Kinetics of adenine nucleotide catabolism in coronary circulation of rats
Fleetwood, G.; Coade, S.B.; Gordon, J.L.; Pearson, J.D.
American Journal of Physiology 256(6 Pt 2): H1565-H1572
1989
ISSN/ISBN: 0002-9513 PMID: 2544108 Document Number: 333433
We have used the rat isolated, perfused heat to study the metabolism of adenine nucleotides on a single passage through the coronary circulation. Low doses (3-30 nmol) of ATP, ADP, or AMP injected as a bolus were extensively catabolized by ectoenzymes. Increasing doses of each nucleotide demonstrated saturability of catabolism that occurred at significantly lower doses of AMP than of ADP or ATP. The patterns of catabolites formed in each case were consistent with the major pathway of metabolism being sequential dephosphorylation of ATP .fwdarw. ADP .fwdarw. AMP .fwdarw. adenosine, although from experiments in which [3H]ATP was co-injected with unlabeled ADP, it appears that some direct conversion of ATP .fwdarw. AMP can occur. Furthermore, particularly in the presence of excess unlabeled ATP, [3H]ADP was phosphorylated to [3H]ATP, indicating that ectoenzymes capable of interconverting nucleotides are present. By evaluating recovery and metabolism in serial samples collected rapidly after bolus injection, we were able to use the integrated form of the Michaelis-Menten equation as developed by Bronikowksi et al. (Math. Biosci. 61: 237-266, 1982) to derive Michaelis constant (Km) and maximum velocity time capillary plasma volume (Amax) values for adenosinetriphosphatase, adenosine diphosphatase, and 5'-nucleotidase (450, 300, and 93 .mu.M; and 5.3, 5.9, and 1.7 .mu.mol/min, respectively). This analysis also indicated that there is high degree of heterogeneity of path lengths within the coronary circulation. We conclude that the main pathway for adenine nucleotide catabolism in the coronary bed is sequential dephosphorylation, most likely by distinct endothelial ectonucleotides that we have previously characterized in cultured endothelial cells from large vessels, and that this process, coupled with adenosine uptake by endothelium and red cells, regulates the circulating levels of vasoactive derivatives in the coronary circulation.