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 microM; Vmax, 0.20 +/− 0.08 mmol (l cells)‐1 h‐1; means +/− S.E.M., n = 5). Guanine inhibited hypoxanthine influx (apparent Ki, 24 +/− 6 microM), 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 (approximately 15%) whose erythrocytes fail to transport hypoxanthine. This variation appears to be under genetic control, but the precise nature of the control is unknown.