Apocynin-induced vasodilation involves Rho kinase inhibition but not NADPH oxidase inhibition.
OBJECTIVE
The present study was designed to test the hypothesis that NADPH oxidase inhibition with apocynin would lower blood pressure and improve endothelial function in spontaneously hypertensive rats (SHRs). Although apocyin effectively dilated arterial segments in vitro, it failed to lower blood pressure or improve endothelial function. Further experiments were performed in normotensive rats and in NADPH oxidase subunit knock-out mice to test if apocynin-induced vasodilation depends on NADPH oxidase inhibition at all.


RESULTS
SHRs were treated with apocynin orally or i.v. Arterial pressure was recorded directly. Rat and mouse arterial function was investigated in vitro by small vessel wire myography. NADPH oxidase activity was measured in human granulocytes and in rat vascular preparations. Rho kinase activity was determined by Western blot analysis. Apocynin did not reduce arterial pressure acutely in SHR when given at 50, 100, or 150 mg kg(-1) day(-1) orally over 1-week intervals or when given i.v. Apocynin potently inhibited granulocyte NADPH oxidase but not vascular NADPH-oxidase-dependent oxygen radical formation unless exogenous peroxidase was added to vascular preparations. Apocynin dilated rat intrarenal and coronary arteries independently of pharmacological interventions that reduce vascular superoxide radical abundance and actions. Aortic rings from p47phox(-/-) mice were more sensitive to apocynin-induced dilation than wild-type aortic rings. Rho kinase inhibition reduced or prevented the inhibitory effect of apocynin on agonist-induced vasoconstriction and apocynin inhibited the phosphorylation of Rho kinase substrates.


CONCLUSIONS
Apocynin per se does not inhibit vascular NADPH-oxidase-dependent superoxide formation. Its in vitro vasodilator actions are not due to NADPH oxidase inhibition but may be explained at least in part by inhibition of Rho kinase activity. The discrepancy between apocynin-induced vasodilation in vitro and the failure of apocynin to lower arterial pressure in SHR suggests opposing effects on arterial pressure-regulating systems in vivo. Its use as a pharmacological tool to investigate vascular NADPH oxidase should be discontinued.
Schlüter
Torsten
T
Department of Physiology, University of Greifswald, Greifswalder Street 11c, D17495 Greifswald, Karlsburg, Germany.
Steinbach
Antje C
AC
Steffen
Anja
A
Rettig
Rainer
R
Grisk
Olaf
O
D000098:Acetophenones / Q000494:pharmacology*; D000367:Age Factors; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D004195:Disease Models, Animal; D004305:Dose-Response Relationship, Drug; D004730:Endothelium, Vascular / Q000187:drug effects* / Q000201:enzymology / Q000503:physiopathology; D005260:Female; D006098:Granulocytes / Q000187:drug effects / Q000201:enzymology; D006801:Humans; D006973:Hypertension / Q000188:drug therapy* / Q000201:enzymology / Q000503:physiopathology; D008297:Male; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D000074662:NADPH Oxidase 2; D019255:NADPH Oxidases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D047428:Protein Kinase Inhibitors / Q000494:pharmacology*; D051381:Rats; D011916:Rats, Inbred F344; D011918:Rats, Inbred SHR; D015398:Signal Transduction / Q000187:drug effects; D014664:Vasodilation / Q000187:drug effects*; D014665:Vasodilator Agents / Q000494:pharmacology*; D054460:rho-Associated Kinases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D020742:rhoA GTP-Binding Protein / Q000378:metabolism
D000098:Acetophenones; D008562:Membrane Glycoproteins; D047428:Protein Kinase Inhibitors; D014665:Vasodilator Agents; C056165:acetovanillone; C488842:Cybb protein, mouse; D000074662:NADPH Oxidase 2; D019255:NADPH Oxidases; C060505:neutrophil cytosolic factor 1; D054460:rho-Associated Kinases; D020742:rhoA GTP-Binding Protein