Adenosine and hypoxanthine transport in horse erythrocytes: Evidence for a polymorphism in the transport of hypoxanthine via a sodium-dependent cotransporter

The inward transport of two purines, adenosine and hypoxanthine, at 37 degrees C by horse erythrocytes was compared. No mediated transport of adenosine was detected in horse erythrocytes, nor was saturable, high-affinity binding of the potent facilitated-diffusion inhibitor nitrobenzylthioinosine demonstrable in horse erythrocyte membranes. In contrast, erythrocytes from most horses possessed a saturable sodium-dependent hypoxanthine transporter (apparent K-m, 100 +/- 28 mu M; V-max, 0.20 +/- 0.08 mmol (1 cells)(-1) h(-1); means +/- S.E.M., n = 5). Guanine inhibited hypoxanthine influx (apparent K-i, 24 +/- 6 mu M), but adenine and xanthine had no effect. Unlike human erythrocytes, no sodium-independent hypoxanthine transporter was detected in horse erythrocytes. There are, however, a small number of animals (similar to 15 %) whose erythrocytes fail to transport hypoxanthine. This variation appears to be under genetic control, but the precise nature of the control is unknown.