The Comparative Physiology of Diving in North American Freshwater Turtles. II. Plasma Ion Balance during Prolonged Anoxia

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Abstract
Plasma ionic composition was measured during submergence in anoxic water ( ) at 10 C of four species of North American freshwater turtles (Chrysemys picta, Chelydra serpentina, Sternotherus odoratus, and Trionyx spiniferus). As we reported in the previous paper, the species rank in the order listed in their capacity to survive prolonged anoxia at this temperature. The objectives of this study were to examine the changes in plasma ions in response to the anoxia and to relate these responses to the overall blood acid-base picture. The qualitatively similar pattern of ionic changes in each species included a primary increase in [lactate⁻] and decrease in [HCO₃⁻]. In addition, there were increases in [K⁺], total [Ca⁺⁺], and total [Mg⁺⁺] and decrease in [Cl⁻]. These latter strong ion changes are interpreted as compensatory changes that served to balance the lactate charge and that were associated with exchanges of weak ions that buffered the H⁺. The overall importance of these ionic changes to acid-base balance was evaluated by comparing the observed change in plasma strong ion difference (SID) with the calculated SID if lactate had risen as observed but all other strong ions had remained at control values. On this basis, the plasma ion adjustments of Chrysemys and Sternotherus balanced about 60% of the added lactate, while in Chelydra and Trionyx the compensation was only 47% and 34%, respectively. In addition to strong ion adjustments, three other factors are identified from measurements on blood that influenced the development of anoxic acidosis: (1) the rate of lactate accumulation, (2) the initial plasma weak ion buffering, and (3) the magnitude of the respiratory acidosis. These factors are discussed, and their contributions to the observed acidosis in each species are compared. We conclude that the rank order tolerance to anoxia is mainly related to the rate at which lactate accumulates in the body fluids, and this rate is probably a function of anaerobic metabolic rate.