Transport of Bicarbonate, other Ions and Substrates Across the Red Blood Cell Membrane of Hagfishes

Summary

We review here the membrane transport studies that have appeared for the red cells of hagfishes Myxine glutinosa and Eptatretus stoutii. In the first part the transport of substrates of energy metabolism across the red blood cell membrane is considered. Hagfish red cells possess highly efficient transport systems for glucose, pyruvate and several amino acids. In the case of glucose, this property may be related to the animal’s hypoxia tolerance and its capacity for anaerobic glycolysis. Transport systems for small inorganic ions include the Na⁺, K⁺-ATPase and a K⁺-C1^- symport requiring the presence of Na⁺. A striking feature is the absence of a Cl^--HCO₃ ^--exchanger, which constitutes in almost all red cells, except those of hagfishes and lampreys, the major pathway for Cl^- and for HCO₃ ^-. Consequently, Cl^- permeability in hagfish red cells is orders of magnitude lower than in mammalian and most other red cells. We report our mass spectrometric determinations of hagfish red blood cell permeability for HCO₃ ^- and find its value to be not significantly different from zero. This implies that CO₂ transport in hagfish blood operates quite differently from that in the blood of most other species. Hagfish red cells, due to the presence of intracellular haemoglobin and carbonic anhydrase, can rapidly convert the CO₂ taken up to HCO₃ ^- and H⁺, but they cannot transfer most of the HCO₃ ^- to the plasma, as red cells of most species do.