Aims: Improving NO bioavailability is essential to improve post-ischemic recovery in functional and structural aspects (inhibition of adverse remodelling). Increasing arginine supplementation of post infarct patients however is associated with higher postinfarction mortality (VINTAGE-MI trial). Ischemia induces arginase and shifts the arginine metabolism from NO synthesis to polyamine metabolism a process that is further enhanced by increasing the ODC/eNOS expression ratio in this setting. This study hypothesized that post-ischemic arginine supplementation feeds the polyamine metabolism that reduces arginine bioavailability for eNOS. Therefore, we investigated whether arginase inhibition during or after ischemia modifies post-ischemic recovery of the heart in a favourable way.

Methods: 

Isolated rat hearts (n=12 per group) were exposed to 45 min global warm no-flow ischemia and 120 min of reperfusion. Arginase inhibition was performed by administration of Nor-NOHA. Controls received no additional supplement. Arginase expression was analyzed by qRT-PCR and immunoblot technique. Isolated cardiomyocytes and non-myocytes were used to identify the cell type of arginase expression. Arginase expression was also analyzed in rats undergoing in vivo ischemia (30 min) and reperfusion (1–7 days).

Results: 

Arginase 1 expression was increased 2-fold during reperfusion. Inhibition of arginase increased arginase 1 mRNA expression 6.3- and 7.4-fold if the inhibitor was given prior to ischemia or exclusively during reperfusion, respectively and 1.3-fold on the protein level. Arginase 1 mRNA expression and the ODC/eNOS ratio were significantly enhanced in rats 1 day after infarction but not after 7 days. Functional recovery (left ventricular developed pressure and rate pressure product) were significantly enhanced in hearts in which arginase was inhibited during the onset of reperfusion but reduced in hearts in which the enzyme was blocked during ischemia. Coronary resistance was not affected nor was the increase of heart wet weight during reperfusion (oedema formation) modified by arginase inhibition. Arginase 1 was preferentially expressed in cardiac non-myocytes in mice but equally expressed in non-myocytes and cardiomyocytes in rats.

Conclusion: 

During ischemia arginine is accumulating due to the functional loss of eNOS in the absence of oxygen. Arginase converts arginine to ornithine that can either be converted to citrulline and serving as a buffer for arginine in the reperfused heart or further converted to putrescine and improve directly early post-infarct recovery. However inhibition of the enzyme leads to accumulation of arginine that attenuates eNOS activity during reperfusion and reduces the substrate for polyamine metabolism so that both can no longer contribute to improved recovery. Inhibition during the onset of reperfusion, however, attenuates the shift of arginine metabolism from eNOS to polyamine metabolism and this improves the outcome. Thus, timing of arginase inhibition is the most critical point in possible beneficial effect of arginase inhibition. Although the principle to shift arginine metabolism back to NO formation has been successfully shown we still need to identify the best pharmacological tool to use this knowledge.