Characteristics of the anion transport system in sea turtle erythrocytes

Erythrocytes of Kemp's ridley sea turtle (Lepidochelys kempi) contain a 100- to 105-kDa protein that is reactive with a monoclonal antibody to the membrane domain of human erythrocyte band 3. Based on inhibition of membrane HCO₃^--Cl^- exchange with 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS), sea turtle erythrocytes were found to contain 4 x 10⁶ copies of band 3 per cell. Unidirectional HCO₃^- transfer, specifically HCO₃^-(out→in)-Cl^-(in→out) exchange, where subscript in→out represents transfer from inside to outside and subscript out→in represents transfer from outside to inside, was characterized by a maximal exchange rate of 1.0-1.1 nmol·cm^-2·s^-1, substrate affinity coefficients of 0.1-0.2 mM for HCO₃^- and 1.6 mM for Cl^-, and an apparent inhibition constant for SITS of 0.6-1.0 μM (10°C, pH 7.6). Under physiological conditions (30°C, pH 7.4), the rate of net HCO₃^- transfer (i.e., the difference between HCO₃^-(in→out)-Cl^-(out→in) and HCO₃^-(out→in)-Cl^-(in→out) was 1.13 nmol·cm^-2·s^-1 for cells subjected to a 5-mM decrement in CO₂ content. This yields a rate coefficient for the ''physiological'' anion shift in sea turtle blood of 1.7 s^-1, indicating that the anion shift may require 2.6 s to reach 99% completion in vivo. The erythrocyte anion shift appears to be a potential rate-limiting step for capillary CO₂ exchange in these turtles.