Differences in the regional distribution and response to ischaemia of adenosine-regulating enzymes in the heart

The activities of the adenosine-generating enzyme 5'-nucleotidase and the adenosine-degrading enzyme adenosine deaminase were determined for four regions of rat hearts prior to and following 10-60 min of ischaemia. Whereas adenosine deaminase was uniformly active throughout the heart, 5'-nucleotidase was twice as active in atrial than in ventricular myocardium, and more active in the right than in the left ventricles in normoxic tissues. In isolated heart preparations normoxic perfusion decreased adenosine deaminase and increased 5'-nucleotidase activity compared to levels in vivo. Global ischaemia for 10 min elevated adenosine deaminase activity but had no effect on 5'-nucleotidase activity. However, 30 min of ischaemia decreased 5'-nucleotidase activity by 50% in all regions of the heart. These changed levels were not altered by 10 min of reperfusion. The fall in 5'-nucleotidase activity with ischaemia occurred only in the 90% of this enzyme which is membrane-bound. The reasons for the marked differences in distribution and responses to ischaemia of these two enzymes have yet to be elucidated but metabolic inhibitors seem unlikely to be involved.