Impaired endothelium-dependent relaxation of dog coronary arteries after myocardial ischaemia and reperfusion: prevention by amlodipine, propranolol and allopurinol

Coenzyme Q10 Reduces Infarct Size in Animal Models of Myocardial Ischemia-Reperfusion Injury: A Meta-Analysis and Summary of Underlying Mechanisms

Objective

Effective interventions that might limit myocardial ischemia-reperfusion (I/R) injury are still lacking. Coenzyme Q₁₀ (CoQ₁₀) may exert cardioprotective actions that reduce myocardial I/R injury. We conducted this meta-analysis to assess the potential cardioprotective effect of CoQ₁₀ in animal models of myocardial I/R injury.

Methods

We searched PubMed and Embase databases from inception to February 2022 to identify animal studies that compared the effect of CoQ₁₀ with vehicle treatment or no treatment on myocardial infarct size in models of myocardial I/R injury. Means and standard deviations of the infarct size measurements were pooled as the weighted mean difference with 95% confidence interval (CI) using the random-effects model. Subgroup analyses were also conducted according to animals' species, models' type, and reperfusion time.

Results

Six animal studies (4 in vivo and 2 ex vivo) with 116 animals were included. Pooled analysis suggested that CoQ₁₀ significantly reduced myocardial infarct size by −11.36% (95% CI: −16.82, −5.90, p < 0.0001, I² = 94%) compared with the control group. The significance of the pooled effect estimate was maintained in rats, Hartley guinea pigs, and Yorkshire pigs. However, it became insignificant in the subgroup of rabbits −5.29% (95% CI: −27.83, 17.26; I² = 87%). Furthermore, CoQ₁₀ significantly reduced the myocardial infarct size regardless of model type (either in vivo or ex vivo) and reperfusion time (either ≤ 4 h or >4 h).

Conclusion

Coenzyme Q₁₀ significantly decreased myocardial infarct size by 11.36% compared with the control group in animal models of myocardial I/R injury. This beneficial action was retained regardless of model type and reperfusion time.

Antioxidant activity contributes to flavonol cardioprotection during reperfusion of rat hearts

The mechanism of flavonol-induced cardioprotection is unclear. We compared the protective actions of a flavonol that inhibits calcium utilization and has antioxidant activity, 3',4'-dihydroxyflavonol (DiOHF); a flavonol that affects only calcium activity, 4'-OH-3'-OCH(3)-flavonol (4'-OH-3'-OCH(3)F); and a water-soluble flavonol with selective antioxidant activity, DiOHF-6-succinamic acid (DiOHF-6-SA), in isolated, perfused rat hearts. Hearts were subjected to global ischemia for 20 min followed by 30 min reperfusion and were treated with vehicle (0.05% DMSO), DiOHF, 4'-OH-3'-OCH(3)F, or DiOHF-6-SA (all 10 μM, n=5-8 per group). Flavonols were infused for 10 min before ischemia and during reperfusion. In vehicle-treated hearts, left-ventricular (LV) +dP/dt was reduced by 60% at the end of reperfusion compared to the preischemic level. Lactate dehydrogenase (LDH) release was elevated and endothelial NO synthase (eNOS) expression was lower in vehicle-treated hearts compared to shams. In comparison, DiOHF treatment improved LV function upon reperfusion, decreased LDH, and preserved eNOS expression. The antioxidant DiOHF-6-SA also preserved contractility, reduced LDH, and preserved eNOS expression. In contrast, hearts treated with 4'-OH-3'-OCH(3)F showed a degree of contractile impairment similar to that of the vehicle group. DiOHF and DiOHF-6-SA also exerted cardioprotection when given only during reperfusion and not when administered only before ischemia. Flavonol-induced cardioprotection relies on antioxidant activity and is mainly exerted during reperfusion.

Parstatin: a cryptic peptide involved in cardioprotection after ischaemia and reperfusion injury

AIMS

Thrombin activates protease-activated receptor 1 by proteolytic cleavage of the N-terminus. Although much research has focused on the activated receptor, little is known about the 41-amino acid N-terminal peptide (parstatin). We hypothesized that parstatin would protect the heart against ischaemia-reperfusion injury.

METHODS AND RESULTS

We assessed the protective role of parstatin in an in vivo and in vitro rat model of myocardial ischaemia-reperfusion injury. Parstatin treatment before, during, and after ischaemia decreased infarct size by 26%, 23%, and 18%, respectively, in an in vivo model of ischaemia-reperfusion injury. Parstatin treatment immediately before ischaemia decreased infarct size by 65% and increased recovery in ventricular function by 23% in an in vitro model. We then assessed whether parstatin induced cardioprotection by activation of a Gi-protein-dependent pathway. Gi-protein inactivation by pertussis toxin completely abolished the cardioprotective effects. The cardioprotective effects were also abolished by inhibition of nitric oxide synthase (NOS), extracellular signal-regulated kinases 1/2 (ERK1/2), p38 mitogen-activated protein kinase (p38 MAPK), and K(ATP) channels in vitro. Furthermore, parstatin increased coronary flow and decreased perfusion pressure in the isolated heart. The vasodilatory properties of parstatin were confirmed in rat coronary arterioles.

CONCLUSION

A single treatment of parstatin administered prior to ischaemia confers immediate cardioprotection by recruiting the Gi-protein activation pathway including p38 MAPK, ERK1/2, NOS, and K(ATP) channels. Parstatin exerts effects on both the cardiomyocytes and the coronary circulation to induce cardioprotection. This suggests a potential therapeutic role of parstatin in the treatment of cardiac injury resulting from ischaemia and reperfusion.

Therapeutic effects of xanthine oxidase inhibitors: renaissance half a century after the discovery of allopurinol

The prototypical xanthine oxidase (XO) inhibitor allopurinol, has been the cornerstone of the clinical management of gout and conditions associated with hyperuricemia for several decades. More recent data indicate that XO also plays an important role in various forms of ischemic and other types of tissue and vascular injuries, inflammatory diseases, and chronic heart failure. Allopurinol and its active metabolite oxypurinol showed considerable promise in the treatment of these conditions both in experimental animals and in small-scale human clinical trials. Although some of the beneficial effects of these compounds may be unrelated to the inhibition of the XO, the encouraging findings rekindled significant interest in the development of additional, novel series of XO inhibitors for various therapeutic indications. Here we present a critical overview of the effects of XO inhibitors in various pathophysiological conditions and also review the various emerging therapeutic strategies offered by this approach.

Oxypurinol improves coronary and peripheral endothelial function in patients with coronary artery disease

Coronary endothelial dysfunction is a powerful prognostic marker in patients with coronary artery disease (CAD) that is centrally related to oxidative inhibition of nitric oxide (NO)-dependent vascular cell signaling. Xanthine oxidase (XO), which both binds to and is expressed by endothelial cells, generates superoxide and hydrogen peroxide upon oxidation of purines. Whether inhibition of xanthine oxidase activity results in improved coronary vasomotor function in patients with CAD, however, remains unknown. We assessed coronary and peripheral (brachial artery) endothelial function in 18 patients (pts; 65+/-8 years, 86% male) with angiographically documented CAD, preserved left ventricular function, and non-elevated uric acid levels (233+/-10 microM). Patients received incremental doses of intracoronary acetylcholine (ACh; 10(-7) to 10(-5) microM), and minimal lumen diameter (MLD) and coronary blood flow (CBF) were assessed before and after intravenous administration of oxypurinol (200 mg). Oxypurinol inhibited plasma XO activity 63% (0.051+/- 0.001 vs 0.019+/- 0.005 microU/mg protein; p<0.01). In pts who displayed endothelial dysfunction as evidenced by coronary vasoconstriction in response to ACh (n=13), oxypurinol markedly attenuated ACh-induced vasoconstriction (-23+/- 4 vs -15+/- 4% at ACh 10(-5) microM, p<0.05) and significantly increased CBF (16+/-17 vs 62+/-18% at ACh 10(-5) microM, p<0.05), whereas in patients with preserved coronary endothelial function, oxypurinol had no effect on ACh-dependent changes in MLD (+2.8+/- 4.2 vs 5.2+/- 0.7%, p>0.05) or CBF (135+/-75 vs 154+/-61%, p>0.05). Flow-mediated dilation of the brachial artery, assessed in eight consecutive patients, increased from 5.1+/-1.5 before to 7.6+/-1.5% after oxypurinol administration (p < 0.05). Oxypurinol inhibition of XO improves coronary vascular endothelial dysfunction, a hallmark of patients with CAD. These observations reveal that XO-derived reactive oxygen species significantly contribute to impaired coronary NO bioavailability in CAD and that XO inhibition represents an additional treatment concept for inflammatory vascular diseases that deserves further investigation.

Ischemia–Reperfusion Impairs Ascending Vasodilation in Feed Arteries of Hamster Skeletal Muscle

OBJECTIVE

Vasodilation originating within the microcirculation ascends into proximal feed arteries during muscle contraction to attain peak levels of muscle blood flow. Ascending vasodilation (AVD) requires an intact endothelium, as does conducted vasodilation in response to acetylcholine (ACh). Whereas ischemia-reperfusion (I-R) can affect endothelial cell function, the effect of I-R on AVD is unknown. The authors tested the hypothesis that I-R (1h-1h) would impair AVD.

METHODS

Using the retractor muscle of anesthetized hamsters, contractions were evoked using field stimulation (200 ms at 40 Hz every 2 s for 1 min) and ACh was delivered using microiontophoresis (1 microm tip, 500-4000 ms pulse at 800 nA). Feed artery responses were monitored 500-1500 microm upstream.

RESULTS

Neither resting (51 +/- 4 microm) nor maximal diameter (81 +/- 5 microm; 10 microm sodium nitroprusside) following I-R (n = 8) were different from time-matched controls (n = 10). With peak active tension of 23 +/- 4 mN x mm(-2), control AVD was 26 +/- 2 microm. Following I-R, active tension fell by 48% (p < .05) and AVD by 57% (p < .05). Stimulation at 70 Hz restored active tension but AVD remained depressed by nearly half (p < .05), as did local and conducted responses to ACh. Nevertheless, control responses to 500 ms ACh were restored by increasing stimulus duration to 4000 ms.

CONCLUSIONS

Ischemia-reperfusion impairs the initiation of feed artery dilation with muscle contraction and with ACh while conduction along the vessel wall is preserved. Respective components of endothelial cell signaling events may differ in their susceptibility to I-R.

Postischemic recovery and oxidative stress are independent of nitric-oxide synthases modulation in isolated rat heart

During myocardial ischemia and reperfusion, nitric oxide ((.)NO) was shown to exert either beneficial or detrimental effects. Uncoupled (.)NO synthases (NOS) can generate superoxide anion under suboptimal concentrations of substrate and cofactors. The aim of our study was to investigate the role of NOS modulation on 1) the evolution of functional parameters and 2) the amount of free radicals released during an ischemia-reperfusion sequence. Isolated perfused rat hearts underwent 30 min of total ischemia, followed by 30 min of reperfusion in the presence of N(G)-nitro-D- or L-arginine methyl ester (NAME, 100 microM) or of D- or L-arginine (3 mM). Functional parameters were recorded and coronary effluents were analyzed with electron spin resonance to identify and quantify the amount of alpha-phenyl-N-tert-butylnitrone spin adducts produced during reperfusion. The antioxidant capacities of the compounds were determined with the oxygen radical absorbance capacity test. L-NAME-treated hearts showed a reduction of coronary flow and contractile performance, although neither L-NAME nor L-arginine improved the recovery of coronary flow, left end diastolic ventricular pressure, rate pressure product, and duration of reperfusion arrhythmia, compared with their D-specific enantiomers. A large and long-lasting release of alkyl/alkoxyl radicals was detected upon reperfusion, but no differences of free radical release were observed between D- and L-NAME or D- and L-arginine treatment. These results may indicate that, in our experimental conditions, cardiac NOS might not be a major factor implicated in the oxidative burst that follows a global myocardial ischemia.

Localization of inducible nitric oxide synthase to mast cells during ischemia/reperfusion injury of skeletal muscle

Nitric oxide contributes to tissue necrosis after ischemia-reperfusion (IR). A biochemical and immunohistochemical study was made of the amounts and localization of both Ca++-independent nitric oxide synthase (NOS) II and Ca++-dependent (NOS I and NOS III) in rat skeletal muscle after ischemia and 0.5, 2, 8, 16, and 24 hours reperfusion. NOS II was not detectable in control muscle or during ischemia, was first detected after 2 hours reperfusion, increased further by 8 hours, and remained elevated at 24 hours. Both NOS II and nitrotyrosine, a marker of peroxynitrite formation, were localized exclusively to mast cells except after 24 hours reperfusion when some macrophages and neutrophils also showed positive immunoreactivity. Mast cells underwent extensive degranulation during reperfusion. NOS I was not detected in injured or control muscle. The level of NOS III, which was localized to the endothelium of blood vessels of all sizes in control muscle, decreased progressively during ischemia and reperfusion to reach undetectable levels after 16 hours reperfusion. These findings indicate that most of the nitric oxide formed during IR injury is generated by NOS II located almost exclusively in mast cells.

Enhanced role for the opening of potassium channels in relaxant responses to acetylcholine after myocardial ischaemia and reperfusion in dog coronary arteries

1. Anaesthetized dogs were subjected to 1 h occlusion of the left circumflex coronary artery followed by 2 h of reperfusion. Relaxant responses were examined in coronary artery rings removed proximal (nonischaemic) or distal (ischaemic) to the site of occlusion. 2. Relaxant responses to acetylcholine (ACh) were similar in nonischaemic and ischaemic artery rings. In addition ACh-induced relaxation of nonischaemic and ischaemic artery rings was equally susceptible to inhibition of nitric oxide (NO) synthase using L-N(G)-nitroarginine (L-NOARG, 10(-4) M), or to inhibition of soluble guanylate cyclase using 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one (ODQ, 10(-5) M). 3. In nonischaemic arteries, the relaxation to ACh was unaffected by high K+ (67 mM) but in ischaemic arteries, the maximum relaxation to ACh was significantly reduced from 113+/-6 to 60+/-2% (ANOVA, P<0.05). Tetraethylammonium (TEA, 10(-3) M), an inhibitor of large conductance calcium activated potassium (BK(Ca)) channels did not inhibit the response to ACh in nonischaemic arteries but in ischaemic arteries TEA significantly shifted the concentration response curve to ACh to the right (pEC(50); nonischaemic, 7.07+/-0.25; ischaemic, 6.54+/-0.21, P<0.01, ANOVA) without decreasing the maximum relaxation. TEA did not affect the responses to sodium nitroprusside in either nonischaemic or ischaemic arteries. 4. In conclusion, ischaemia/reperfusion did not change the sensitivity of endothelium-dependent relaxation to L-NOARG or ODQ indicating that ischaemia did not affect the contribution of NO or cyclic GMP to ACh-induced relaxation. However, in ischaemic arteries the opening of the BK(Ca) channels contributed to relaxation caused by ACh whereas TEA had no effect in nonischaemic arteries. The factor responsible for the opening of this potassium channel was a factor other than NO and may be endothelium derived hyperpolarizing factor (EDHF).

Preconditioning improves myocardial function and reflow, but not vasodilator reactivity, after ischaemia and reperfusion in anaesthetized dogs

1. The present study examines whether three cycles of brief coronary artery occlusion and reperfusion (i.e. ischaemic preconditioning; PC) can prevent vasodilator dysfunction and the impairment of myocardial reflow caused by prolonged ischaemia. Coronary blood flow, left ventricular dP/dt, systemic arterial blood pressure and heart rate were measured in open-chest anaesthetized dogs. 2. Sixty minute occlusion of the left circumflex coronary artery (LCx) and 60 min LCx reperfusion (ISC/REP; group 1) significantly reduced resting coronary blood flow (CBF, initial 29 +/- 3 mL/min; ISC/REP 20 +/- 3 mL/min, P < 0.05 vs initial) and increased coronary vascular resistance (CVR, initial 4.1 +/- 0.6 mmHg/min per mL; ISC/REP 5.8 +/- 1.0 mmHg/min per mL, P < 0.05 vs initial). By contrast CBF and CVR were not affected in dogs subjected to preconditioning before ischaemia (group 2: CBF, initial 24 +/- 4 mL/min; PC+ISC/REP 23 +/- 4 mL/min; CVR, initial 4.7 +/- 0.6 mmHg/min per mL; PC+ ISC/REP 5.3 +/- 1.0 mmHg/min per mL). These data suggest that ischaemic preconditioning prevents the ischaemia-induced impairment of myocardial reflow. 3. Ischaemia and reperfusion impaired coronary dilator responses to the endothelium-dependent dilator acetylcholine (delta CBF, after ISC/REP: 50 +/- 6% of initial) and the endothelium-independent dilator glyceryl trinitrate (delta CBF, ISC/REP: 46 +/- 6% of initial). Despite the improvement in reperfusion in the preconditioned group, there was no significant improvement in responses to acetylcholine (PC+ISC/REP 52 +/- 6% of initial) or glyceryl trinitrate (PC+ISC/REP 59 +/- 6% of initial) after ischaemia and reperfusion. 4. The reduction in left ventricular dP/dt after ischaemia and reperfusion was significantly smaller in the preconditioned group indicating a lower level of impairment of cardiac contractility. In addition, we confirmed that preconditioning caused a significant reduction in infarct size and a reduction in the release of lactate dehydrogenase indicating less cardiac injury. 5. These results suggest that although ischaemic preconditioning was able to improve both myocardial reperfusion and contractility, it was not able to preserve vasodilator function. Such a reduction in vasodilator reserve could prevent adequate myocardial perfusion under conditions of elevated oxygen demand.

Prevention of ischaemia-induced coronary vascular dysfunction

Myocardial ischaemia and reperfusion cause dysfunction of the coronary vasculature leading to a sustained reduction in coronary blood flow and an impairment of responses to both endothelium-dependent and endothelium-independent vasodilators. In contrast, when previously ischaemic arteries are removed from the myocardium and vascular function is examined in vitro, it is evident that while endothelial function is impaired, smooth muscle reactivity remains intact. Therefore, other changes must be responsible for the general reduction in vasodilator reserve. Examination of the vasculature in the ischaemic myocardium by electron microscopy reveals adhesion of leukocytes and plugging of capillaries. There also is evidence that polymorphonuclear leukocytes (PMNs) release a factor that constricts coronary arterioles, and that release of this factor is increased by atherosclerosis. The identity of this factor remains uncertain, but the calcium antagonist amlodipine prevents the coronary vasoconstriction. Amlodipine is also able to prevent the impaired perfusion and the reduction in vasodilator reserve that occurs after myocardial ischaemia and reperfusion in the dog. In addition, amlodipine prevents the endothelial dysfunction observed in isolated arteries after ischaemia and reperfusion. The interaction between the endothelium and activated PMNs may be a suitable target for pharmacological intervention to improve postischaemic vascular function.

Relation between serum uric acid and lower limb blood flow in patients with chronic heart failure

OBJECTIVE

To determine whether lower limb blood flow is related to serum uric acid concentrations in patients with chronic heart failure, taking into account the hyperuricaemic effects of diuretic treatment and insulin resistance.

DESIGN

Lower limb blood flow was measured at rest and after maximum exercise followed by a five minute period of ischaemia (maximum blood flow) using strain gauge venous occlusion plethysmography. All patients underwent a metabolic assessment, which included an intravenous glucose tolerance test (IVGTT)-to obtain an index of insulin sensitivity- and measurement of serum uric acid.

SETTING

University and hospital departments specialising in cardiology and metabolic medicine.

SUBJECTS

22 patients with chronic heart failure.

RESULTS

Mean (SEM) resting and maximum blood flow values were 2.87 (0.23) and 24.00 (1.83) ml/100 ml/min, respectively. Patients in the upper tertile of serum uric acid had lower maximum blood flow than those in the lowest tertile (15.6 (2.2) v 31.0 (2.1) ml/100 ml/min, P = 0.003). Serum uric acid correlated with maximum blood flow (r = -0.86, P < 0.001), but not with resting blood flow. In stepwise regression analysis, uric acid emerged as the only predictor of maximum blood flow (standardised coefficient = -0.83 (P < 0.001), R2 = 0.68 (P < 0.001)), independently of diuretic dose, age, body mass index, plasma creatinine, fasting and IVGTT glucose and insulin, insulin sensitivity, maximum oxygen uptake and exercise time during the treadmill exercise test, and alcohol intake.

CONCLUSIONS

There is a strong inverse relation between serum uric acid concentrations and maximum leg blood flow in patients with chronic heart failure. Further studies are needed to determine whether serum uric acid can be used as an index of vascular function in cardiovascular diseases.

Reperfusion injury: a review of the pathophysiology, clinical manifestations and therapeutic options

Lack of blood supply or ischaemia underlies many of the most important cardiovascular and cerebrovascular diseases faced by clinicians in their daily practice. Many of these ischaemic episodes can be reversed at an early stage by surgical or pharmacological means with the ultimate aim of preventing infarction and cell necrosis in the ischaemic tissues. However, reperfusion of ischaemic areas, in particular the readmission of oxygen, may contribute to further tissue damage (reperfusion injury). For example, the use of thrombolytic therapy in acute myocardial infarction and other revascularisation procedures, such as percutaneous transluminal angioplasty and coronary artery bypass surgery, may be associated with reperfusion of ischaemic myocardium. Such ischaemia and reperfusion may result in injury to one of more of the biochemical, cellular and microvascular components of the heart. Our understanding of the significance of reperfusion injury is however restricted by the profuse literature in animal models and limited literature in the clinical situation. This article reviews the pathophysiology, clinical manifestations of reperfusion injury to the heart and discusses the possible therapeutic approaches to avoiding any adverse effects.

Augmented endothelium-dependent constriction to hypoxia early and late following reperfusion of the canine coronary artery

1. Canine coronary arteries with intact endothelium respond to hypoxaemia or serotonin infusion with dilatation, but when the endothelium is injured or dysfunctional, these stimuli can cause constriction. The present studies investigated whether or not regional ischaemia and reperfusion alter endothelium-dependent responsiveness of canine coronary arteries in vivo and in vitro. 2. In organ chamber experiments, isolated control and reperfused coronary artery rings were contracted with prostaglandin F2 alpha and exposed to hypoxia (PO2 less than 5 mmHg). 3. Hypoxia augmented the response of reperfused arteries more than that of controls. The hypoxic augmentation was blocked by NG-monomethyl-L-arginine, an inhibitor of nitric oxide synthesis from L-arginine. 4. These findings demonstrate that early following coronary reperfusion the hypoxic augmentation, which is mediated by a nitric oxide-dependent pathway in the endothelium, is facilitated. 5. In vivo studies revealed hyperconstriction of reperfused arteries in response to hypoxaemia (PO2 = 30-40 mmHg) and administration of either serotonin or ergonovine. 6. Twelve weeks following reperfusion injury, coronary arteries still exhibited augmented endothelium-dependent hypoxic augmentations in vitro, which were inhibited by NG-monomethyl-L-arginine. 7. Furthermore, resting coronary segments with endothelium displayed hypoxia-induced contractions that could not be inhibited by indomethacin, the lipoxygenase inhibitor AA861, superoxide dismutase plus catalase, deferoxamine, ouabain, or NG-monomethyl-L-arginine. 8. These endothelium-dependent hypoxic response may play a role in the pathogenesis of hyperconstriction (vasospasm) following coronary reperfusion.

Nitric oxide in coronary artery disease: roles in atherosclerosis, myocardial reperfusion and heart failure

Nitric oxide (NO), derived from the vascular endothelium or other cells of the cardiovascular system, has an important role in physiological regulation of blood flow and has pathophysiological functions in cardiovascular disease. The mechanisms and enzymes involved in the biosynthesis of NO and biological actions of NO, including vasodilatation, cytotoxicity and inflammation, are briefly reviewed. These reactions involving NO cause pathological disturbances of arterial function, coronary blood flow regulation, and may contribute to cardiac myocyte dysfunction. NO and prostacyclin (PGI2), which is also released from the endothelium, act synergistically to inhibit platelet aggregation and adhesion, and in some arteries these mediators also synergise in terms of vasodilatation. In addition, NO is capable of hyperpolarizing vascular smooth muscle, but activation of the endothelium may cause hyperpolarization and may thus promote vasodilatation by an additional mechanism. After myocardial ischemia and reperfusion, production of NO and superoxide radicals represent important mechanisms of cytotoxicity, causing injury to the coronary endothelium and myocytes and compromising ventricular contractile function. Moreover, upon reperfusion endothelium-dependent vasodilatation is impaired and the coronary arteries constrict, leading to irregular myocardial perfusion. This is a consequence of the accumulation of activated leucocytes that we found to generate endogenous inhibitors of NO. These factors have yet to be fully characterised, but clearly they may have a role in irregularities of myocardial reperfusion and cellular injury. Chronic heart failure is associated both with impairment of endothelium-dependent vasodilatation and with excess production of NO via the inducible NO synthase (iNOS), although it is unclear whether the latter assists or compromises ventricular contractile performance under these conditions. Disturbances in the activity of isoforms of NO synthase in the artery wall also accompany the development of atherosclerosis, providing conditions propitious for vasospasm and thrombosis, and perhaps contributing to cell proliferation. Reversing these NO defects with therapeutic agents including angiotensin converting enzyme (ACE) inhibitors offers promise in protecting against some manifestations of vascular disease.

Vasoconstrictor and vasodilator effects of hypoxia

Hypoxia has marked effects on artery calibre, which reflects important physiological control mechanisms that are altered in disease states. Hypoxia modifies the release of mediators, especially from the endothelium, and influences smooth muscle membrane potential and Ca2+ regulation. In this review, Roger Wadsworth evaluates the vasoconstrictor and vasodilator effects of hypoxia studied in vitro. In the future, drugs developed to act on the mediators or smooth muscle may be beneficial in the therapy of, for example, pulmonary hypertension or coronary vasospasm.

Myocardial ischaemia: what happens to the coronary arteries?

Study of the effects of myocardial ischaemia and reperfusion has largely been confined to the impairment of cardiac contractility. However, emerging recognition of the importance of endothelium-derived vasoactive factors in blood flow regulation has stimulated interest in the effect of pathological states such as ischaemia on coronary vascular function and is discussed in this review by Christopher Sobey and Owen Woodman. Neutrophils may play a key role in impaired reperfusion and endothelial damage, and pharmacological intervention to preserve endothelial function could significantly improve coronary blood flow and cardiac function after an ischaemic attack.

Support of renal blood flow after ischaemic-reperfusion injury by endogenous formation of nitric oxide and of cyclo-oxygenase vasodilator metabolites

1. Ischaemia-reperfusion injury in the kidney is associated with a loss of autoregulation, an increase in renal vascular resistance (RVR), a decrease of renal blood flow (RBF) and ultimately acute renal failure. The aim of this study was to investigate the role of the release of endogenous nitric oxide (NO) in the recovery of RBF after ischaemic injury of the renal vascular bed. 2. Anaesthetized rats (thiopentone sodium; 120 mg kg-1, i.p.) were submitted to acute renal ischaemia followed by 2 or 6 h of reperfusion (I/R). Reperfusion was associated with a significant reduction in RBF, an increase in RVR, and an impairment of the vasodilator effect of acetylcholine (ACh). 3. NG-nitro-L-arginine methyl ester (L-NAME, 30 micrograms kg-1 min-1, i.v., n = 5) significantly prevented the recovery of RBF after I/R injury. Similarly, inhibition of prostanoid formation with indomethacin (5 mg kg-1, i.v., n = 4) significantly enhanced the rise in RVR associated with I/R injury. 4. Infusion of L-arginine (L-Arg; 1 or 3 mg kg-1 min-1, i.v., n = 5 and 4, respectively) or D-Arg (1 mg kg-1 min-1, i.v., n = 6), starting 30 min after occlusion, did not improve the recovery of RBF. Furthermore, infusion of L-Arg (20 mg kg-1 min-1 for 15 min; n = 4) had no effect on the I/R-induced impairment of the vasodilator responses to ACh. 5. To elucidate the relative importance of the constitutive and inducible NO synthase isoforms for the formation of NO after I/R, calcium-dependent (constitutive) and calcium-independent (inducible) NO synthase activities were measured in kidney homogenates obtained from ischaemic or non-ischaemic kidneys. A calcium-independent NO synthase activity was not detectable in kidney homogenates obtained from either sham-operated control rats or from animals subjected to I/R. Moreover, dexamethasone(3 mg kg-1, i.v., 60 min prior to I/R, n = 6), an inhibitor of the induction of NO synthase,had no effect on either RBF or RVR in rats subjected to I/R. In contrast to I/R, lipopolysaccaride(LPS, endotoxin; 5 mg kg-1, i.p., n = 3) caused a significant induction of a calcium-independent NO synthase activity in the kidney.6. These results confirm the importance of the release of vasodilator cyclo-oxygenase metabolites in the compromised renal circulation and indicate that the formation of NO derived from the constitutive, but not the inducible NO synthase, is also important for the maintenance of RBF after I/R injury of the renal vascular bed.

Allopurinol and amlodipine improve coronary vasodilatation after myocardial ischaemia and reperfusion in anaesthetized dogs

1. We have assessed the effect of allopurinol, amlodipine and propranolol pretreatment on both endothelium-dependent and endothelium-independent coronary vasodilatation in vivo, by comparing pre-ischaemic responses with those measured after 60 min of coronary artery occlusion and 30 min of reperfusion in anaesthetized dogs. 2. In 15 untreated dogs ischaemia and reperfusion attenuated the increases in coronary blood flow produced by either acetylcholine (0.01-0.05 micrograms kg-1, i.a.) or glyceryl trinitrate (0.05-0.2 micrograms kg-1, i.a.), to an average of 39 +/- 4% and 42 +/- 5% of the pre-ischaemic control response, respectively (both P < 0.05). 3. In 5 dogs treated with allopurinol (25 mg kg-1, orally, 24 h previously, plus 50 mg kg-1, i.v., 5 min before occlusion), the increases in coronary blood flow after ischaemia and reperfusion (acetylcholine: 78 +/- 12%, glyceryl trinitrate: 60 +/- 3% of pre-ischaemic response) were significantly larger than post-ischaemic responses in untreated dogs (both P < 0.05). 4. Similarly, amlodipine treatment (3 micrograms kg-1 min-1, i.v., starting 90 min before occlusion) in 5 dogs improved post-ischaemic increases in blood flow (acetylcholine: 58.5%, glyceryl trinitrate: 66 +/- 6% of pre-ischaemic response, significantly greater than post-ischaemic responses in untreated dogs, P < 0.05). 5. In contrast, in a further 6 dogs pretreated with propranolol (1 mg kg-1, i.v., 30 min before occlusion,plus 0.5 mg kg-1 h-1, i.v.), blood flow responses after ischaemia and reperfusion were not different from post-ischaemic responses in untreated dogs (acetylcholine: 46 +/- 6%, glyceryl trinitrate: 46 +/-6% of pre-ischaemic response).6. These results suggest that allopurinol and amlodipine protect against the post-ischaemic impairment of endothelium-dependent and endothelium-independent coronary vasodilatation in vivo by mechanisms additional to endothelial protection.