Morphology and water permeability of red blood cells from green sea turtle (Chelonia mydas)

Abstract

The morphology and diffusional water permeability (P _d) of red blood cells (RBCs) from green sea turtle (GST) (Chelonia mydas) are presented for the first time. The RBCs had an ellipsoidal shape with full-axis lengths (diameters): D = 14.4 μm; d = 10.2 μm; h = 2.8 μm. The values of P _d (cm s⁻¹) were 5.1 × 10⁻³ at 15 °C, 5.7 × 10⁻³ at 20 °C, 6.3 × 10⁻³ at 25 °C, 6.8 × 10⁻³ at 30 °C, and 7.9 × 10⁻³ at 37 °C (i.e., significantly higher than in human RBCs in which it was measured to be 4.2 × 10⁻³ at 25 °C, 5.0 × 10⁻³ at 30 °C, and 6.2 × 10⁻³ at 37 °C). There was a lack of inhibition of P _d of GST RBCs by p-chloromercuribenzoate (PCMB), a well-known inhibitor of the RBC water channel proteins (WCPs). The activation energy of water diffusion (E _a,d) in GST RBCs was 15.0 ± 1.6 kJ mol⁻¹ which is lower than the E _a,d for human RBCs (~25 kJ mol⁻¹). These results indicate that in the membrane of GST RBCs, there were no WCPs that were inhibited by the mercurial reagent, while the lipid bilayer of this membrane is unusually permeable to water. This is likely to be a phylogenetically old trait, like that found in amphibians and even the later birds, all of which have nucleated erythrocytes; and it is also likely to be a result of the animal’s adaptation to a herbivorous diet (algae and seagrasses).