Oxygen equilibria of ectotherm blood containing multiple hemoglobins

Complete isocapnic O2 equilibrium curves (O2EC's) and related blood-gas properties are reported for whole blood of the bullfrog (Rana catesbeiana) and the aquatic turtle (Pseudemys scripta) at temperatures ranging from 5 to 35 degrees C. P50's for bullfrog and turtle blood at physiological pH and 25 degrees C were 36.6 Torr (pH 7.83) and 19.3 Torr (pH 7.55), respectively. Elevation of blood temperature significantly reduced hemoglobin oxygen affinity in both species (delta H = -8.1 and -7.8 kcal/mol O2 for Rana and Pseudemys, respectively). Bullfrog and turtle oxygen equilibrium data revealed non-standard curve shapes when compared with the Severinghaus curve for human blood (1979); ectotherm O2EC's rose more steeply below P50 (less sigmoid) and were distinctly flattened (linear) above 50% saturation. The CO2-Bohr effect for bullfrog and turtle blood varied significantly as a function of saturation. In addition, both species exhibited non-linear Hill relationships (logS/1-s vs. log PO2). These results indicate that the oxygen binding properties of the multiple hemoglobin bloods of Rana and Pseudemys (demonstrated by isoelectric focusing) are more complex than those exhibited by normal human blood. As a consequence, these ectotherm blood oxygen data are not well characterized by the limited number of simple descriptive parameters (P50, Hill's n and delta log P50/delta pH) commonly used to delineate predominantly single hemoglobin systems.