Within the past 10 years the field of signaling via extracellular nucleotides has witnessed a breath-taking expansion. Molecular cloning of receptors and physiological and pharmacological analyses revealed that essentially every cell in a vertebrate organism carries surface-located receptors for ATP (Abbracchio and Burnstock 1998; Fischer 1999). In addition, other nucleotides have been recognized as signaling molecules. These include ADP, UTP, UDP and a variety of diadenosine polyphosphates such as Ap4A to Ap5A (diadenosine tetraphosphate to diadenosine pentaphosphate; Miras-Portugal et al. 1998). Intercellular signaling pathways generally require mechanisms of signal inactivation. Besides receptor desensitization and receptor downregulation effective mechanisms for removing or inactivating the extracellular signaling molecule have evolved. These include cellular reuptake as in the case of catecholamines or amino acid transmitters or extracellular hydrolysis and salvage of hydrolysis products as in the case of peptides, acetylcholine or nucleotides. Nucleotides are hydrolyzed by an extracellular hydrolysis cascade that results in the formation of the respective nucleoside and free phosphate. The latter can be recycled by surrounding cells and reused for nucleotide resynthesis (Pastor-Anglada et al. 1998). In the case of adenine nucleotides the hydrolysis product adenosine can initiate additional receptor-mediated functions (Fredholm et al. 1996).