Chronic administration of N-acetylcysteine fails to prevent nitrate tolerance in patients with stable angina pectoris

1. Reduced availability of sulphydryl groups in vascular smooth muscle cells may contribute to the development of tolerance to the action of the organic nitrovasodilators. 2. Eight patients with stable angina were treated with 10 mg of transdermal glyceryl trinitrate per 24 h, together with 400 mg N-acetylcysteine, a sulphydryl group donor, or matching placebo three times daily in a double-blind randomised crossover manner for two periods of 4 days with intervening washout period of 3 days off these drugs. Other therapy remained unaltered during the study. 3. Blood pressure, heart rate and symptom-limited treadmill walking time were measured in the pre-treatment control state and 4 h after starting treatment on day 1 and day 4 of each glyceryl trinitrate treatment period. 4. The changes seen on day 1 were attenuated by day 4 to an equal extent in N-acetylcysteine and placebo treatment periods. 5. These results suggest that chronic oral administration of N-acetylcysteine fails to prevent the development of tolerance to the anti-anginal or haemodynamic effects of glyceryl trinitrate.

Does endocardium modulate myocardial contractile performance?

It is now clear that the endocardium releases at least two agents which exert opposing effects on myocardial contraction. One appears to be endothelium-derived relaxing factor (EDRF), as in vascular endothelium, but the identity of the other is unknown. The mechanism by which these agents exert their characteristic but opposing effects on the duration of contraction likewise remains unknown. It is unlike that of other inotropic interventions and merits further investigation. Alterations in the time of onset of relaxation have important implications for diastolic filling.

Calcific aortic stenosis in twins: a clue to its pathogenesis?

A pair of identical twins presented almost simultaneously at age 62 years with calcific aortic stenosis requiring surgery, one with bicuspid and one with tricuspid aortic valve. This suggests that bicuspid morphology may perhaps not be the cause, but may be a genetically linked marker of an inherited predisposition to calcific stenosis.

EDRF and the regulation of vascular tone

Vascular endothelium is now known to produce a vasodilator agent, known as endothelium derived relaxing factor (EDRF) and recently shown to be nitric oxide. It acts locally on subjacent vascular smooth muscle by stimulating soluble guanylate cyclase to increase cyclic GMP levels and so reduce cytosolic free calcium. EDRF activity varies widely between different artery types and can be stimulated by various pharmacological agents and by flow. Studies in the intact vascular bed of the buffer-perfused rabbit ear show that EDRF activity is high in resistance vessels (particularly those of ca. 150 microns diameter), that EDRF is responsible for a 4th power relationship of diameter to flow in response to acute changes in flow (so limiting pressure gradients needed to increase flow rate), and that basal EDRF activity maintains geometric similarity of vessel diameters (implying that the spatial distribution of flow remains constant at different flow rates).

N-acetylcysteine fails to attenuate haemodynamic tolerance to glyceryl trinitrate in healthy volunteers

1. The effects of chronic dosing with N-acetylcysteine (NAC), on nitrate-induced haemodynamic changes during the acute and chronic treatment of healthy volunteers with glyceryl trinitrate (GTN) patches (Transiderm nitro) has been investigated. 2. Seven volunteers were treated in a double-blind randomised crossover manner for two periods of 4 days with 20 mg of transdermal GTN/24 h together with NAC (200 mg three times daily) or matching placebo. There was a washout period of greater than 3 days between treatment periods. 3. Haemodynamic measurements (blood pressure (BP); heart rate (HR] at rest and following maximal treadmill exercise were performed before treatment and 4 h after starting treatment on days 1 and 4. 4. Significant haemodynamic changes as evidenced by a fall in BP and rise in HR, were seen on day 1 in both the NAC and placebo phases. By day 4 the haemodynamic changes had returned towards the pre-treatment values during both the NAC and placebo phases suggesting the development of tolerance in both treatment groups. 5. These findings suggest that concurrent administration of NAC fails to prevent the development of tolerance to GTN.

Syndrome X

The problem of anginalike chest pain with normal coronary arteriographic findings is briefly reviewed. This common clinical presentation (ca. 20% of patients investigated by coronary arteriography) is usually due to noncardiac causes (e.g., thoracic root or esophageal pain) but may represent myocardial ischemia attributable to reduced coronary dilator capacity downstream from the epicardial vessels and of unknown pathogenesis--Syndrome X (? less than 0.1% of such patients).

Vascular endothelium in ischemic heart disease: possible role for endothelium-derived relaxing factor

The nature, mechanism of action, and roles of endothelium-derived relaxant factor (EDRF) are reviewed, particularly in relation to the coordination of vascular behavior in response to changes in flow, coronary spasm, and platelet aggregation. Vascular endothelium performs a multiplicity of roles. It is an active sieve for macromolecules and leukocytes, a negatively charged "lubricant" for passage of negatively charged red cells and platelets, and a factory for Von Willebrand factor, glycoaminoglycans, and plasminogen activator and its inhibitor. It is also a processing plant that metabolizes adenosine nucleotides to adenosine and activates angiotensin. Endothelium also produces prostacyclin and endothelium-derived relaxant factor, which act synergistically and through different pathways to the common ends of relaxing vascular smooth muscle and inhibiting platelet aggregation. Most recently it has been shown to also produce a constrictor agent called endothelin, a peptide whose structure has now been elucidated. This review will concentrate on EDRF, the recently discovered vasodilator agent that is continuously released by all vascular endothelium. It would be premature to define the role of EDRF in ischemic heart disease. It may, however, be timely to consider the ways in which EDRF might be relevant, based on a review of what is at present known.

Glyceryl trinitrate and platelet aggregation: effects of N-acetyl-cysteine

The concentration range of GTN causing inhibition of platelet aggregation in vitro is much higher than the plasma concentrations achieved clinically. This action is potentiated by the sulphydryl donor N-acetylcysteine. We have investigated the effects of GTN given with and without N-acetylcysteine on ex vivo platelet aggregation in man. In a double-blind randomised crossover trial eight healthy volunteers were treated with 20 mg of transdermal GTN/24 h, together with N-acetylcysteine 200 mg three times daily or matching placebo. Platelet aggregation, measured ex vivo by whole blood impedance aggregometry in response to adenosine diphosphate, was not significantly altered by GTN acutely or after 4 days' treatment with or without N-acetylcysteine. Platelet cyclic guanosine monophosphate levels were not significantly altered by GTN either in the absence or presence of N-acetylcysteine. This result implies that previously reported beneficial effects of GTN in myocardial infarction or unstable angina are unlikely to be attributable to direct pharmacological inhibition of platelet aggregation.

Inotropic effects of endothelin in ferret ventricular myocardium

The characteristics of the inotropic response to endothelin were studied in isolated ferret papillary muscle preparations. Endothelin (0.1-10 nM) induced concentration-dependent positive inotropic effects. At 10 nM, isometric tension increased by 17%, maximum rate of tension development by 18%, maximum velocity of unloaded shortening by 23% and peak isotonic shortening by 11%. Time to peak isometric tension and half isometric relaxation time were unaltered by endothelin. This pattern of change is similar to that of elevating extracellular calcium concentration.

The role of EDRF in flow distribution: a microangiographic study of the rabbit isolated ear

A microangiographic technique was used to study the influence of endothelium-derived relaxing factor (EDRF) on vasomotor control mechanisms in resistance vessels of intact buffer-perfused rabbit ear. Selective inhibition of EDRF activity by hemoglobin unmasked an intrinsic ("myogenic") constrictor response to sudden increases in flow rate. EDRF activity was greatest in arteries in which calculated shear stress and hydraulic resistance were maximal, namely the central ear artery and its first generation of branch arteries: these are proximal "feed" vessels (150-700 microns internal diameter) in this bed. The findings are consistent with enhancement of EDRF release by the physical stimulus of shear stress in resistance vessels as previously demonstrated in conduit vessels-a phenomenon which is likely to exert a major influence on flow in vascular networks. EDRF activity thus reduced perfusion pressure and power losses, particularly in highly constricted preparations. Shear-induced release of EDRF may provide an integrating link between flow and arterial topography by optimizing perfusion characteristics over a wide range of flow rates.

Post-infarction ventricular septal defect: the importance of right ventricular coronary perfusion in determining surgical outcome

Mortality from a post-infarction ventricular septal defect (VSD) can be reduced by surgery, selection for which would be helped by knowledge of factors affecting the postoperative prognosis. We reviewed our 9-year experience (1978-1987) of 40 surgically treated patients, comparing preoperative characteristics in those who died postoperatively (n = 15, 37%) and those who survived (n = 25, 63%), all still alive. Sex, age, infarct size as assessed by peak serum creatine kinase values, left ventricular end-diastolic pressure and pulmonary/systemic flow ratio (2.9 +/- 0.2 vs 3.1 +/- 0.3) were similar. Survivors had a lower incidence of inferior infarct than those who died (6, 24% vs 9, 60%, P less than 0.05), a lower incidence of cardiogenic shock (7, 28% vs 10, 67%, P less than 0.05), less elevation of right ventricular end-diastolic pressure (10.1 +/- 0.9 vs 14.7 +/- 1.0 mm Hg, P less than 0.01) and less impairment of right ventricular coronary supply as determined by a coronary angiography-derived myocardial score (0.9 +/- 0.3 vs 4.7 +/- 0.7, P less than 0.001). The data suggest that right ventricular coronary perfusion influences prognosis. The proposed angiographic score may help to identify preoperatively those patients most likely to benefit from surgery.

Effects of enalapril on myocardial noradrenaline overflow during exercise in patients with chronic heart failure

The effects of the angiotensin converting enzyme inhibitor enalapril on myocardial sympathetic tone, as represented by noradrenaline overflow, were studied in 14 men with congestive heart failure (mean ejection fraction 20%) in a double blind crossover comparison with placebo. Arterial and coronary sinus catecholamine concentrations and oxygen content, and coronary sinus blood flow, were measured at rest and during peak symptom limited upright exercise on a bicycle ergometer. There were no significant changes four hours after the first dose of enalapril, but after six weeks of treatment (10-20 mg/day) enalapril reduced myocardial overflow of noradrenaline at peak exercise. The external workload (exercise duration) increased from baseline values after both placebo and enalapril, and there was no difference between placebo and enalapril at six weeks. Heart work, however, was lower after enalapril: stroke work index was reduced at rest and the double product was lower at peak exercise. The reduction in maximal myocardial oxygen consumption after enalapril did not reach statistical significance. Coronary sinus adrenaline concentrations after enalapril and after placebo were not significantly different. The long term reduction of myocardial sympathetic activity on exercise may represent a significant benefit from angiotensin converting enzyme inhibition in heart failure and may reflect a reduced cardiac workload.

Endothelium-derived relaxing factor

This article reviews what is known of endothelium-derived relaxing factor and its possible physiologic and pathophysiologic roles. This relaxing factor is now thought to be nitric oxide or a ready source of it. It acts as an endogenous nitrovasodilator, stimulating soluble guanylate cyclase to increase cyclic guanosine monophosphate (GMP) levels in vascular smooth muscle and platelets, with consequent relaxant and anti-aggregatory effects (predominantly when stimulated through receptor-operated channels). Its actions are thus synergistic with those of cyclic adenosine monophosphate (AMP)-mediated stimulation (for example, adenosine, prostacyclin). Endothelium-derived relaxing factor is unstable and is thought to act only very locally in vivo. Its release is continuous in the basal state and is stimulated by a number of neuropeptides and by agents released during platelet activation and thrombosis--with large differences in activity among different vessels. Endothelium-derived relaxing factor activity is also flow related, thereby coordinating vasomotor behavior in an intact vascular tree in response to changes in flow. Endothelium-derived relaxing factor activity is reduced in several pathologic states, including atherosclerosis.

In vivo EDRF activity influences platelet function

The administration of carbachol to rabbits to stimulate the release of endothelium derived relaxing factor (EDRF) results in inhibition of platelet aggregation and elevation of platelet cyclic GMP content. These effects are reversed by simultaneous administration of the EDRF inhibitors methylene blue or haemoglobin. The data provide the first direct biochemical evidence of in vivo EDRF activity.

Endothelium-derived relaxing factor and nitroprusside compared in noradrenaline- and K+-contracted rabbit and rat aortae

1. The effects of endothelium-derived relaxing factor (EDRF) (as stimulated by acetylcholine in the presence of endothelium), sodium nitroprusside and 8-bromocyclic GMP on mechanical relaxation, calcium (45Ca) influx and cyclic GMP levels were studied in isolated rabbit aortic preparations pre-contracted either by noradrenaline or by high (120 mM) extracellular potassium. 2. The results confirmed a relatively greater effect of these three interventions on mechanical relaxation and on reducing calcium influx in noradrenaline-contracted than in potassium-contracted preparations. 3. The increase in cyclic GMP levels induced by sodium nitroprusside, contrary to previous reports, was no greater in noradrenaline-stimulated preparations than in potassium-stimulated preparations, a finding confirmed in rat aortic preparations, and relaxation was not associated with a significant reduction of calcium influx in the potassium-stimulated preparations. 4. Cyclic GMP-mediated relaxation of potassium contraction thus appears to be due to actions of cyclic GMP other than on calcium influx. 5. These findings suggest that cyclic GMP reduces calcium influx more through receptor-operated channels than through voltage-operated channels. 6. The endothelium-dependent acetylcholine-induced elevation of cyclic GMP was reduced both by noradrenaline and by high extracellular potassium, possibly by altering release or activity of EDRF. 7. The sensitivity of the soluble guanylate cyclase system to stimulation by EDRF and nitrovasodilators appears to be greater in rat than rabbit aortic preparations.

Evidence that inhibitory factor extracted from bovine retractor penis is nitrite, whose acid-activated derivative is stabilized nitric oxide

1. Unactivated extracts of bovine retractor penis (BRP) contains 3-7 microM nitrite. Acid-activation of these extracts at pH 2 for 10 min followed by neutralization generates the active form of inhibitory factor (IF; assayed by its vasodilator action on rabbit aorta), and is associated with partial loss of nitrite. 2. Increasing the time of acid-activation at pH 2 from 10 to 60 min with intermittent vortex mixing generates greater vasodilator activity and increases nitrite loss. 3. When acid-activated and neutralized extracts are incubated at 37 degrees C or 30 min or boiled for 5 min, vasodilator activity is lost and nitrite content increased. Reactivation of these samples at pH 2 for 10 min followed by neutralization leads to partial recoveries of vasodilator activity with loss in nitrite content. 4. Addition of sodium nitrite to BRP extracts increases acid-activatable vasodilator activity pro rata. 5. Acid-activation of aqueous sodium nitrite solutions results in less loss of nitrite and generation of less vasodilator activity than BRP extracts. Vasodilatation is only transient and is rapidly abolished on neutralization, whereas responses to acid-activated BRP extracts are more prolonged and activity is stable on ice. 6. Bovine aortic endothelial cells yield vasodilator activity that is indistinguishable from that isolated from BRP. It is activated by acid, stable on ice, abolished by boiling or by haemoglobin, and appears to be due to the generation of nitric oxide (NO) from nitrite. 7. The data provide confirmatory evidence that nitrite in BRP extracts is IF, that acid-activation of BRP extracts yields NO which is responsible for its vasodilator action, and that inactivation occurs by decay of NO to nitrite and nitrate. They further suggest that BRP extracts contain a NO-stabilizing agent which favours conversion of nitrite to NO. 8. The finding that bovine aortic endothelial cells yield an agent indistinguishable from IF suggests that nitrite in endothelial cells may likewise be the precursor of endothelium-derived relaxing factor (EDRF), itself identified as NO.

A randomized cross-over study of enalapril in congestive heart failure: haemodynamic and hormonal effects during rest and exercise

We performed a randomized double-blind placebo controlled cross-over study of enalapril in 16 patients with chronic congestive heart failure, to assess haemodynamic and hormonal effects at rest and on exercise. Acute effects were measured 4 h after enalapril 10 mg, and chronic effects after 6 weeks treatment with enalapril 10-20 mg per day. Exercise tolerance, assessed by the duration of a maximal bicycle ergometer test, was not altered by enalapril. Mean blood pressure was reduced after enalapril, at rest and on exercise, acutely by 7% and 8% respectively, and chronically by 14% and 16%. Systemic vascular resistance was reduced by 16% at rest both acutely (NS) and chronically (p less than 0.05). The resting pulmonary capillary wedge pressure was reduced by 28% with chronic treatment. In the acute study, total body oxygen consumption on exercise was 26% higher after enalapril. Chronically, resting oxygen consumption was reduced by 13% after enalapril, with mixed venous oxygen saturation increasing by 16%. In the acute study enalapril increased plasma renin activity at rest and on exercise by 181% and by 189%, and reduced aldosterone by 49% (NS) and 39% (p less than 0.05), and these effects were sustained after 6 weeks. Enalapril increased antidiuretic hormone concentrations at rest acutely by 73% (NS) and chronically by 34% (p less than 0.05) but not on exercise; the increase in the acute study correlated with plasma enalaprilat levels (r = 0.66, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelium-derived relaxing factor and atriopeptin II elevate cyclic GMP levels in pig aortic endothelial cells

1. Two directly-acting stimulants of soluble guanylate cyclase, glyceryl trinitrate (0.1 microM) and sodium azide (10 microM), and a receptor-mediated stimulant of particulate guanylate cyclase, atriopeptin II (10 nM), each elevated the cyclic GMP content of primary cultures of pig aortic endothelial cells without affecting the cyclic AMP content. 2. Two receptor-mediated stimulants of adenylate cyclase, glucagon (1 microM) and isoprenaline (10 microM), had no effect on the cyclic AMP or cyclic GMP content of these cells, but the directly acting stimulant, forskolin (30 microM), induced a small increase in cyclic AMP content. 3. Three agents that release endothelium-derived relaxing factor (EDRF); bradykinin (0.1 microM), ATP (10 microM) and ionophore A23187 (0.1 microM), each markedly elevated the cyclic GMP content of pig aortic endothelial cells, but acetylcholine (1 microM) had no effect. None of these agents had any effect on cyclic AMP content. 4. Two agents that potentiate the actions of EDRF; M & B 22948 (100 microM) and superoxide dismutase (30 units ml-1), each elevated the cyclic GMP content of pig aortic endothelial cells without affecting the cyclic AMP content. Pretreating cells with catalase (100 units ml-1) did not affect the rise in cyclic GMP content induced by superoxide dismutase (30 units ml-1). 5. Pretreatment of pig aortic endothelial cells with haemoglobin (10 microM) reduced the resting content of cyclic GMP and blocked the increase in cyclic GMP content induced by glyceryl trinitrate (0.1 microM), sodium azide (10 microM), bradykinin (0.1 microM), ATP (10 microM), ionophore A23187 (0.1 microM), M & B 22948 (100 microM) and superoxide dismutase (30 units ml-1), but not that induced by atriopeptin II (10 nM). 6. Pretreatment of pig aortic endothelial cells with an inhibitor of soluble guanylate cyclase, methylene blue (20 microM), had no effect on the resting content of cyclic GMP. Methylene blue (20 microM) blocked the increase in cyclic GMP content induced by glyceryl trinitrate (0.1 microM), M & B22948 (100 microM) and bradykinin (0.1 microM), but not that induced by atriopeptin II (10 nM). 7. The data show that soluble guanylate cyclase, particulate guanylate cyclase and adenylate cyclase are present in pig aortic endothelial cells. They further suggest that EDRF, produced spontaneously or in response to vasoactive agents, elevates endothelial cyclic GMP content by stimulating soluble guanylate cyclase. It is possible that this may serve as a feedback loop by which the endothelial cell modulates EDRF production.

A report and review of recurrent left atrial myxoma: not always such 'a benign tumor'

We report a patient whose left atrial myxoma recurred three times and review 28 other reports of recurrent myxoma. Inadequate resection and multifocal growth appear to be two separate causes for recurrence.