Characterization of sugar transport in the pigeon red blood cell.

Sugar transport in pigeon red blood cells is mediated by two pathways. One is saturable, shows competition between sugars, is inhibited by phloretin and cytochalasin B, and shows many of the properties of 'carrier-mediated' transport, characterized in the human red blood cell. The other is not saturable, and shows no competition between sugars. The saturable pathway is virtually absent from freshly drawn cells, but may be stimulated by pre-incubation with 2 mM-NaCN, or by Ca and the ionophore A 23187. The non-saturating pathway is stimulated only slightly by CN, but considerably by Ca and A 23187. The inhibition of sugar transport by cytochalasin B is antagonized competitively by sugars acting at the inner surface of the membrane. External sugars have no effect, as in the human red blood cell (Widdas, 1980). The binding of cytochalasin B to the cells shows a limited number of high-affinity sites. These are unrelated to inhibition of sugar transport as binding, but not transport, is prevented by the presence of cytochalasin E.