Mechanisms of nitrate-induced vasodilatation and tolerance

New insights into nitroglycerin effects and tolerance: role of calcitonin gene-related peptide

It has been shown that calcitonin gene-relate peptide plays an extensive role in cardiovascular system. CGRP is a potent vasodilator and plays an important role in mediation of nitroglycerin-induced vascular relaxation. Recently, calcitonin gene-relate peptide is emerging as a potential player in nitroglycerin tolerance. There is increasing evidence that the decreased depressor effect of nitroglycerin in tolerant states is closely related to a decrease in calcitonin gene-relate peptide release. The reduced release of calcitonin gene-relate peptide in nitroglycerin tolerance is associated with the decreased nitroglycerin biotransformation due to the mitochondrial dysfunction. Recent work has been shown that the inhibited activity of mitochondrial isoform of aldehyde dehydrogenase and the upregulation of phosphodiesterase 1A1 are the key factors that lead to the decreased nitroglycerin biotransformation in nitroglycerin tolerance, with a subsequently reduced release of calcitonin gene-relate peptide.

Melatonin inhibits nitrate tolerance in isolated coronary arteries

(1) The present study was designed to test the hypothesis that melatonin inhibits nitrate tolerance in coronary arteries. (2) Rings of porcine coronary arteries were suspended in organ chambers for isometric tension recording. Nitrate tolerance was induced by incubating the tissues with nitroglycerin (10(-4) M) for 90 min, followed by repeated rinsing for 1 h. Control rings that had not been exposed previously to nitroglycerin, but were otherwise treated identically, were studied simultaneously. The rings were contracted with U46619 (1-3 x 10(-9) M) and concentration-response curves to nitroglycerin (10(-9)-10(-4) M) were obtained. (3) Nitrate tolerance was evident by a 15- to 20-fold rightward shift in the concentration-response curve to nitroglycerin in rings with and without endothelium exposed previously to the drug for 90 min. Addition of melatonin (10(-9)-10(-7) M) to the organ chamber during the 90-min incubation period with nitroglycerin partially inhibited nitrate tolerance in coronary arteries with intact endothelium; however, melatonin had no effect on nitrate tolerance in coronary arteries without endothelium. (4) The effect of melatonin on nitrate tolerance in coronary arteries with endothelium was abolished by the melatonin receptor antagonist, S20928 (10(-6) M). In contrast to melatonin, the selective MT(3)-melatonin receptor agonist, 5-MCA-NAT (10(-8)-10(-7) M), had no effect on nitrate tolerance in coronary arteries. (5) The results demonstrate that melatonin, acting via specific melatonin receptors, inhibits nitrate tolerance in coronary arteries and that this effect is dependent on the presence of the vascular endothelium.

Reversal of tolerance to nitroglycerin with N-acetylcysteine or captopril: a role of calcitonin gene-related peptide

Previous studies have shown that the development of tolerance to nitroglycerin is related to a decrease in the release of endogenous calcitonin gene-related peptide (CGRP). In the present study, we explored whether endogenous CGRP is involved in reversal of tolerance to nitroglycerin with N-acetylcysteine or captopril in rats in vivo and vitro. Tolerance was induced by exposure to nitroglycerin (4.4 x 10(-6) M) for 10 min in vitro or by pretreatment with nitroglycerin (10 mg/kg, s.c.) three times a day for 8 days in vivo. Nitroglycerin (3 x 10(-9)-10(-6) M) caused a concentration-dependent relaxation in the isolated rat thoracic aorta, an effect that was reduced by CGRP-(8-37) (3 x 10(-7) M) or capsaicin (3 x 10(-7) M). Preincubation with nitroglycerin for 10 min significantly decreased its vasodilation, which was restored in the presence of N-acetylcysteine (10(-5) M) or captopril (10(-5) M). Nitroglycerin (150 microg/kg, i.v.) produced a depressor effect and an increase in concentrations of nitric oxide and CGRP, and the effects of nitroglycerin disappeared after pretreatment with nitroglycerin for 8 days. However, tolerance to nitroglycerin in vivo also was partially restored in the presence of N-acetylcysteine or captopril. The present results suggest that reversal of tolerance to nitroglycerin with N-acetylcysteine or captopril is related to the increased release of CGRP in the rat.

Involvement of calcitonin gene-related peptide in the development of tolerance to nitroglycerin in the rat

Previous studies have shown that the depressor effect of nitroglycerin is related to stimulation of endogenous calcitonin gene-related peptide (CGRP) release. In the present study, we explored whether endogenous CGRP is involved in the development of tolerance to nitroglycerin in the rat. Tolerance was induced by treatment with nitroglycerin (10 mg/kg, subcutaneous [s.c.]) three times a day for 8 days and confirmed by a reduction in hypotensive responses to intravenous (i.v.) nitroglycerin. Nitroglycerin (30 or 150 microg/kg, i.v.) significantly decreased blood pressure concomitantly with an increase in plasma concentration of nitric oxide (NO) and CGRP, and these effects of nitroglycerin disappeared after pretreatment with nitroglycerin for 8 days. However, the nitroglycerin-induced depressor effect and elevation of NO and CGRP content were restored, partially or completely, 4 or 8 days after nitroglycerin removal in the tolerant rat. The present study suggests that the development of tolerance to nitroglycerin is related to the decreased release of CGRP in the rat.

Effects of N-acetylcysteine on nitroglycerin-induced relaxation and protein phosphorylation of porcine coronary arteries

We investigated the effects of the sulfhydryl-donor, N-acetylcysteine (NAC), on nitroglycerin (NTG)-induced relaxation of the vascular smooth muscle. Addition of histamine to isolated porcine coronary arteries induced an initial rapid contraction followed by a gradual decrease in tonic contraction. NTG applied to the coronary artery strips before histamine caused relaxation of the histamine-induced rapid (3 min) and tonic (48 min) contraction. The inhibition of the tonic contraction by NTG was less at 48 min than at 3 min. Application of NAC (NTG-NAC) enhanced the relaxing effects of NTG on the histamine-induced tonic contraction rather than the acute contraction. In phosphorylation studies, changes in the phosphorylation of an intermediate filament, desmin, were parallel with changes in contraction in NTG-treated and NTG-NAC samples at 48 min. These phosphorylation changes of desmin at 48 min, which might be responsible for tonic phase contraction, were more extensive than those of myosin light chain (MLC) phosphorylation at 3 min, which might be responsible for acute contraction. These results suggest that treatment with the sulfhydryl donor, NAC, inhibited the phosphorylation of desmin associated with the enhancement of NTG-induced relaxation, which might be related to the mechanisms of recovery from NTG tolerance by sulfhydryl groups.

Tolerance induction by transdermal glyceryl trinitrate in rats

Mechanisms of mild glyceryl trinitrate tolerance in rats (transdermal application; 15 mg/day/2 days) were examined in isolated aortic rings contracted with phenylephrine. Tolerance to glyceryl trinitrate was comparable in both endothelium-intact and -denuded vessels; the maximum relaxation decreased to 70-80% and the EC50 increased 3-4-fold. There was minimal cross-tolerance to acetylcholine (1.7-fold increase in EC50) and none to sodium nitroprusside. The results suggest that mild tolerance to glyceryl trinitrate in rats is mediated by mechanisms which are predominantly endothelium-independent and which produce little activation of the cellular mechanism responsible for cross-tolerance.

Effects of guanylyl cyclase and protein kinase G inhibitors on vasodilatation in non-tolerant and tolerant bovine coronary arteries

The effects in bovine coronary arteries of the soluble guanylyl cyclase inhibitor 1 H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) were examined in order to establish the relative importance of the enzyme (a) in the vasodilator actions of glyceryl trinitrate and S-nitroso-N-acetylpenicillamine and (b) in induction of tolerance to these agents. ODQ strongly inhibited responses to both relaxants with IC50's of the order of 0.5 microM; in contrast, the protein kinase G inhibitor, 8-bromoguanosine-3',5'-monophosphorothioate (Rp-8-Br-cGMPS) had little effect on the responses. Tolerance after pre-incubation with glyceryl trinitrate (10 microM) was unaffected by co-pre-incubation with ODQ (1.0 microM), but similar experiments with S-nitroso-N-acetylpenicillamine were inconclusive because tolerance was associated with depressed contractile activity. It is concluded that in bovine coronary arteries soluble guanylyl cyclase is essential for vasorelaxation to both glyceryl trinitrate and S-nitroso-N-acetylpenicillamine but is unimportant for induction of tolerance to glyceryl trinitrate. Our results add weight to the hypothesis of impaired biotransformation rather than guanylyl cyclase desensitisation as the mechanism of in vitro nitrate tolerance.

Salt-sensitive hypertension: lessons from animal models

The underlying etiology of salt-sensitive hypertension has been elusive, in part because the term represents a syndrome rather than a specific disease entity and in part because of the difficulty in completely defining the characteristics of the syndrome. The introduction of inbred models of salt-sensitive hypertension has facilitated understanding blood pressure response to dietary salt. Careful examination of one of these models, the Dahl/Rapp rat, has shown that the L-arginine:nitric acid (NO) pathway is integrally involved in production of hypertension in response to an increase in dietary salt. This review provides an overview of NO, salt sensitivity, and the role of NO in the pathogenesis of salt-sensitive hypertension.

Effects of chronic oral administration of a high dose of nicorandil on in vitro contractility of rat arterial smooth muscle

Nicorandil, which is structurally a nitrate and also a nicotinamide, has a vasodilator action by stimulating cyclase and ATP-sensitive K+ channel. The aim of present study was to examine the effects of chronic oral administration of a high dose of nicorandil on in vitro vascular reactivity. Nicorandil (30 mg/kg), at a dose 6-10-times higher than to decrease blood pressure in rat, was orally administered 2-times daily for a 2-4 weeks to the rats. At the end of the administration period, thoracic aorta was isolated for in vitro study. Treatment with nicorandil for 4 weeks markedly reduced the relaxant effect of nicorandil itself and other vasodilators including sodium nitroprusside, nitric oxide, endothelium-derived relaxing factor released by carbachol, 8-Br-cyclic guanosine 3',5'-monophosphate (cGMP), a K+ channel opener, levcromakalim, and forskolin. Increase in cGMP content induced by nicorandil and sodium nitroprusside was less in the aorta from nicorandil-treated rat than in the vehicle-control rat. Chronic administration of nicorandil altered neither the contractile responses to norepinephrine nor the vasodilator effect of verapamil. On the other hand, a 4-week treatment with a dose of nicorandil (2 mg/kg) sufficient to decrease blood pressure in rat showed no change in aortic response. These results suggest that in vivo chronic treatment with a high dose of nicorandil inactivates not only the guanylate cyclase activity but also the mechanism mediated by cGMP; it also attenuates the sensitivity of K+ channels to levcromakalim. Prolonged activation of the specific site may desensitize its site of action.

Studies of vascular tolerance to nitroglycerin: effects of N-acetylcysteine, NG-monomethyl L-arginine, and endothelin-1

Development of vascular tolerance to nitroglycerin (NTG) has been attributed to sulfhydryl (SH) depletion, guanylate cyclase desensitization, or both. Controversy regarding the precise contribution of these mechanisms may be due to variations in experimental design. To examine further the biochemical basis of NTG tolerance, norepinephrine (NE)-precontracted rat aortic rings were exposed to NTG (10(-5)M), which resulted in 84 +/- 6% relaxation. Other rings were first superfused with NTG (10(-6)M) and then contracted with NE. These rings showed a marked tolerance to the vasorelaxant effects of NTG (maximal relaxation 20 +/- 5%, n = 15, p < 0.001 vs. control rings). Similar tolerance to NTG was observed when the vascular rings were first superfused with acetylcholine (ACh 10(-6)M), indicating cross-tolerance between ACh and NTG. Treatment of NTG-tolerant rings with N-acetylcysteine (NAC) (10(-5)M) did not restore vascular smooth muscle (VSM) relaxation in response to NTG (maximal relaxation 23 +/- 5%, n = 8), suggesting that SH depletion may not be the basis of NTG tolerance in these experiments. Parallel sets of NTG-tolerant aortic rings were contracted with endothelin-1 (ET-1, n = 5) or the endothelium-derived relaxing factor (EDRF) synthase inhibitor NG-monomethyl L-arginine (L-NMMA, 10(-4)M, n = 8). In both ET-1- and L-NMMA-contracted rings, vascular relaxation in response to NTG was preserved (80 +/- 6 and 88 +/- 8% relaxation, respectively). Measurement of cyclic GMP in aortic rings showed marked accumulation on initial exposure of tissues to NTG (310 +/- 10 fmol/mg), whereas the NTG-tolerant rings showed much less cyclic GMP accumulation (48 +/- 29 fmol/mg). Rings contracted with L-NMMA or ET-1, but not NE, accumulated cyclic GMP when exposed to NTG (280 +/- 20 fmol/mg). These data indicate that NTG tolerance develops on exposure of vascular rings superfused with NTG or ACh and is probably not related to tissue SH depletion. Contraction of NTG-tolerant rings with ET-1 or L-NMMA restores NTG-mediated relaxation.

Baroreflex resetting but no vascular tolerance in response to transdermal glyceryl trinitrate in conscious rabbits

1. We investigated whether acute (5 h) and chronic (3 days) transdermal glyceryl trinitrate (GTN) patches could cause the development of tolerance in terms of haemodynamics and vascular reactivity in the conscious rabbit. The effects of haemodynamic tolerance were assessed on arterial pressure, heart rate and the baroreflex control of heart rate, while hindquarter vascular reactivity in response to dilator and constrictor drugs and reactive hyperaemia were used to assess vascular tolerance. 2. Seven days prior to experiments, an inflatable cuff, a pulsed Doppler flow probe and an indwelling intra-aortic catheter (for i.a. agonist infusions) were implanted around the lower abdominal aorta. 3. In acute experiments, the effects of 0-5 h treatment with transdermal GTN (0 Sham), 10 or 20 mg 24 h-1) on MAP, HR and the baroreflex were examined. Chronic experiments were performed on three separated days (days 0 - before, 4 - with GTN patch and 8 - recovery). On each day, the baroreflex, reactive hyperaemic responses and hindquarter vascular dose-response curves to i.a. GTN, adenosine, acetylcholine, S-nitroso-N-acetylpenicillamine (SNAP) and methoxamine were assessed. On days 1-4, GTN was administered transdermally via a patch(es) (10 mg 24 h-1 (low dose) or 20 mg 24 h-1 (high dose); renewed every 24 h). 4. Acute treatment with 20 mg GTN 24 h-1, but not with 0 (n = 4) or 10 mg GTN 24 h-1 (n = 4), caused a significant fall in MAP (8 +/- 1 mmHg; n = 4) and resetting of the baroreflex by 5 h. Chronic GTN caused a significant fall in MAP of 8 +/- 2 and 8 +/- 2 mmHg on day 4 with low (n = 8) and high dose (n = 8), respectively, with no change in HR. There was no significant change to hindquarter vascular reactivity to i.a. infusion of GTN, nor were there any significant differences in the reactivity to i.a. adenosine, acetylcholine, SNAP or methoxamine with either low or high doses of GTN. 5. Chronic GTN treatment with low and high dose patches caused a parallel leftward shift ('resetting') of the baroreflex on day 4. By day 8, the baroreflex had still not recovered from this leftward shift 6. In the rabbit, chronic exposure to clinical nitrate patches caused haemodynamic compensation and baroreflex resetting but no evidence of vascular reactivity tolerance. Novel NO donor drugs and delivery regimens which provide intermittent dosing may prevent the development of haemodynamic resetting rather then preventing vascular tolerance, a commonly perceived difficulty in chronic nitrate therapy.

Tolerance to nitroglycerin in vascular smooth muscle cells is not affected by the level of intracellular glutathione or L-cysteine

A major hypothesis for the mechanism of tolerance to nitroglycerin (NTG) is that continued use causes a decrease in thiol donors within the vascular smooth muscle cell that are essential for the effect of NTG. We tested this idea directly in the target cell. NTG tolerance, measured as reduced formation of intracellular cyclic guanosine monophosphate (cGMP), was induced in pig coronary smooth muscle cells. The consequence of altering intracellular levels of the thiol donors, glutathione (GSH) and L-cysteine (L-cys), was determined. Incubating cells with 100 microM NTG for 1 h caused an 83% reduction in cGMP formation in response to acute readministration of 200 microM NTG for 2 min but was not associated with a reduction in intracellular GSH or L-cys. This result was not altered when intracellular GSH levels were increased three-fold by including 1 mM GSH in the incubation buffer. Also, recovery from tolerance was not affected by supplementation with GSH. Further, the response of cGMP to NTG was not altered by inhibiting the synthesis of GSH and lowering intracellular levels of GSH by 77%. Similar findings were made with supplemental L-cys or N-acetyl-L-cysteine. These results do not support the hypothesis that tolerance to NTG is the result of a reduction of the thiol donors GSH and L-cys within vascular smooth muscle cells.

Potentiation of isosorbide dinitrate effects with N-acetylcysteine in patients with chronic heart failure

BACKGROUND

Supply of sulfhydryl groups with the administration of N-acetylcysteine (NAC) has been reported to reverse tolerance to nitroglycerin but not to isosorbide dinitrate (ISDN). Lack of interaction between NAC and ISDN was suggested as an explanation for these findings. The present study was therefore designed to further evaluate this hypothesis. For this purpose, we compared the hemodynamic and hormonal effects of ISDN when given alone and in combination with NAC.

METHODS AND RESULTS

We performed a randomized, cross-over design evaluation of the hemodynamic and hormonal effects of ISDN and ISDN + NAC in 14 patients with chronic congestive heart failure due to left ventricular systolic dysfunction. The findings of this study demonstrated a substantial NAC-mediated potentiation of ISDN effect on mean right atrial pressure (-11 +/- 21% versus -38 +/- 27%, -17 +/- 20% versus -34 +/- 27%, and -7 +/- 20% versus -25 +/- 26% at 2, 3, and 4 hours, respectively; all P < .05), mean pulmonary artery wedge pressure (-18 +/- 16% versus -33 +/- 14%, -15 +/- 25% versus -33 +/- 19%, -14 +/- 22% versus -25 +/- 22%, and -16 +/- 16% versus -26 +/- 16% at 2, 3, 4, and 5 hours, respectively; all P < .05), mean pulmonary artery pressure (-8 +/- 11% versus -20 +/- 15% at 3 hours, P < .05), and cardiac output (an increase of 2 +/- 16% versus 25 +/- 20% at 4 hours, P < .05). Although there were no significant changes in serum catecholamine levels and plasma renin concentration with both regimens, ISDN + NAC resulted in a greater fall in plasma levels of atrial natriuretic peptide (296 +/- 251 pg/mL after ISDN versus 202 +/- 118 pg/mL after ISDN + NAC, P < .05).

CONCLUSIONS

The results of this study provide strong evidence for the existence of an interaction between thiols and ISDN and further support the role of sulfhydryl groups in the activation and therapeutic action of organic nitrates. The discrepancy between the results of this study demonstrating NAC-induced potentiation of ISDN effects and a previous study showing failure to reverse ISDN tolerance with NAC may suggest that ISDN-NAC interaction requires normal intracellular levels of sulfhydryl groups and does not occur after intracellular sulfhydryl group depletion.

Nitrate tolerance in vivo is not associated with depletion of arterial or venous thiol levels

Results from in vitro experiments suggest that development of nitrate tolerance is due to a depletion of vascular thiol compounds (ie, cysteine and glutathione [GSH]) necessary for the bioconversion of organic nitrates. However, it is unknown whether in vivo tolerance development is associated with changes in thiol levels. This study measures plasma and vessel tissue GSH and cysteine levels in nontolerant rats, nitrate-tolerant rats, and rats treated with the two characteristically different thiol donors N-acetyl-L-cysteine and L-2-oxothiazolidine-4-carboxylic acid (OXO). Chronically catheterized conscious rats received an intravenous infusion of either nitroglycerin (NTG, 0.2 mg/h) or matching placebo for 3 days. At day 3, the hypotensive effect of 2.5 mg NTG/kg was decreased by 74 +/- 6% (mean +/- SEM, P < .05) in the NTG-treated group (n = 7), indicating the development of tolerance. No change in the hypotensive effect of NTG was seen in the placebo group (n = 6, P > .05). Hemodynamic tolerance is not associated with changes in aorta cysteine or GSH levels as compared with the placebo group (cysteine, 77 +/- 14 versus 57 +/- 11 [mean + SEM] nmol/g; GSH, 414 +/- 62 versus 399 +/- 89 nmol/g; P > .05). However, the increase in vascular thiol levels seen after OXO treatment in nontolerant rats is completely absent in nitrate-tolerant animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute effects of nitroglycerin depend on both plasma and intracellular sulfhydryl compound levels in vivo. Effect of agents with different sulfhydryl-modulating properties

BACKGROUND

Changes in sulfhydryl (SH) compound availability may alter the hemodynamic effect of nitroglycerin (NTG). Data on the relation between NTG effect and thiol levels are, however, limited to in vitro experiments. The present study investigates how intracellular and extracellular changes in SH group concentrations (cysteine and glutathione [GSH]) affect the responsiveness to NTG in vivo.

METHODS AND RESULTS

GSH and cysteine levels in plasma, vena cava, and aorta were measured after administration of N-acetylserine (placebo, n = 6), N-acetylcysteine (NAC, extracellular and intracellular SH donor, n = 6), oxothiazolidine (OXO, intracellular SH donor, n = 6), buthionine sulfoximine (BSO, intracellular GSH-depleting agent, n = 6), BSO+NAC (n = 6), and BSO+OXO (n = 6) in chronically catheterized conscious rats. In addition, the effect of 2.5 mg NTG/kg i.v. on mean arterial pressure (MAP) was determined before and after the same treatment. NAC (5 mmol/kg i.v. for 2 hours) significantly (p < 0.05) increased extracellular cysteine and GSH levels and potentiated the hypotensive effect of NTG (from 26 +/- 3 to 31 +/- 4 mm Hg [mean +/- SEM], p < 0.05). OXO (5 mmol.kg-1 x hr-1 i.v. for 2 hours) significantly increased intracellular cysteine and GSH levels but had no effect on NTG responsiveness (p > 0.05). BSO (1 g i.p. three times within 24 hours) significantly decreased intracellular GSH levels (p < 0.05) and attenuated the effect of NTG (from 28 +/- 3 to 16 +/- 2 mm Hg).

CONCLUSIONS

The results suggest that the acute hypotensive effect of NTG in vivo is: 1) increased by high extracellular GSH and/or cysteine levels (NAC), 2) decreased by low intracellular GSH levels (BSO), and 3) unaffected by high intracellular levels of cysteine and GSH (OXO).

Pharmacologic mechanisms of nitrates in myocardial ischemia

Nitrates exert both hemodynamic and nonhemodynamic effects that help explain the mechanisms by which these drugs benefit patients with myocardial ischemia. The hemodynamic effects of nitrates include relaxation of conduit arteries, increased arterial compliance, increased venous capacitance, dilation of collateral vessels in the myocardium, and, possibly, increased myocardial compliance. A growing body of evidence suggests that the nonhemodynamic effects of these agents include inhibition of vascular smooth muscle growth and of myocyte hypertrophy and ventricular remodeling. Since endothelial function appears to be abnormal in patients with myocardial ischemia and nitrates replicate many of the effects of endothelium-derived relaxing factor, these drugs may be viewed as a pharmacologic replacement for deficient endogenous activity.

Preventive administration of intravenous N-acetylcysteine and development of tolerance to isosorbide dinitrate in patients with angina pectoris

BACKGROUND

Development of tolerance to organic nitrates may be related to depletion of sulfhydryl groups in vascular smooth muscle. N-Acetylcysteine (NAC), a sulfhydryl donor, has been reported to potentiate the effect of nitroglycerin and reverse tolerance in humans. However, its ability to prevent or delay the development of nitrate tolerance in patients with angina pectoris has not been established.

METHODS AND RESULTS

Ten patients with stable angina pectoris were treated with intravenous isosorbide dinitrate (ISDN; 5 mg/hr) combined with NAC (2 g i.v. over 15 minutes followed by 5 mg/kg/hr) or matching placebo for 30 hours in a double-blind, randomized, crossover study with a washout interval of 8 days. Bicycle exercise tests were performed before and at 1 1/2, 8, 20, 24, and 30 hours after start of treatment. After 24 hours of infusion, exercise parameters were not significantly different from pretreatment values (p greater than 0.05) during ISDN plus placebo, indicating development of tolerance to ISDN. In contrast, time to onset of angina, time to 1-mm ST segment depression, and total amount of ST segment depression were still significantly improved after 24-hour infusion of ISDN plus NAC (p less than 0.05). In addition, compared with placebo, a significant difference (p less than 0.05) in favor of NAC was observed regarding time to angina (507 +/- 63 versus 445 +/- 69 seconds, mean +/- SEM), time to 1-mm ST segment depression (435 +/- 43 versus 407 +/- 45 seconds), and total ST segment depression (1.8 +/- 0.9 versus 3.1 +/- 0.4 mm).

CONCLUSIONS

These results suggest that infusion of high doses of NAC in combination with ISDN for 30 hours affects and partially prevents the development of tolerance to antianginal effects normally observed during infusion with ISDN.

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).