In a previous report, we have demonstrated that simultaneous inhibition of nucleoside transport and adenosine deaminase accumulates endogenous adenosine and protects the myocardium against stunning. The differential cardioprotective effects of erythro-9(2-hydroxy-3-nonyl)-adenine (EHNA), a potent inhibitor of adenosine deamination but not transport, and p-nitrobenzylthioinosine (NBMPR), a selective blocker of adenosine and inosine transport, are not known. Thirty-seven anaesthetized adult dogs were instrumented to monitor left ventricular performance using sonomicrometery. Dogs were randomly assigned into four groups. The control group (n = 8) received only the vehicle solution. Treated groups received saline containing 100 microM EHNA (EHNA-group, n = 7), 25 microM NBMPR (NBMPR-group, n = 7), or a combination of 100 microM EHNA and 25 microM NBMPR (EHNA/NBMPR-group, n = 10). Hearts were subjected to 30 min of normothermic global ischaemia and 60 min of reperfusion while on bypass. Adenine nucleotides, nucleosides, oxypurines and NAD+ were determined in extracts of transmural myocardial biopsies using HPLC. TTC staining revealed the absence of necrosis in this model. Drug administration did not affect myocardial ATP metabolism and cardiac function in the normal myocardium. Ischemia caused about 50% ATP depletion and accumulation of nucleosides. The ratio between adenosine/inosine at the end of ischemia was 1:10, 1:1, 1:1 and 10:1 in the control, EHNA-, NBMPR- and EHNA/NBMPR-group, respectively. Upon reperfusion, both nucleosides washed out from the myocardium in the control and EHNA-group while retained in the myocardium in the NBMPR and EHNA/NBMPR groups. Ventricular dysfunction 'stunning' persisted in the control group (52%) and in the EHNA-treated group (32%) after 30 min of reperfusion. Significant improvement of function was observed in the EHNA group only after 60 min of reperfusion. LV function recovered in the NBMPR- and EHNA/NBMPR-treated groups during reperfusion. ATP recovery occurred only when animals were pretreated with the combination of EHNA/NBMPR and remained depressed in the control group and EHNA and NBMPR-treated groups. At post mortem, TTC staining revealed the absence of myocardial necrosis. Superior myocardial protection was observed with inhibition of nucleoside transport by NBMPR alone or in combination with inhibition of adenosine deaminase by EHNA. Selective blockade of nucleoside transport by NBMPR is more cardioprotective than inhibition of adenosine deaminase alone in attenuating myocardial stunning. It is not known why EHNA partially inhibit adenosine deaminase, in vivo.