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Aims  Myeloperoxidase ( MPO ) catalyses the formation of a wide variety of oxidants, including hypochlorous acid ( HOC l), and contributes to cardiovascular disease progression. We hypothesized that during its action  MPO  evokes substantial vasomotor responses.    Methods  Following exposure to  MPO  (1.92 mU mL −1 ) in the presence of increasing concentrations of hydrogen peroxide (H 2 O 2 ), changes in arteriolar diameter of isolated gracilis skeletal muscle arterioles ( SMA s) and coronary arterioles ( CA s) and in the isometric force in basilar arteries ( BA s) of the rat were monitored.    Results  Myeloperoxidase increased vascular tone to different degrees in  CA s,  SMA s and  BA s. The mechanism of increased vasoconstriction was studied in detail in  SMA s.  MPO ‐evoked vasoconstrictions were prevented by the  MPO  inhibitor 4‐aminobenzhydrazide (50  μ  m ), by endothelium removal in the  SMA s. Surprisingly, the  HOC l scavenger L‐methionine (100  μ  m ), the thromboxane A2 ( TXA 2) antagonist  SQ ‐29548 (1  μ  m ) or the non‐specific cyclooxygenase ( COX ) antagonist indomethacin (1  μ  m ) converted the  MPO ‐evoked vasoconstrictions to pronounced vasodilations in  SMA s, not seen in the presence of H 2 O 2 . In contrast to noradrenaline‐induced vasoconstrictions, the  MPO ‐evoked vasoconstrictions were not accompanied by significant increases in arteriolar [Ca 2+ ] levels in  SMA s.    Conclusion  These data showed that H 2 O 2 ‐derived  HOC l to be a potent vasoconstrictor upon  MPO  application.  HOC l activated the  COX  pathway, causing the synthesis and release of a  TXA 2‐like substance to increase the Ca 2+  sensitivity of the contractile apparatus in vascular smooth muscle cells and thereby to augment H 2 O 2 ‐evoked vasoconstrictions. Nevertheless, inhibition of the  HOC l– COX – TXA 2 pathway unmasked the effects of additional  MPO ‐derived radicals with a marked vasodilatory potential in  SMA s.

Myeloperoxidase evokes substantial vasomotor responses in isolated skeletal muscle arterioles of the rat