Creatine transport into red blood cells

The permeation of creatine from plasma into normal human red blood cells was investigated by means of 1-[14C]-creatine. Two statistically different Vmax and Km values were found for lower and for higher creatine concentrations of the plasma, respectively, indicating two types of transport with different affinities and capacities. It is suggested that the high affinity process, which accounts for 1/4 of the capacity and has an affinity constant of 0.087 +/- 0.032 mM is an active transport, while the low affinity transport represents an exchange diffusion. There is little, if any, effect of pH in the range of 6.9-7.9 on the transport. The total creatine concentration of the red cells did not change significantly even with high creatine concentration of the plasma during 6 h incubation at 37 degrees C. The in vitro experiments showed a daily exchange of cellular creatine of 20%, the t0.5 being about 2.5 days.