Mechanisms of nitrate tolerance: a review

Vascular redox signaling, eNOS uncoupling and endothelial dysfunction in the setting of transportation noise exposure or chronic treatment with organic nitrates

SIGNIFICANCE

Cardiovascular disease and drug-induced health side effects are frequently associated with - or even caused by - an imbalance between the concentrations of reactive oxygen and nitrogen species (RONS) and antioxidants respectively determining the metabolism of these harmful oxidants.

RECENT ADVANCES

According to the "kindling radical" hypothesis, initial formation of RONS may further trigger the additional activation of RONS formation under certain pathological conditions. The present review will specifically focus on a dysfunctional, uncoupled endothelial nitric oxide synthase (eNOS) caused by RONS in the setting of transportation noise exposure or chronic treatment with organic nitrates, especially nitroglycerin. We will further describe the various "redox switches" that are proposed to be involved in the uncoupling process of eNOS.

CRITICAL ISSUES

In particular, the oxidative depletion of tetrahydrobiopterin (BH4), and S-glutathionylation of the eNOS reductase domain will be highlighted as major pathways for eNOS uncoupling upon noise exposure or nitroglycerin treatment. In addition, oxidative disruption of the eNOS dimer, inhibitory phosphorylation of eNOS at threonine or tyrosine residues, redox-triggered accumulation of asymmetric dimethylarginine (ADMA) and L-arginine deficiency will be discussed as alternative mechanisms of eNOS uncoupling.

FUTURE DIRECTIONS

The clinical consequences of eNOS dysfunction due to uncoupling on cardiovascular disease will be summarized also providing a template for future clinical studies on endothelial dysfunction caused by pharmacological or environmental risk factors.

FK409, a new nitric-oxide donor, suppresses smooth muscle proliferation in the rat model of balloon angioplasty

The effect of FK409, a new nitric-oxide (NO) donor, on neointimal formation of rat carotid arteries following balloon injury was studied. The intimal thickening at 14 days was strongly suppressed by twice daily administration of FK409 at 10 mg/kg from 2 days before to 13 days after injury. The neointima area and neointima/media ratio were decreased by 48.0% (P < 0.01) and 38.5% (P < 0.01), respectively, compared with control. On the other hand, isosorbide dinitrate (ISDN), a classical nitro-vasodilator, did not suppress intimal thickening even at 100 mg/kg twice a day. An in vivo 5-bromo-2'-dedoxyuridine (BrdU) uptake study revealed that FK409 inhibited the proliferative response of smooth muscle cells (SMC) in media at early stage of injury. In fact, the neointimal formation at 14 days was inhibited by the short term administration of FK409 only from the day of injury to 4 days after at 10 mg/kg twice a day. In cultured rat SMC, FK409 (1-10 mumol/l) markedly enhanced intracellular c-GMP and inhibited the proliferation in 10% FBS-containing medium. These results suggest that FK409 suppresses intimal thickening following balloon injury of the rat carotid artery by inhibition of SMC proliferation.

Tolerance to the vascular effect of a novel nitric oxide-donating vasodilator, FK409

We investigated whether tolerance develops to the vasorelaxant effects of a new vasodilator, (+-)-(E)-4-ethyl-2-[(E)-hydroxy-imino]-5-nitro-3-hexenamide (FK409), in isolated canine coronary artery strips and to its hypotensive effect in rats, and whether FK409 activates soluble guanylate cyclase isolated from vascular tissues in the absence of L-cysteine. No tolerance to FK409 (0.46 nM to 0.46 microM or 1-1000 micrograms/kg, i.v.) or cross-tolerance between FK409 and glyceryl trinitrate was demonstrated in in vitro and in vivo experiments, whereas the tolerance to glyceryl trinitrate (0.44 nM to 4.4 microM or 1-1000 micrograms/kg, i.v.) was marked in both conditions. In addition, FK409 (0.1-10 microM) activated soluble guanylate cyclase without L-cysteine, but glyceryl trinitrate (1-100 microM) required the addition of L-cysteine (5 mM) for the activation of the enzyme. The results suggest that FK409 may be advantageous compared to tolerance-producing nitrates currently in clinical use, and that this property of FK409 is probably due to its independence of a sulfhydryl group donor.

Effects of continuous intravenous infusion of isosorbide dinitrate on development of tolerance to vasodilating action in human epicardial coronary arteries

This study was performed to determine the effects of long-term intravenous infusion on the coronary vasodilating actions of continuous intravenous and bolus intracoronary administration of isosorbide dinitrate (ISDN). With quantitative coronary angiography, the coronary diameter and the vasodilating response to intracoronary ISDN (1 mg) at angiographically normal segments were studied before and after intravenous administration of ISDN, 10 to 60 micrograms/min for 1 hour, 2 days, or 5 days. The vasodilating effects of intravenous ISDN were 72% +/- 13%, 65% +/- 21%, and 6% +/- 11% of the response to intracoronary ISDN in the baseline study in each group. Irrespective of the duration of intravenous infusion, subsequent intracoronary ISDN dilated coronary arteries to extent similar to that observed in each baseline study. In conclusion, significant coronary vasodilating effects of intravenous ISDN were observed after a 2-day infusion, whereas tolerance to the vasodilating effects apparently developed within 5 days of infusion. The vasodilating response to bolus intracoronary ISDN was preserved even when the vasodilating effects of intravenous ISDN were no longer present.

Pharmacokinetic optimisation of angiotensin converting enzyme (ACE) inhibitor therapy

Angiotensin converting enzyme (ACE) inhibitors are increasingly used to treat hypertension and congestive heart failure. Recently, several new ACE inhibitors with pharmacokinetic features different from earlier agents such as captopril or enalapril have come into use. This review discusses the use of pharmacokinetics to optimise ACE inhibitory therapy in various patient groups. Among the pharmacokinetic characteristics of ACE inhibitors the route of excretion and to a lesser degree the half-life appear to be the most clinically relevant. There is no evidence that being a prodrug offers a significant clinical advantage. The importance of varying tissue penetration also remains to be determined. Knowledge of ACE inhibitor pharmacokinetics is particularly important in patients with renal or hepatic dysfunction in whom the major route of excretion of these agents is impaired. This might also be the case in elderly patients or those with severe congestive heart failure. However, for most ACE inhibitors, major changes in the drug dosage (amount or interval) are necessary only when the glomerular filtration rate falls below 30 ml/min (1.80 L/h). The occurrence of adverse effects due to overdosage or drug interactions may be prevented by adapting the prescription of an ACE inhibitor to its pharmacokinetic characteristics.

Factors affecting the regional haemodynamic responses to glyceryl trinitrate and molsidomine in conscious rats

1. A series of experiments was performed in conscious, unrestrained, male, Long Evans rats, chronically instrumented for the measurement of regional haemodynamics. 2. Infusion of glyceryl trinitrate (GTN, 0.1 mg kg-1 min-1, i.v.) for 10 min elicited tachycardia, but no sustained change in mean arterial blood pressure. Renal haemodynamics were unaffected, but there were reductions in hindquarters flow and vascular conductance together with substantial increases in flow and conductance in the mesenteric vascular bed. 3. In the presence of captopril (2 mg kg-1 bolus, and 1 mg kg-1 h-1 infusion, i.v.) GTN elicited significant hypotension and increases in renal blood flow and vascular conductance, indicating that activation of the renin-angiotension system opposed the dilator effects of GTN in this vascular bed. However, the mesenteric and hindquarters haemodynamic effects of GTN were not affected by captopril. In contrast, in the presence of enalaprilat (2 mg kg-1 bolus, and 1 mg kg-1 h-1 infusion, i.v.) there was significant enhancement of the mesenteric, as well as renal, haemodynamic effects of GTN. Hence, these results provide no evidence for the sulphydryl groups in captopril exerting a specific effect to enhance the haemodynamic actions of GTN in our experimental protocols. 4. Administration of molsidomine alone (1 mg kg-1, i.v. bolus) elicited tachycardia and hypotension; there were no changes in mesenteric or hindquarters haemodynamics, but renal flow and vascular conductance fell. Thus, the hypotensive effect of molsidomine was probably due to a reduction in cardiac output, consequent upon venodilatation. 5. In the presence of captopril or enalaprilat, molsidomine evoked renal and mesenteric vasodilatations in association with hypotension, indicating that activation of the renin-angiotensin system contributed to the lack of vasodilator responses to administration of molsidomine alone. However, since the effects of enalaprilat were more marked than those of captopril (in spite of the dose of both drugs being supramaximal for inhibition of angiotensin-converting enzyme), other factors must have been involved. 6. In a separate experiment, pretreatment with the nitric oxide synthesis inhibitor, N0-nitro-L-arginine methyl ester (1 mg kg- 1 h-1, i.v.), enhanced the mesenteric vasodilator effect of molsidomine. Collectively, these results are consistent with in vitro data showing that endogenous nitric oxide can inhibit the vasodilator effects of nitric oxide derived from molsidomine, and that the sulphydryl groups of captopril can protect endogenous nitric oxide from inactivation by oxygen-derived free radicals, thereby enhancing the inhibitory effect of endogenous nitric oxide on the vasodilator responses to exogenous nitric oxide derived from molsidomine (or GTN).

Comparison of the effects of the novel vasodilator FK409 with those of nitroglycerin in isolated coronary artery of the dog

1. The vasorelaxant effects of FK409, a new nitrovasodilator synthesized from a microbial product, were compared with those of nitroglycerin in isolated coronary artery rings of the dog contracted with U46619 (10(-7) M). 2. FK409 (10(-11)-10(-5) M) and nitroglycerin (10(-9)-10(-4) M) each produced a concentration-dependent relaxation. Comparison of EC50 values showed that FK409 was about 25 times more potent than nitroglycerin. 3. Submaximum concentrations of nitroglycerin (10(-6) M) and FK409 (3 x 10(-8) M) elevated guanosine 3':5'-cyclic monophosphate (cyclic GMP) levels, effects associated with vasorelaxation. Adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels were unaffected. 4. The concentration-relaxation curves for nitroglycerin and FK409 were shifted to the right by methylene blue (3 x 10(-6) - 3 x 10(-5) M), an inhibitor of soluble guanylate cyclase, and to the left by M&B22,948 (3 x 10(-6) - 3 x 10(-5) M), an inhibitor of cyclic GMP phosphodiesterase. 5. After exposure of coronary arteries to the maximally-effective concentration of nitroglycerin (10(-4) M), the mean EC50 value of FK409 did not change significantly, although that of nitroglycerin increased about 60 fold. After exposure to the maximally-effective concentration of FK409 (10(-5) M), the mean EC50 value of FK409 increased about 6 fold and that of nitroglycerin about 11 fold. 6. These results suggest that the vasorelaxant effect of FK409, like that of nitroglycerin, is due to activation of soluble guanylate cyclase and a resultant increase in intracellular cyclic GMP. However, compared with nitroglycerin, there was less self-tolerance to the relaxant effects of FK409 and relatively little cross-tolerance between the two agents.