The Hydrolysis of Extracellular Adenine Nucleotides by Arterial Smooth Muscle Cells. Regulation of Adenosine Production at the Cell Surface

The extracellular reaction sequence ATP----ADP----AMP----adenosine participates in regulating the time course of cellular response during crisis or signaling events, such as thrombus formation or neurotransmission. We have investigated the whole time course of hydrolysis of ATP to adenosine by recirculating adenine nucleotide substrates over smooth muscle cells attached to polystyrene beads. Kinetic parameters were estimated for each reaction by fitting observed time courses to models of the pathway. In spite of the inhibition of 5'-nucleotidase by ADP, adenosine was produced very rapidly by smooth muscle cells. Comparisons of the apparent Km values of ADPase and 5'-nucleotidase (determined from experiments in which each substrate was used as the initial substrate with Km values observed when each substrate was supplied from the upstream reaction) suggest that the local concentrations of substrate supplied from the preceding reactions are very much higher than those in the bulk phase. This enhancement of efficiency overcomes the effect of the feed-forward inhibition to give rise to very rapid adenosine production from ADP or ATP. These observations are in marked contrast to our previous findings with endothelial cells (Gordon, E. L., Pearson, J. D., and Slakey, L. L. (1986) J. Biol. Chem. 261, 15496-15504), on which feed-forward inhibition causes a profound lag in adenosine production from adenine nucleotides and on which there are no apparent surface effects on substrate delivery.