Specificity of different organic nitrates to elicit NO formation in rabbit vascular tissues and organs in vivo

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Abstract
1 In the present study we assessed the formation of nitric oxide (NO) from classical and thiol‐containing organic nitrates in vascular tissues 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 isolated cytochrome P450 on NO formation from organic nitrates was investigated. 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‐cysteine ethylester (SPM 3672), S‐carboxyethyl‐N‐(3‐nitratopivaloyl)‐L‐cysteine ethylester (SPM 5185), at 10 mg kg−1 each. After 20 min the animals were killed, blood vessels and organs were removed, and subsequently analyzed for spin‐trapped NO by cryogenic electron spin resonance (e.s.r.) spectroscopy. 3 In the saline‐treated control group, NO remained below the detection limit in all vessels and organs. In contrast, all of the nitrates tested elicited measurable NO formation, which was higher in organs (liver, kidney, heart, lung, spleen) (up to 4.8 nmol 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 mesenteric bed and the vena cava, while the SPM compounds elicited comparable NO formation in veins and arteries. 4 Using a similar spin trapping technique, NO formation was assessed in vitro in phenylephrine‐precontracted rabbit aortic rings. The maximal relaxation elicited by a first exposure (10 min) to NTG (0.3 to 10 μm) was positively correlated (r=0.8) with the net increase (NTG minus basal) of NO spin‐trapped during a second exposure to the same concentration of NTG in the presence of DETC. 5 Cytochrome P450 purified from rabbit liver enhanced NO formation in a NADPH‐dependent fashion from NTG, but not from the other nitrates, as assessed by activation of purified soluble guanylyl cyclase. 6 We conclude that the vessel selective action of different organic nitrates in vivo reflects differences in vascular NO formation. Thus, efficient preload reduction by classical organic nitrates can be accounted for by higher NO formation in venous capacitance as compared to arterial conductance and resistance vessels. In contrast, NO is released from cysteine‐containing nitrates (SPMs) to a similar extent in arteries and veins, presumably independently of an organic nitrate‐specific biotransformation. Limited tissue bioavailability of NTG and ISDN might account for low NO formation in the aorta, while true differences in biotransformation seem to account for differences in NO formation in the other vascular tissues.

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