L-ANTIGENS OF SHEEP RED-BLOOD-CELL MEMBRANES AND MODULATION OF ION-TRANSPORT

Sheep are polymorphic with respect to the intracellular Na+ and K+ con centrations of their erythrocytes. Erythrocytes of sheep of the high-K + (HK) phenotype have high K+ and low Na+ concentrations; erythrocytes from sheep of the allelic low-K+ (LK) phenotype have abnormally low K + and high Na+ concentrations. The difference is due to differences in rates of cation transport: higher Na+-K+ pump flux in HK cells and hi gher K+-Cl- cotransport in LK cells. The HK/LK polymorphism is associa ted with a polymorphism of red blood cell antigens: the L antigen is o nly on LK cells, and HK cells have only the M antigen. There are two c lasses of L antigen that assort together: L(p), which is associated wi th Na+-K+ pumps, and L(l), which is associated with K+-Cl- cotransport ers. There are functional consequences of these associations: anti-L(p ) antibody stimulates the pump and anti-L(l) antibody inhibits cotrans port. The use of these antibodies has permitted delineation of the rol es of the antigens in modulating the function of the transporters. In this review we summarize the evidence that these antigens are entities distinct from the pump. The L(p) antigen reacts reversibly with the N a+-K+ pump; the antigen inhibits the pump, mainly by promoting nonspec ific inhibition by intracellular K+. The antigen also modulates pump d ifferentiation in immature cells. In contrast, the L(l) antigen stimul ates K+-Cl- cotransport. The evidence suggests that the two polymorphi sms are controlled by a single genetic locus and that all of the disti nct properties of ion transporters in LK cells are attributable to int eractions with L antigens.