Oxygen binding in alligator blood related to temperature, diving, and "alkaline tide"

Blood of Alligator mississippiensis has a relatively high O2 affinity, the half-saturation O2 tension (P50) = 24.5 Torr at pH 7.495, the arterial, normocapnic pH at 25 degrees C. Although the overall temperature sensitivity of P50 at 15, 25, and 35 degrees C and constant pH is low, the effect on P50 almost doubles when measured at the in vivo pH of each temperature (delta Happ = -24 and -47 kJ/mol, respectively). The CO2 Bohr effect (theta CO2 = -0.95) is 5.5 times greater than the fixed acid Bohr effect (theta FA), and the Haldane effect is small (approximately 0.03 pH units). The relatively high O2 affinity may ensure efficient utilization of the lung O2 reserve during breath holding and diving, whereas its pronounced in vivo temperature sensitivity may be adaptive to the high temperature quotients of the organismic O2 requirement. The large difference between theta CO2 and theta FA will favor constancy in blood O2 affinity in the face of large activity-induced increases in blood lactate and pronounced feeding-induced alkaloses. These "alkaline tides," which result from an exchange of plasma Cl- for HCO-3 across the gut wall, appear to be only slightly compensated by increased blood CO2 tensions. The results are additionally discussed in terms of allosteric modulation of hemoglobin-O2 affinity in crocodilians.