Cation transport and regulatory volume increase in red blood cells of northern fur seals (Callorhinus ursinus)

Abstract

We investigated the membrane transport of Na and K ions in red blood cells (RBCs) of northern fur seal (Callorhinus ursinus) by measurement of unidirectional fluxes. Like red blood cells of other carnivores, those of northern fur seal contain high Na and low K concentrations, which result from the lack of Na–K ATPase activity on their membranes. In physiological conditions, activities of bumetanide-sensitive Na, K–Cl cotransport and amiloride-sensitive Na/H exchange were measured. K–Cl cotransport and Na–Cl cotransport were not detected. Hypertonicity activated only Na/H exchange. We further examined the ion transport systems for regulatory volume increase (RVI) in red blood cells. In the hyperosmotic condition, shrunken RBCs restored their original cell volume in Na medium but not in Na-free medium, and this restoration with Na medium was inhibited by amiloride. From these results, it is suggested that RVI in northern fur seal RBCs are performed by amiloride-sensitive Na/H exchanger but not Na, K–Cl cotransporter.