Species differences in adenosine metabolic sites in the heart

5'-Nucleotidase and purine nucleoside phosphorylase, two key enzymes in nucleoside metabolism, have been localized electronmicroscopically in left ventricular myocardium of the human, dog, pig, rabbit, guinea pig and rat. Ectonucleotidase activity was present in all species at the plasma membrane of pericytes. Reactive endothelial cells in the microcirculatory bed were restricted to those covering resistance arterioles. Cardiomyocytes were reactive only in the rat. Purine nucleoside phosphorylase was localized uniformly in the vascular endothelium of all species. The strongest activity was seen in the pericytes of guinea pig, rat and dog. Pericytes of rabbit and pig were virtually unreactive, whereas a minority of cells in human samples were positive. Cardiomyocytes were unreactive in all species. These variations in the distribution pattern of adenosine metabolic sites may have definite consequences for disposal and recovery of adenylates and their breakdown products in ischaemia and for the effects to be expected from interference with nucleoside transport inhibition.