Platelet nitric oxide and superoxide release during the development of nitrate tolerance: effect of supplemental ascorbate.
BACKGROUND
The therapeutic benefits that accompany the continuous administration of organic nitrates are attenuated by the development of tolerance to the compounds. Altered superoxide production and NO bioavailability have been implicated in contributing to the development of tolerance, an effect that may be ameliorated by the administration of antioxidants.


RESULTS
We studied the effect of 3 days of continuous transdermal administration of nitroglycerin (NTG) (10 mg/24 hours) on platelet free radical (NO and superoxide anion [O2*-] activity) with and without coadministration of supplemental ascorbate (2.4 g/24 hours). NAD(P)H oxidase activity, nitric oxide synthase (NOS) activity, and cyclic guanosine monophosphate (cGMP) content were also assessed. Radial artery pressure pulse waveforms were used to track the hemodynamic actions of NTG. Three days of NTG/placebo was associated with a significant increase in platelet NO and O2*- production from 1.0+/-1.17 to 2.52+/-0.88 pmol/10(8) platelets and 13.2+/-4.8 to 72.5+/-34.4 pmol/10(8) platelets, respectively (P<0.01 for both). These changes were accompanied by increased platelet NADH oxidase activity from 47.9+/-11.0 to 65.3+/-13.6 pmol O2*- min/mg protein and cGMP content from 0.60+/-0.10 to 0.89+/-0.16 pmol/10(9) platelets (P<0.05 for both). Administration of NTG/ascorbate attenuated both NO and O2*- release in platelets.


CONCLUSIONS
Three days of continuous transdermal administration of NTG was accompanied by increased platelet NO and O2*- production and NADH oxidase activity that was suppressed by coadministration of oral ascorbate. Although a significant degree of tolerance would be expected during continuous nitrate administration, a residual hemodynamic action could be identified by arterial pulse contour analysis.
McVeigh
Gary E
GE
Department of Therapeutics and Pharmacology, The Queen's University of Belfast, North Ireland, UK. g.mcveigh@qub.ac.uk
Hamilton
Paul
P
Wilson
Martin
M
Hanratty
Colm G
CG
Leahey
William J
WJ
Devine
Adrian B
AB
Morgan
David G
DG
Dixon
Lana J
LJ
McGrath
Lawrence T
LT
D000328:Adult; D000975:Antioxidants / Q000494:pharmacology*; D001205:Ascorbic Acid / Q000097:blood / Q000494:pharmacology*; D001792:Blood Platelets / Q000378:metabolism*; D018592:Cross-Over Studies; D006152:Cyclic GMP / Q000096:biosynthesis; D004311:Double-Blind Method; D004361:Drug Tolerance; D005609:Free Radicals / Q000378:metabolism*; D006439:Hemodynamics / Q000187:drug effects; D006801:Humans; D008297:Male; D008875:Middle Aged; D009243:NAD / Q000378:metabolism; D019255:NADPH Oxidases / Q000378:metabolism; D009566:Nitrates / Q000627:therapeutic use; D009569:Nitric Oxide / Q000096:biosynthesis; D019001:Nitric Oxide Synthase / Q000378:metabolism; D009573:Nitrites / Q000032:analysis; D005996:Nitroglycerin / Q000008:administration & dosage / Q000494:pharmacology*; D013481:Superoxides / Q000378:metabolism; D013997:Time Factors; D014665:Vasodilator Agents / Q000008:administration & dosage / Q000494:pharmacology*
D000975:Antioxidants; D005609:Free Radicals; D009566:Nitrates; D009573:Nitrites; D014665:Vasodilator Agents; D009243:NAD; D013481:Superoxides; D009569:Nitric Oxide; D019001:Nitric Oxide Synthase; D019255:NADPH Oxidases; D005996:Nitroglycerin; D006152:Cyclic GMP; D001205:Ascorbic Acid