pH, protein charge and acid-base-relevant ion exchange in air-breathing ectotherms. Plasma and skeletal muscles in cane toads (Bufo marinus) and bullfrogs (Rana catesbeiana) were examined at 30, 20 and 10 °C. In addition, skeletal muscle ion concentrations were examined in black racer snakes (Coluber constrictor) at 30 and 10 °C. Cooling the amphibians produced a reduction in most of the plasma ion concentrations (Na+, K+, Ca2+, Cl-, SO42-) and in protein concentration because of increased hydration. Between 30 and 10 °C, total plasma osmolality fell by 14 % in the toads and by 5 % in the frogs. Plasma protein charge, calculated using the principle of electroneutrality, was unaffected by temperature, except possibly for the toads at 10 °C. The in vivo skeletal muscle ΔpHi/ΔT ratio, where pHi is intracellular pH and T is temperature, between 30 and 20 °C averaged −0.014 °C-1 in the toads and −0.019 °C-1 in the frogs. Between 20 and 10 °C, there was no change in pHi in the toads and a −0.005 °C-1 change in the frogs. The in vitro skeletal muscle ΔpHi/ΔT averaged −0.011 °C-1 in both toads and frogs. In all three species, skeletal muscle inulin space declined with cooling. Intracellular ion concentrations were calculated by subtracting extracellular fluid ion concentrations from whole-muscle ion concentrations. In general, temperature had a large effect upon intracellular ion concentrations (Na+, K+, Cl-) and intracellular CO2 levels. The relevance of the changes in intracellular ion concentration to skeletal muscle acid-base status and protein charge and the possible mechanisms producing the adjustments in intracellular ion concentration are discussed. It is concluded that ion-exchange mechanisms make an important contribution to adjusting pH with changes in temperature.