SPECIFICITY OF DIFFERENT ORGANIC NITRATES TO ELICIT NO FORMATION IN RABBIT VASCULAR TISSUES AND ORGANS IN-VIVO

1 In the present study we assessed the formation of nitric oxide (NO) from classical and thiol-containing organic nitrates in vascular tissu es and organs of anaesthetized rabbits, and established a relationship between the relaxant response elicited by nitroglycerin (NTG) and NO formation in the rabbit isolated aorta. Furthermore, the effect of iso lated cytochrome P450 on NO formation from organic nitrates was invest igated. 2 Rabbits received diethyldithiocarbamate (DETC; 200 mg kg(-1) initial bolus i.p. and 200 mg kg(-1) during 20 min, i.v.) and either saline, or one of the following organic nitrates: nitroglycerin (NTG, 0.5 mg kg(-1)), isosorbide dinitrate (ISDN), N-(3-nitratopivaloyl)-L-c ysteine ethylester (SPM 3672), S-carboxyethyl-N-(3-nitratopivaloyl)-L- cystine ethylester (SPM 5185), at 10 mg kg(-1) each. After 20 min the animals were killed, blood vessels and organs were removed, and subseq uently analyzed for spin-trapped NO by cryogenic electron spin resonan ce (e.s.r.) spectroscopy. 3 In the saline-treated control group, NO re mained below the detection limit in all vessels and organs. In contras t, all of the nitrates tested elicited measurable NO formation, which was higher in organs (liver, kidney, heart, lung, spleen) (up to 4.8 n mol g(-1) 20 min(-1)) than in blood vessels (vena cava, mesenteric bed , femoral artery, aorta) (up to 0.7 nmol g(-1) 20 min(-1)). Classical organic nitrates (NTG, ISDN) formed NO preferentially in the mesenteri c bed and the vena cava, while the SPM compounds elicited comparable N O formation in veins and arteries. 4 Using a similar spin trapping tec hnique, NO formation was assessed in vitro in phenylephrine-precontrac ted rabbit aortic rings. The maximal relaxation elicited by a first ex posure (10 min) to NTG (0.3 to 10 mu M) was positively correlated (r = 0.8) with the net increase (NTG minus basal) of NO spin-trapped durin g a second exposure to the same concentration of NTG in the presence o f DETC. 5 Cytochrome P450 purified from rabbit liver enhanced NO forma tion in a NADPH-dependent fashion from NTG, but not from the other nit rates, as assessed by activation of purified soluble guanylyl cyclase. 6 We conclude that the vessel selective action of different organic n itrates in vivo reflects differences in vascular NO formation. Thus, e fficient preload reduction by classical organic nitrates can be accoun ted for by higher NO formation in venous capacitance as compared to ar terial conductance and resistance vessels. In contrast, NO is released from cysteine-containing nitrates (SPMs) to a similar extent in arter ies and veins, presumably independently of an organic nitrate-specific biotransformation. Limited tissue bioavailability of NTG and ISDN mig ht account for low NO formation in the aorta, while true differences i n biotransformation seem to account for differences in NO formation in the other vascular tissues.