A dopamine transporter in human erythrocytes: modulation by insulin.

Red blood cells are capable of transport and accumulation of catecholamines. The aim of this paper is to characterize the catecholamine transport system in the human red blood cell and in particular that of dopamine. Dopamine, noradrenaline and adrenaline enter the red blood cell by a similar process, which shows saturation kinetics with Vmax values of 0.54 +/- 0.12, 0.48 +/- 0.08 and 0.63 +/- 0.13 mumol (1 cells)-1 min-1, respectively, and K(m) values of 15.62 +/- 1.19, 5.81 +/- 1.19 and 12.00 +/- 2.97 nM, respectively. Observations based on the dependence of dopamine influx on the transmembrane H+ gradient, and the effect of transport inhibitors such as DMA (dimethyl-amiloride), DIDS (4,4'-diisothiocyanatostilbene 2,2'-disulphonic acid), reserpine, GBR 12909 (1-(2-(di(4-fluoro-phenyl)-methoxy)-ethyl)-4-(3-phenylpropyl)piperazine) , GBR 12935 (1-(2-(diphenyl-methoxy)-ethyl)-4-(3-phenyl-propyl)piperazine), and cyanine suggest that catecholamine transport is not mediated by the Na(+)-H+ exchanger, the anion exchanger or a system similar to that responsible for dopamine uptake in either synaptosomes or the proximal tubule. However, choline inhibits the influx of dopamine with an IC50 value of 17 microM and stimulates the efflux of dopamine with a K(m) value of 8.20 microM. These results strongly suggest that dopamine is transported by the choline exchanger previously reported to be present in red blood cells. Probenecid inhibits dopamine uptake with an IC50 of 0.63 microM. The presence of insulin receptors in human red blood cells, and the relationship between insulin and catecholamine levels in the plasma led us to investigate the effect of insulin on catecholamine transport. In fasting subjects, dopamine, adrenaline and noradrenaline influxes were higher than in fed subjects. Furthermore, the addition of exogenous insulin to red blood cells from fasting subjects significantly reduced the influx of catecholamines while no effect was observed when insulin was added to red blood cells obtained from fed subjects. The present study shows that catecholamines, and in particular dopamine, are transported in red blood cells via an exchanger which is possibly the choline transport system. The activity of this transporter is regulated by insulin. These results support a role for red blood cells as a storage pool for circulating catecholamines.