ACTIVATION OF NITRIC-OXIDE SYNTHASE BY ISCHEMIA IN THE PERFUSED HEART

Objective: Recent data have indicated that the activity of nitric oxid e synthase (NO synthase), the enzyme producing NO from L-arginine, cou ld be modified by ischaemia. The aim of the present work was therefore to study whether ischaemia activated NO synthase. Methods: NO synthas e activity was measured by the conversion of radioactive arginine into citrulline in extracts of isolated perfused rabbit hearts submitted t o low-flow ischaemia and reperfusion. Results: When measured in heart homogenates, NO synthase activity was significantly increased during i schaemia. This activation was already detectable after 5 min of ischae mia and was maintained during the whole ischaemic period. After cell f ractionation, NO synthase was recovered in cytosolic and membrane frac tions. The increase in NO synthase activity by ischaemia was related t o an activation of the cytosolic activity, white the membrane-bound NO synthase activity remained constant. Conclusion: NO synthase activity in the heart is rapidly stimulated by ischaemia and this stimulation is maintained during the whole ischaemic episode. This activation is f ound only in the cytosolic fraction, whereas the particulate activity is not affected by ischaemia.