Fig. 3. Extracellular adenosine uptake. Extracellular adenosine is taken up from the extracellular to the intracellular space  via nucleoside transporters. Functionally, extracellular adenosine uptake is mainly achieved through equilibrative nucleoside transporters (ENTs), ENT1 and ENT2. These transporters represent diffusion-limited channels that allow adenosine to freely cross the cell membrane after a concentration gradient.