Endothelial and neuro-humoral control of coronary blood flow in health and disease

Pharmacologic inhomogeneity between the reactivity of intramural coronary arteries and arterioles

We hypothesized that because of their size, anatomic location, and hemodynamic function, coronary arteries and arterioles would respond differently to vasoactive substances. Intramural arteries (281.7 +/- 23.1 microm) and arterioles (77.3 +/- 6.6 microm) of the left anterior descending coronary of rats were isolated and cannulated. Spontaneous tone was lower in arteries than in arterioles (81.1 +/- 5.7 vs. 53.0 +/- 3.9% of passive diameter, p < 0.05 at 60 mm Hg intraluminal pressure). Arterial tone was adjusted by the thromboxane receptor agonist U46619 (5 x 10(-8) M ) to reach an active tone close to that of arterioles. Bradykinin elicited dilations in both types of vessels. Acetylcholine (10(-6) - 10(-5) M ) dilated arteries (by 42.6 +/- 11.5 microm) but constricted arterioles (by 16.4 +/- 9.3 microm). Sodium nitroprusside and adenosine elicited significantly greater dilations in arterioles than in arteries (by 7.9 and 11.9%, respectively, p < 0.05), whereas dilations to norepinephrine were similar. Inhibition of nitric oxide synthesis caused a significantly smaller constriction in arteries (10.2 +/- 3.31%) than in arterioles (31.6 +/- 6.9%) and completely blocked bradykinin-and acetylcholine-induced dilations, whereas it did not affect dilations to sodium nitroprusside, adenosine, and norepinephrine. Compared with arteries, arterioles have a greater spontaneous tone and enhanced nitric oxide modulation of basal tone and exhibit greater responsiveness to nitric oxide and adenosine. In addition, nitric oxide synthase is activated differently by pharmacologic stimuli in these segments. The qualitative and quantitative differences among vasoactive responses of coronary arteries and arterioles demonstrated in this study suggest segment-specific roles for endothelial and metabolic factors in regulation of coronary vascular resistance.

Role of cellular acidosis in production of nitric oxide in canine ischemic myocardium

We tested the hypothesis that cellular acidosis modulates the production of nitric oxide (NO) in ischemic hearts. In canine hearts, we decreased coronary blood flow (CBF) to one third of the control by reduction of coronary perfusion pressure (105+/-3 to 41+/-5 mmHg), and thereafter we maintained CBF constant (89.8+/-1.6 to 30.0+/-0.5 ml/100 g/min) with an intracoronary administration of either saline, atropine, rauwolscine, HOE140, 8-sulfophenyltheophylline (8SPT), NaHCO3, or HOE642 (the inhibitor of Na+/H+ exchange). The cardiac NO levels defined as the differences of the nitrate and nitrite levels between coronary venous and arterial blood increased in the saline administration (2.9+/-0.2 to 12.7+/-1.7 micromol/l), and the extents of increases were identical in the condition of either saline, atropine, rauwolscine, HOE140 or 8SPT administration. In the condition with either NaHCO3 or HOE642, the increases in the cardiac NO levels were blunted (4.5+/-0.7 and 4.8+/-0.4 micromol/l, respectively). Cyclic GMP content of epicardial coronary artery in the ischemic area increased, which was also attenuated by either NaHCO3 or HOE642. We confirmed the acidosis-induced NO production in a more severe ischemic myocardium, and also showed that cellular acidosis produced by infusion of HCl increased NO production in non-ischemic myocardium. We conclude that cellular acidosis and subsequent activation of Na+/H+ exchanges modulate production of endogenous NO in canine ischemic myocardium.

Effect of endothelial dysfunction on regional perfusion in myocardial territories supplied by normal and diseased vessels in patients with coronary artery disease

BACKGROUND

Endothelium-dependent regulation of coronary tone affects both conduit and resistance coronary arteries. However, little is known about the usefulness of myocardial perfusion imaging in evaluating coronary endothelial function. We evaluated the relation between invasive angiographic measurements of coronary vasomotion in response to intracoronary acetylcholine and the presence of regional perfusion abnormalities assessed by technetium 99m sestamibi imaging.

METHODS AND RESULTS

We studied 11 patients (9 men and 2 women) with suspected coronary artery disease (48 +/- 8 years, mean +/- standard deviation). We used quantitative coronary angiography to delineate the vasomotor response to increasing doses of acetylcholine given intracoronary. Regional myocardial perfusion was assessed by planar Tc-99m sestamibi imaging during and after the administration of acetylcholine. In the 11 patients, 23 coronary artery territories were analyzed: 13 were angiographically normal, and 10 showed varying degrees of luminal narrowing. Four (31%) of 13 angiographically normal coronary arteries had a positive vasomotor response to acetylcholine (> or =20% reduction in luminal diameter) that was associated with a regional perfusion defect. Acetylcholine induced a positive vasomotor response, which was also associated with a regional perfusion defect in 1 of 3 coronary arteries with stenoses of intermediate severity (50% to 69%). Likewise, acetylcholine induced a positive vasomotor response in 6 of 7 coronary arteries with significant luminal narrowing (> or =70%), 5 of which showed a corresponding regional perfusion defect.

CONCLUSIONS

In patients with coronary artery disease, noninvasive measurements of regional myocardial perfusion by Tc-99m sestamibi correlate well with invasive measurements of coronary endothelial function. These findings may have implications for monitoring the effects of interventions designed to improve endothelial function and microvascular function in patients with coronary artery disease.

New insights into nitric oxide and coronary circulation

Since its discovery over 20 years ago as an intercellular messenger, nitric oxide (NO), has been extensively studied with regard to its involvement in the control of the circulation and, more recently, in the prevention of atherosclerosis. The importance of NO in coronary blood flow control has also been recognized. NO-independent vasodilation causes increased shear stress within the blood vessel which, in turn, stimulates endothelial NO synthase activation, NO release and prolongation of vasodilation. Reactive hyperemia, myogenic vasodilation and vasodilator effects of acetylcholine and bradykinin are all mediated by NO. Ischemic preconditioning, which protects the myocardium from cellular damage and arrhythmias, is itself linked with NO and both the first and second windows of protection may be due to NO release. Exercise increases NO synthesis via increases in shear stress and pulse pressure and so it is likely that NO is an important blood flow regulatory mechanism in exercise. This phenomenon may account for the beneficial effects of exercise seen in atherosclerotic individuals. Whilst NO plays a protective role in preventing atherosclerosis via superoxide anion scavenging, risk factors such as hypercholesterolemia reduce NO release leading the way for endothelial dysfunction and atherosclerotic lesions. Exercise reverses this process by stimulating NO synthesis and release. Other factors impacting on the activity of NO include estrogens, endothelins, adrenomedullin and adenosine, the last appearing to be a compensatory pathway for coronary control in the presence of NO inhibition. These studies reinforce the pivotal role played by the substance in the control of coronary circulation.

Metabolic adaptation of endothelial cells to substrate deprivation

Endothelial cells are known to be metabolically rather robust. To study the mechanisms involved, porcine aortic endothelial cells (PAEC), cultured on microcarrier beads, were perfused with glucose (10 mM) or with substrate-free medium. Substrate-free perfusion for 2 h induced an almost complete loss of nucleoside triphosphates (31P-NMR) and decreased heat flux, a measure of total energy turnover, by >90% in parallel microcalorimetric measurements. Heat flux and nucleoside triphosphates recovered after addition of glucose. Because protein synthesis is a major energy consumer in PAEC, the rate of protein synthesis was measured ([14C]leucine incorporation). Reduction or blockade of energy supply resulted in a pronounced reduction in the rate of protein synthesis (up to 80% reduction). Intracellular triglyceride stores were decreased by approximately 60% after 2 h of substrate-free perfusion. Under basal perfusion conditions, PAEC released approximately 30 pmol purine. mg protein-1. min-1, i.e., 16% of the cellular ATP per hour, while ATP remained constant. Substrate deprivation increased the release of various purines and pyrimidines about threefold and also induced a twofold rise in purine de novo synthesis ([14C]formate). These results demonstrate that PAEC are capable of recovering from extended periods of substrate deprivation. They can do so by a massive downregulation of their energy expenditure, particularly protein synthesis, while at the same time using endogenous triglycerides as substrates and upregulating purine de novo synthesis to compensate for the loss of purines.

A Ca channel blocker, benidipine, increases coronary blood flow and attenuates the severity of myocardial ischemia via NO-dependent mechanisms in dogs

OBJECTIVES

This study was undertaken to examine whether a dihydropyridine Ca channel blocker, benidipine, increases cardiac NO levels, and thus coronary blood flow (CBF) in ischemic hearts.

BACKGROUND

Benidipine protects endothelial cells against ischemia and reperfusion injury in hearts.

METHODS AND RESULTS

In open chest dogs, coronary perfusion pressure (CPP) of the left anterior descending coronary artery was reduced so that CBF decreased to one-third of the control CBF, and thereafter CPP was maintained constant (103+/-8 to 42+/-1 mmHg). Both fractional shortening (FS: 6.1+/-1.0%) and lactate extraction ratio (LER: -41+/-4%) decreased. Ten minutes after the onset of an intracoronary infusion of benidipine (100 ng/kg/min), CBF increased from 32+/-1 to 48+/-4 ml/100g/ min during 20 min without changing CPP (42+/-2 mmHg). Both FS (10.7+/-1.2%) and LER (-16+/-4%) also increased. Benidipine increased cardiac NO levels (11+/-2 to 17+/-3 nmol/ml). The increases in CBF, FS, LER and cardiac NO levels due to benidipine were blunted by L-NAME. Benidipine increased cyclic GMP contents of the coronary artery of ischemic myocardium (139+/-13 to 208+/-15 fmol/mg protein), which was blunted by L-NAME.

CONCLUSION

Thus, we conclude that benidipine mediates coronary vasodilation and improves myocardial ischemia through NO-cyclic GMP-dependent mechanisms.

Involvement of ATP in the non-adrenergic non-cholinergic inhibitory neurotransmission of lamb isolated coronary small arteries

1. The involvement of non-adrenergic non-cholinergic (NANC) transmitters, such as nitric oxide (NO) and adenosine 5'-triphosphate (ATP), in the neurogenic relaxation of lamb coronary small arteries was investigated in vessel segments with an internal lumen diameter of 200-550 microns, isolated from the left ventricle of the heart, and suspended for isometric tension recording in microvascular myographs. 2. In both endothelium-intact and -denuded coronary small arteries treated with phentolamine (3 x 10(-6) M), propranolol (3 x 10(-6) M), and atropine (10(-6) M) and contracted to 3 x 10(-7) M of the thromboxane analogue U46619, electrical field stimulation (EFS) evoked frequency-dependent relaxations, which were markedly reduced in the presence of tetrodotoxin (10(-6) M). 3. Exogenous NO added as acidified sodium nitrite (10(-6)-10(-3) M) and L-nitrosocysteine induced potent relaxations of lamb coronary small arteries. However, both inhibition of NO synthase with NG- nitro-L-arginine (L-NOARG, 3 x 10(-5) M), and mechanical endothelial cell removal increased rather than inhibited relaxations to EFS. In small arteries processed for NADPH-diaphorase histochemistry, activity was only observed within endothelial cells. 4. In arteries contracted to U46619, exogenously added ATP caused concentration-dependent relaxations with pD2 and maximum responses of 4.72 +/- 0.12 and 89.6 +/- 3.8% (n = 12), respectively. ADP and the P2Y-agonist, 2-methylthio-ATP, induced relaxations equipotent to ATP, while the P2x-agonist, alpha, beta-methylene ATP (10(-9)-10(-4) M), and the P2U-agonist, UTP (10(-9)-10(-4) M) only caused small transient relaxations at the highest concentrations (10(-4) and 10(-3) M). 5. ATP and EFS-induced relaxations were unchanged in the presence of the P1-purinoceptor antagonist, 8-phenyltheophylline (10(-5) M), while this antagonist inhibited the concentration-dependent relaxations to adenosine. In contrast, the P2-purinoceptor antagonist, suramin (3 x 10(-5) M), markedly reduced the relaxations to EFS. 6. After desensitization of P2x-purinoceptors with alpha, beta-methylene ATP (2 x 10(-5) M), the relaxations to exogenous added ATP were enhanced, but this procedure did not influence the relaxations to EFS. In contrast, the P2y-purinoceptor antagonist, basilen blue E-3G (3 x 10(-5) M, earlier named reactive blue 2) significantly inhibited the concentration-relaxation curves to ATP and almost abolished the EFS-induced relaxations. 7. Mechanical removal of the endothelium significantly inhibited ATP-induced maximal relaxations without affecting sensitivity, pD2 and maximum relaxations being 4.72 +/- 0.12 and 89.7 +/- 3.8% (n = 10), and 5.45 +/- 0.38 and 48.0 +/- 8.6% (P < 0.05, paired t test, n = 10) in endothelium-intact and -denuded coronary small arteries, respectively. However, incubation with L-NOARG did not change relaxations elicited by ATP. 8. The present study suggests that in NANC conditions neurogenic relaxations of coronary small arteries are mediated by ATP, which relaxes coronary small arteries through P2Y-purinoceptors. A prejunctional modulation of these relaxations by endothelial-derived NO cannot be excluded.

Altered vasodilator response of coronary microvasculature in pacing-induced congestive heart failure

To characterize vasodilator capacity of small coronary arteries (200-350 microm diameter) in the setting of congestive heart failure, we examined relaxation responses to acetylcholine (10(-9)-10(-4) M) and nitroglycerin (10(-9)-10(-4) M), in the absence and presence of the nitric oxide precursor, L-arginine (10(-4) M). Congestive heart failure was reliably induced in dogs by rapid ventricular pacing (250 beats.min(-1) for 4 weeks). Maximum relaxations (means +/- S.E.) to each vasodilator are expressed as a percentage of the relaxation response to papaverine (10(-4) M). Relaxation responses to the endothelium-dependent relaxing agent, acetylcholine, were not altered at heart failure, or in the presence of L-arginine. Contrary to acetylcholine, relaxations to nitroglycerin were significantly enhanced in heart failure compared to control (83 +/- 25% vs. 25 +/- 6%, respectively, P < 0.05). Although L-arginine, alone, did not cause any vasodilator response in coronary microvessels, it was able to potentiate nitroglycerin relaxations at control (no L-arginine: 25 +/- 6% vs. L-arginine: 135 +/- 66%). In contrast, at heart failure, L-arginine diminished nitroglycerin relaxations (no L-arginine: 83 +/- 25%, vs. L-arginine: 48 +/- 15%). These data indicate a unique vasodilator profile in small coronary arteries at heart failure: endothelium-dependent relaxations are unaltered, whereas responses to nitroglycerin are augmented. Addition of the nitric oxide precursor, L-arginine, did not affect acetylcholine relaxation, yet surprisingly had a differential effect in response to nitroglycerin. Moreover, inhibition of nitric oxide synthase with N(omega)-nitro-L-arginine elicited concentration-dependent constriction in heart failure but not control coronary microvessels. In summary, our study suggests an important role for nitric oxide in vasodilator control of coronary microvessels, which may modify nitrovasodilator therapy in congestive heart failure.

Endothelial function in different organs

The endothelial lining represents an organ of 1.5 kg in an adult which is distributed throughout the body and serves multifunctional purposes. It regulates vascular growth processes and adaptations and controls the delicate equilibrium between coagulation-hemostasis and fibrinolysis. The endothelium is not only a simple diffusion barrier between the intravascular and extravascular space of blood and lymph vessels thus regulating permeability (ie, the fluid, metabolite and catabolite exchange), but synthetizes, releases, converts, activates and/or inactivates various vasoactive hormones. Thus, it regulates vascular tone and organ blood supply as well as lymphatic flow and expression of surface receptors for the activation of leukocytes eg, during inflammation. In different organs it has additional, organ specific functions (eg, cerebral endothelial lining/blood brain barrier, endothelium mediated changes in renal, splenic and hepatic function and in skeletal muscle perfusion) by generating various autacoids such as nitric oxide, prostaglandins, endothelins, hyperpolarizing factors, and so on. These autacoids are not only vasoactive compounds but also modulate the activation of transcription factors. The endothelial autacoids exert an important role in vascular homeostasis (eg, by direct inhibition of atherogenesis and by inhibition of proatherogenic genes).

The importance of the hyperpolarizing mechanism increases as the vessel size decreases in endothelium-dependent relaxations in rat mesenteric circulation

Endothelium-dependent relaxations are achieved by a combination of endothelium-derived prostacyclin (PGI2), nitric oxide (NO), and endothelium-derived hyperpolarizing factor (EDHF). However, it remains to be fully clarified whether the relative contribution of these three mechanisms to endothelium-dependent relaxations varies as a function of the vessel size. This study was designed to clarify this point. Acetylcholine (ACh)-induced endothelium-dependent relaxations were examined in isolated blood vessels taken from the aorta and the proximal and distal mesenteric arteries of the rat. The contributions of PGI2, NO, and EDHF were evaluated by the inhibitory effects of indomethacin, N omega-nitro-L-arginine methyl ester (L-NAME) in the presence of indomethacin, and KCl in the presence of indomethacin and L-NAME, respectively. The membrane potentials were recorded with microelectrodes. The expression of endothelial No synthase (eNOS) was examined by both immunostaining and immunoblotting. The contribution of PGI2 was negligible in three different-sized blood vessels. The contribution of NO was most prominent in the aorta, whereas that of EDHF was most prominent in the distal mesenteric arteries. The resting membrane potential was significantly deeper and the ACh-induced hyperpolarization was greater in the distal mesenteric arteries than those in the aorta. The expression of eNOS was the highest in the aorta and the lowest in the distal mesenteric arteries. These results indicate that the importance of EDHF increases as the vessel size decreases in endothelium-dependent relaxations in the rat mesenteric circulation.

Role of nitric oxide in regulation of coronary blood flow during myocardial ischemia in dogs

OBJECTIVES

This study was undertaken to examine whether nitric oxide released in ischemic myocardium decreases the coronary vascular resistance and attenuates the severity of contractile and metabolic dysfunction.

BACKGROUND

Endothelium-derived relaxing factor, recently identified as nitric oxide, is a potent relaxant of coronary smooth muscle.

METHODS

The left anterior descending coronary artery was perfused through an extracorporeal bypass tube placed in the carotid artery in 56 open chest dogs. After hemodynamic stabilization, we occluded this bypass tube to decrease coronary blood flow to one third of the control flow. Thereafter, we maintained a constant coronary perfusion pressure (40.9 +/- 3.1 mm Hg).

RESULTS

Under ischemic conditions, the coronary arteriovenous differences in nitrate and nitrite (end products of nitric oxide) increased (from 3.5 +/- 0.4 [mean +/- SEM] to 12.9 +/- 2.1 mumol/liter, p < 0.01). NG-Monomethyl L-arginine (3 micrograms/kg body weight per min, intracoronary) decreased the coronary arteriovenous differences in nitrate and nitrite (5.0 +/- 0.9 mumol/liter, p < 0.05) and coronary blood flow (from 29.8 +/- 0.5 to 18.1 +/- 1.1 ml/100 g per min, p < 0.001). Fractional shortening (from 3.7 +/- 1.0 to -1.3 +/- 0.7%, p < 0.001) and lactate extraction ratio (from -44.0 +/- 4.1 to -59.2 +/- 4.9%, p < 0.005) of the perfused area also decreased. These values were restored by the concomitant administration of L-arginine. Blood flow to the endomyocardium was decreased relative to the epimyocardium. A reduction in coronary blood flow and worsening of myocardial contractile and metabolic functions due to the administration of NG-monomethyl L-arginine during ischemia were observed in denervated hearts. A reduction in coronary blood flow in ischemic myocardium was observed with the administration of NW-nitro-L-arginine methyl ester as well, although neither NW-nitro-L-arginine methyl ester nor NG-monomethyl L-arginine changed coronary blood flow and myocardial contractile and metabolic functions in the nonischemic myocardium. The cyclic guanosine monophosphate content of epicardial coronary artery increased due to myocardial ischemia; this increase was attenuated with NG-monomethyl L-arginine treatment.

CONCLUSIONS

We conclude that endogenous nitric oxide predominantly decreases the coronary vascular resistance of ischemic endomyocardium, thereby improving myocardial contractility and metabolic function.

Effect of inhaled nitric oxide on venous admixture depends on cardiac output in patients with acute lung injury and acute respiratory distress syndrome

BACKGROUND

It has been shown that inhaled nitric oxide (NO) reduces intrapulmonary venous admixture (QVA/QT) and improves oxygenation in patients suffering from acute respiratory distress syndrome (ARDS). The change in QVA/QT during NO inhalation varies individually. Factors known to influence the respiratory response to NO are the NO concentration and the level of shunt before NO administration. Other factors that may modify the effect on gas-exchange during NO breathing are unknown.

METHODS

We studied the effect of 40 ppm inhaled NO on pulmonary gas-exchange and haemodynamics in 37 patients with acute lung injury (ALI) and ARDS, respectively, and factors that may influence the respiratory response to NO.

RESULTS

Inhalation of 40 ppm NO produced a decrease in mean pulmonary artery pressure (MPAP) from 33.1 +/- 7.2 to 30.2 +/- 6.8 (mean +/- SD) mmHg (P < 0.0001) while pulmonary artery wedge pressure (PAWP), cardiac output and mean arterial pressure remained constant. Change in QVA/QT during NO inhalation depended on the preinhalation cardiac output and had no association with mixed venous oxygen tension, MPAP-PAWP, and QVA/QT before NO delivery. QVA/QT decreased in 26 patients (group 1) and increased in 11 patients (group 2) during NO inhalation. In group 1, cardiac output was lower than in group 2 (8.6 vs 12.2 l.min-1; P < 0.0005).

CONCLUSION

We conclude that the change in venous admixture during inhalation of 40 ppm NO depends on cardiac output. If preinhalation cardiac output is high, 40 ppm NO can adversely affect gas exchange in patients with ALI and ARDS.

The role of nitric oxide in the initiation and in the duration of some vasodilator responses in the coronary circulation

In the coronary bed vasodilation can be mediated by several mechanisms including endothelium-produced nitric oxide. To examine the contribution of nitric oxide, three different techniques to cause vasodilation in the coronary vessels were used in the anaesthetized dog: intracoronary injection of 1 microgram acetylcholine, sudden reduction of the aortic blood pressure inducing a myogenic response and transient occlusion followed by release of the left circumflex coronary artery causing reactive hyperaemia. Each manoeuvre was performed before and after intracoronary administration of 100 mg N-nitro-L-arginine, an inhibitor of the synthesis of nitric oxide. In contrast to previous investigations, the inhibition of nitric oxide synthesis was prevented from causing an increase in blood pressure by the use of a blood-pressure-compensating device. The results observed during each of the three techniques, suggest that the initial cause of the vasodilatation is not the result of the increase of the production of nitric oxide. However, subsequent to the initiation of vasodilation, an increase in the shear stress can result in an increase in the release of nitric oxide from the vascular endothelium, thus prolonging the vasodilatation obtained using each technique.

Control of coronary blood flow by autacoids

Coronary flow and thus myocardial perfusion is regulated by myogenic, metabolic, humoral and neuro-hormonal factors which closely interact with local autacoids released from the endothelial lining of the coronary bed. In a number of disease states an impaired synthesis and release of autacoids decisively limit the overall capacity of coronary regulation and adaptation of myocardial perfusion to increased metabolic demands. The important factors for these control mechanisms are analyzed and reviewed in this article.

Endothelium, coronary vasodilation, and organic nitrates

Recent investigations have suggested that the vascular endothelium is an active participant in the regulation of arterial tone and blood flow. In a state of health, the endothelium contributes to hemodynamic equilibrium; however, it rapidly becomes dysfunctional in hypercholesterolemia and diabetes mellitus or with exposure to the stress of hypertension or long-term smoking. Among the deficits observed during endothelial dysfunction is a reduction in the synthesis and release or an excessive degradation of EDRF. This potent vasorelaxant is derived from the amino acid L-arginine and has been characterized as NO or a closely related substance. EDRF relaxes vascular smooth muscle by activating guanylate cyclase. A deficiency in the activity of EDRF may be the mechanism of diminished coronary vasodilation in patients with ischemic heart disease. Organic nitrates, which are metabolized to NO or S-nitrosothiol at the cellular level, are often used in the management of myocardial ischemia; they also induce vasodilation by activating guanylate cyclase. The similarities between organic nitrates and endogenous EDRF and their interactions are discussed in this review.

Prolonged hypercholesterolemia induces reversible alterations in venous vasomotor function

Hypercholesterolemia is associated with altered arterial endothelial and smooth muscle cell function. This study examines the influence of hypercholesterolemia on external jugular venous endothelial and smooth muscle cell vasoreactivity. Eighteen New Zealand White rabbits received a 1% cholesterol diet: in nine animals, this diet was continued until harvest at 8 weeks (hypercholesterolemic group), but in the other nine animals, the diet was changed to standard rabbit chow after 4 weeks and continued for a further 4 weeks (cholesterol reduction group). The change in the diet resulted in a 70% decrease in serum cholesterol concentration. Eight animals received standard rabbit chow for 8 weeks. Hypercholesterolemia induced hypersensitivity and increased maximal contractions to norepinephrine and endothelin-1. In addition, the maximal response to bradykinin increased, and a contraction to serotonin was induced in the veins from the hypercholesterolemic animals. Cholesterol reduction induced bradykinin hypersensitivity but had no effect on endothelin-1 sensitivity. Norepinephrine hypersensitivity returned to normal and the serotonin response disappeared. A decrease in the maximal contractile responses to these agonists was also observed. Hypercholesterolemia interfered with dose-dependent, EDRF (endothelium derived relaxing factor)-mediated relaxation induced by acetylcholine but, following the reduction of serum cholesterol, normal acetylcholine-induced, endothelium-dependent relaxation returned. Non-endothelium-dependent relaxation to sodium nitroprusside of precontracted veins was unaffected by the presence of high cholesterol concentrations. There were no morphological changes apparent in the veins of either the hypercholesterolemic or the cholesterol reduction groups. In conclusion, this study suggests that hypercholesterolemia induces reversible functional abnormalities in venous tissue and this ability of the jugular veins to recover may be, in part, linked to the lack of morphological changes.

A kinetic model of coronary reactive hyperemic response to transient ischemia

A kinetic model is proposed to delineate the factors that determine the coronary reactive hyperemic response (RHR) to transient ischemia. The model comprises of myocardial-interstitial (M) and vascular (V) compartments. Vasodilator metabolites (VM) are produced in the M compartment during the interval of coronary occlusion. The rate of VM production is dependent on the flow rate during the ischemic period, the ratio of excess flow above the control level (R) to the loss of flow during occlusion period (D), the amount of oxygen stored and the degree of vasodilation in the V compartment prior to occlusion. Following a complete release of occlusion, VM are transported from the M to V compartment and are washed out or degraded with time. The time course of RHR is determined by the coronary patency which is proportional to VM concentration in the V compartment. Based on a set of numerical constants, the model is tested by simulating RHR to the various occlusion manoeuvres: a pair of 10 sec occlusions separated by brief release, a 15 sec release followed by a second brief occlusion, a brief release of an occlusion followed by restricted inflow and a period of restricted inflow after occlusion. The simulated results fit the experimental R/D and RH durations data of canine hearts. Factors that determine the impairment of RH capacity in coronary stenosis are suggested in terms of the model scheme.

Control of coronary blood flow by endothelial release of nitric oxide

1. Nitric oxide (NO) is released from vascular endothelium following conversion of L-arginine to L-citrulline by calcium-calmodulin-dependent 'constitutive' NO-synthase. 2. Nitric oxide release occurs under basal conditions, in response to chemical stimuli (acetylcholine, bradykinin, thrombin, prostacyclin, serotonin, etc.) and in response to changes in shear stress (effects of blood velocity on vascular endothelium). 3. Analogues of L-arginine inhibit NO and are widely used to study the effects of NO on the cardiovascular system: in intact animals, these inhibitors cause vasoconstriction, leading to an increase in arterial blood pressure (ABP) and bradycardia. 4. Bradycardia induced by NO inhibitors is due, in part, to baroreceptor activity following the increase in ABP and in part to a direct effect on the sino-atrial node. 5. In the intact animals and isolated perfused heart, NO inhibitors cause coronary vasoconstriction and hence a reduction in basal coronary flow. This effect, however, is not seen in isolated coronary vessels. 6. From experiments in which ABP did not change, NO does not appear to have an important role in regulating coronary vasomotor tone under basal conditions. 7. Nitric oxide appears to be involved in the duration of reactive hyperaemia following coronary vascular occlusion but is not involved to any significant extent in the peak amplitude of hyperaemia. 8. Responses to vasodilator stimuli which do not involve NO in the initiation of the vasodilation may be prolonged by the effect of increased blood flow (shear stress) which releases NO and potentiates hyperaemia.

Coronary vasomotor responses: role of endothelium and nitrovasodilators

The endogenous nitrovasodilator endothelium-derived nitric oxide (EDNO) is continuously synthetized enzymatically by NO synthase from L-arginine and is released from endothelial cells. Enhanced, superimposed EDNO release can be stimulated by various local and circulating factors, such as bradykinin, ATP, etc., but also most importantly by viscous drag-induced shear stress of the bloodstream acting on the endothelial lining. Thus luminal release suppresses leukocyte adhesion (expression of adhesion molecules), platelet activation, platelet adhesion, and platelet aggregation, and abluminal release counteracts myogenic and neurogenic coronary constrictor tone, thereby increasing myocardial perfusion and dilating large coronary artery calibers. Thus endothelial impairment and denudation (hypercholesterolemia, atheromatosis, balloon catheter interventions) favor excessive constrictor tone and myocardial ischemia. Under these conditions EDNO can be supplemented by compounds (e.g., nitroglycerin, isosorbide dinitrate) converted by biological systems into NO. In addition, it can be supplemented by compounds that even spontaneously release NO (e.g., sydnonimines such as SIN-1 and sodium nitroprusside). EDNO and exogenously supplemented NO stimulate soluble guanylyl cyclase, increase cGMP levels, and bring about vascular relaxation, particularly in those still compliant sections in which EDNO production is impaired and cGMP levels are thus diminished. Exogenous nitrovasodilators are preferentially converted (in the presence of cysteine) enzymatically in large coronary arteries, improving coronary conductance, and in the venous bed (preload reduction), resulting in an improved O2 supply/demand ratio. During chronic, continuous application, neurohormonal counterregulation and diminished enzymatic biotransformation into NO may reduce their effectiveness, resulting in tolerance, particularly in the most sensitive vascular sections, such as veins and coronary arteries. This drawback can be overcome by applying spontaneously NO-releasing compounds, intermittent therapy, or intermittent interposition of other vasodilator principles.

Transient effects of quick changes in myocardial metabolism and perfusion pressure on coronary vasomotor responses

The effects of transient changes in coronary transmural pressure on the coronary vasomotor tone were studied in 23 anesthetized dogs. Increases and decreases of the coronary transmural pressure were obtained by constrictions of various duration (2 to 20 s) of the descending thoracic aorta. The maneuvers were performed in animals with intact cardiac innervation, with the vagi sectioned and with vagal section together with beta-blockade. In the absence of beta-blockade the increase in the transmural pressure caused a transient increase in the coronary vasomotor tone attributable to a myogenic contractile response and the extravascular compression. This contractile response was not observed when the transmural pressure was increased in the presence of high vasomotor tone after beta-blockade. In all animals a transient hyperemia was seen with its peak 8 to 12 s after the release of the aortic constriction. Since its timing and amplitude were independent of the duration of the constriction, the metabolic effect of the increased ventricular afterload, although it may have contributed to the decrease of the coronary resistance, cannot be considered entirely responsible for the hyperemia, which was otherwise compatible with a myogenic vasodilatory response triggered by the sudden fall of the transmural pressure at the release of the constriction. It is concluded that, in the coronary circulation of the intact dog, transient changes in transmural pressure can induce vasomotor responses in which myogenic and metabolic mechanisms combine together in regulating the coronary flow. Changes in extravascular compression can also affect the flow when the experimental maneuver implies changes in the diastolic left ventricular pressure and volume. With the present experimental procedure the myogenic responses have been evidenced when the metabolic factors would have been expected to produce opposite changes in the vasomotor tone.

Myocardial and cerebral hemodynamics during tachyarrhythmia-induced hypotension in the rat

BACKGROUND

The different vulnerabilities of heart and brain to hypotension and hypoxia have been discussed. Hemorrhagic or cardiogenic hypotension appears to cause greater cerebral lesions than drug-induced hypotension. The present model was established to evaluate myocardial blood flow (MBF) and function of the heart and cerebral blood flow (CBF) during tachyarrhythmias and to characterize the capacity of blood flow regulation in the heart and brain during tachycardia-induced borderline hypotension.

METHODS AND RESULTS

MBF and CBF were determined with radiolabeled microspheres. Coronary and central venous oxygen tensions were measured to estimate myocardial and cerebral oxygen consumption (MVO2 and CVO2). Measurements were performed in 62 Sprague-Dawley rats during sinus rhythm and high-rate left ventricular pacing and after hemorrhage. In control rats, MBF and CBF were 5.08 +/- 1.07 and 1.09 +/- 0.29 mL.g-1.min-1. MBF increased (7.21 +/- 1.98 mL.g-1.min-1, P < .05), whereas CBF decreased (0.99 +/- 0.29 mL.g-1.min-1, P = NS) during normotensive high-rate pacing. MBF and CBF dropped to 4.27 +/- 2.24 mL.g-1.min-1 (P = NS) and 0.68 +/- 0.29 mL.g-1.min-1 (P < .05) during pacing-induced borderline hypotension and decreased further during severe hypotension (1.77 +/- 0.81 mL.g-1.min-1, P < .01; 0.45 +/- 0.18 mL.g-1.min-1, P < .01). During borderline hypotension due to hemorrhage, MBF and CBF were 4.05 +/- 0.95 mL.g-1.min-1 (P = NS) and 0.71 +/- 0.23 mL.g-1.min-1 (P < .05). MVO2 and CVO2 were 72.7 +/- 15.4 and 12.7 +/- 3.3 mL.100 g-1.min-1 in control rats. MVO2 increased during normotensive pacing (100.3 +/- 27.4 mL.100 g-1.min-1, P = NS). Mean MVO2 was reduced during pacing-induced borderline hypotension (64.1 +/- 35.6 mL.100 g-1.min-1, P = NS) and severe hypotension (29.8 +/- 15.4 mL.100 g-1.min-1, P < .05). CVO2 decreased in correlation to CBF. Coronary and cerebrovascular resistance and autoregulation indexes indicated a maintenance of MBF regulation and a failure of CBF regulation during borderline hypotensive tachycardias. These results show a dissociation of MBF and CBF after onset of hypotensive tachycardias. Thus, brain tissue appears to be jeopardized at an earlier stage than myocardial muscle during tachyarrhythmias.

CONCLUSIONS

The proposed hypotension model is suitable to analyze tachyarrhythmia-induced hemodynamic changes and end-organ perfusion in the presence of myocardial dysfunction. It has the potential to test therapeutic strategies in the treatment of tachycardias.

Mediation by M3-muscarinic receptors of both endothelium-dependent contraction and relaxation to acetylcholine in the aorta of the spontaneously hypertensive rat

1. Experiments were designed to characterize the subtype(s) of endothelial muscarinic receptor that mediate(s) endothelium-dependent relaxation and contraction in the aorta of spontaneously hypertensive rats (SHR). 2. Rings of SHR aorta with endothelium were suspended in organ baths for the measurement of isometric force. Ecothiopate (an inhibitor of acetylcholinesterase) was present throughout the experiments. Endothelium-dependent contraction to acetylcholine was studied in quiescent aortic rings in the presence of NG-nitro-L-arginine (to prevent the formation of nitric oxide). Endothelium-dependent relaxation to acetylcholine was obtained during contraction to phenylephrine and in the presence of indomethacin (to inhibit cyclo-oxygenase activity). Responses to acetylcholine were assessed against the non-preferential muscarinic receptor antagonist, atropine, and the preferential antagonists pirenzepine (M1), methoctramine (M2) and 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP; M3). 3. The potency of acetylcholine in inducing endothelium-dependent contraction was 6.54 +/- 0.07 (EC50). Atropine, pirenzepine, methoctramine and 4-DAMP displayed competitive antagonism towards the endothelium-dependent contraction to acetylcholine. The pA2 values for these muscarinic receptor antagonists were estimated from Arunlakshana-Schild plots to be (-log M) 9.48 +/- 0.07, 6.74 +/- 0.22, 6.30 +/- 0.20 and 9.39 +/- 0.22 respectively. The potency of acetylcholine in inducing endothelium-dependent relaxation was 7.82 +/- 0.09 (IC50). Atropine, pirenzepine and 4-DAMP displayed competitive antagonism towards the endothelium-dependent relaxation to acetylcholine but methoctramine had no effect. The pA2 values for atropine and 4-DAMP for the relaxation to acetylcholine were estimated from Arunlakshana-Schild plots to be (-log M) 9.15 +/- 0.23 and 9.63 +/- 0.28, respectively. These results suggest that the muscarinic M3 receptor subtype mediates both endothelium-dependent relaxation and contraction to acetylcholine in SHR aorta.

Involvement of nitric oxide in coronary vascular responses to 5-hydroxytryptamine in the anaesthetized greyhound

1. The effect of the intracoronary (i.c.) injection of 5-hydroxytryptamine (5-HT; 0.1-1.0 micrograms/kg) was examined before and after inhibition of nitric oxide (NO) synthesis with N-nitro-L-arginine (NOLA; 5 mg/kg i.c.) in nine anaesthetized greyhounds. Before administration of NOLA, 5-HT increased coronary blood flow (CBF) but decreased large coronary artery diameter indicating simultaneous dilatation of resistance vessels and constriction of large arteries. 2. The administration of NOLA significantly decreased large coronary artery diameter and increased systemic arterial pressure. There was no significant effect on coronary vascular resistance or heart rate. In the presence of NOLA, the 5-HT-induced constriction of the large coronary artery was enhanced and the dilatation of the resistance vessels was reduced. In addition there was a secondary reduction in CBF, a response that was not observed before NOLA treatment. 3. The response to NOLA suggests that a basal release of NO is important in the regulation of coronary and systemic vascular tone. Nitric oxide is an important mediator of coronary vasodilator responses to 5-HT, and in addition the release of NO modulates 5HT-induced constriction of large coronary arteries.

Reduced nitric oxide formation causes coronary vasoconstriction and impaired dilator responses to endogenous agonists and hypoxia in dogs

We investigated the relative contribution of basal and agonist stimulated EDRF/NO release to the adjustment of coronary tone and myocardial perfusion in conscious dogs by inhibiting coronary endothelial NO formation with NG-nitro-L-arginine methyl ester (L-NAME). Chronically instrumented conscious dogs (n = 9) were prepared for measurement of mean arterial blood pressure (MAP), heart rate (HR), coronary blood flow (CF) and diameter of the left circumflex (CDLC) and left anterior descending (CDLAD) coronary artery, respectively. Intracoronary infusions of L-NAME (30.3 mM; 0.25 ml x min-1) caused significant increases in MAP and decreases in HR. CDLC decreased by 3.8% from 3.01 +/- 0.04 to 2.90 +/- 0.04 mm and CF decreases by 30% from 12.9 +/- 0.2 to 9.1 +/- 0.2 (aU). Peak reactive hyperemia (CFmax) evoked by 20-s-lasting occlusions of the left circumflex coronary artery decreased from 29.9 +/- 0.8 to 25.8 +/- 1.0 aU and maximal flow-dependent coronary dilation were reduced from 2.04 +/- 0.08 to 0.91 +/- 0.12% after inhibition of NO-synthesis. Intracoronary infusions of acetylcholine (ACh), adenosine (Ado), bradykinin (Bk), and papaverine (Pap) caused dose-dependent increases in CDLC and CF. Infusion of L-NAME nearly abolished the dilator effect of Ado on CDLC and reduced those to ACh, Bk and Pap. Increases in CF to ACh, Ado and Bk but not to Pap were reduced by L-NAME. Subsequent intracoronary infusions of L-arginine (303 mM; 0.25 ml x min-1) reduced L-NAME-induced CF-changes partly, but did not reverse coronary constriction. These results suggest that inhibition of the continuous release of nitric oxide markedly reduces myocardial perfusion in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence in support of the spasm of resistance vessel concept of ischemic heart disease: an update in 1993

The goal of this communication is to provide more evidence for the ischemic heart disease (IHD) component of the spasm of resistance vessel (S-RV) concept of IHD and other ischemic diseases. The S-RV concept of IHD is considered to be an alternate paradigm which challenges the accepted understanding of this disorder. The theory asserts that primary S-RV directly induces symptoms in IHD, and this position is in opposition to the accepted view that symptoms are induced directly by primary occlusions of epicardial arteries by coronary artery disease, spasm, and thromboses. The theory, if valid, should be useful in reducing the impact of IHD, as it generally is accepted that the correct appreciation of basic pathophysiological mechanisms helps ensure the most appropriate prevention and treatment of disease.

Alpha 1-adrenergic blockade reduces exercise-induced regional myocardial ischemia in dogs

The effect of selective alpha 1-adrenoceptor blockade on regional myocardial blood flow and contractile function during exercise-induced myocardial ischemia was determined in nine dogs having chronic single vessel coronary stenosis (ameroid constrictor). Treadmill exercise performed after beta-adrenoceptor blockade (1.0 mg/kg i.v. propranolol), to block potential prejunctional effects recently recognized with alpha-blockade, elicited severe regional myocardial ischemia with a steady-state reduction of regional systolic wall thickening of the post-stenotic myocardium from a resting value of 21.8% to 7.0%. Blood flow to the ischemic subendocardium was reduced from 0.76 to 0.36 ml/min/g (microspheres). Prazosin (80 micrograms/kg i.v.) was administered during continued running, and angiotensin II was simultaneously infused to prevent the decrease in systemic arterial pressure caused by prazosin. During the subsequent steady state, with arterial pressure matched to the pre-prazosin level, regional subendocardial blood flow improved to 0.60 ml/min/g and systolic wall thickening in the post-stenotic region increased modestly to 10.2%. No changes in blood flow to the outer portion of the post-stenotic region or to the remote control region were seen, and prazosin caused no effect on regional function in the control area, indicating effective blockade of prejunctional effects of prazosin (demonstrated in pilot studies). The finding that blockade of alpha 1-adrenoceptors resulted in improved ischemic zone subendocardial blood flow and contractile function are consistent with alpha 1-adrenoceptor-mediated reduction of blood flow to the ischemic subendocardium during exercise-induced ischemia in dogs with beta-blockade.

Local and neurohumoral control of coronary blood flow

The powerful local metabolic regulation adjusting coronary blood flow to myocardial oxygen consumption under normal conditions is beyond doubt. However, despite substantial experimental efforts the responsible mediators are still largely unknown. Adenosine, a purported mediator of local metabolic control of coronary blood flow, is probably only involved in transient flow adaptations, but not in steady-state coronary autoregulation. Even below the autoregulatory range a substantial vasodilator reserve persists. Recruitment of such vasodilator reserve results in improved regional myocardial blood flow and attenuated regional ischemic dysfunction. beta-adrenergic coronary dilation is of minor functional importance. alpha-adrenergic coronary constriction acts to attenuate increases in coronary blood flow during sympathetic activation under normal conditions, such that myocardial oxygen extraction increases to match the increased oxygen consumption. alpha-adrenergic coronary constriction remains operative in ischemic myocardium, thus precipitating or contributing to acute myocardial ischemia during sympathetic activation and exercise in experimental animals as well as in patients with stable angina. The vagal transmitter acetylcholine--upon exogenous intracoronary infusion--induces critical constriction of epicardial coronary arteries with endothelial dysfunction and atherosclerosis. However, a vagal initiation of coronary spasm or myocardial ischemia has not been documented so far. Similarly, peptide hormones/transmitters such as NPY, vasopressin, and angiotensin can induce myocardial ischemia upon exogenous administration. Their pathophysiological role in myocardial ischemia and reperfusion, however, remains to be established.

Endothelium-derived hyperpolarizing factor and endothelium-dependent relaxations

The endothelial cells inhibit the tone of the underlying vascular smooth muscle by releasing endothelium-derived relaxing factors (EDRF). The existence of at least two such factors, nitric oxide and endothelium-derived hyperpolarizing factor (EDHF), has been demonstrated. EDHF is an as yet unidentified substance that hyperpolarizes vascular smooth muscle cells and causes their relaxation. The contribution of endothelium-dependent hyperpolarization varies along the vascular tree. Particularly in smaller blood vessels, EDHF acts on vascular smooth muscle in cooperation with nitric oxide. Basal release of EDHF is not likely to occur, at least in vitro. The production and/or release of EDHF is regulated by the cytosolic concentration of Ca2+ ions, derived both from the extracellular space and intracellular stores. Calmodulin may be involved in its production and/or release. EDHF hyperpolarizes the vascular smooth muscle by opening K+ channels. The hyperpolarization closes voltage-dependent Ca2+ channels and, as a consequence, EDHF relaxes blood vessels. In the absence of chemical identification of EDHF, it is difficult to assess its contribution to endothelium-dependent relaxations in vivo.

Nitric oxide is an important determinant of coronary flow in the isolated blood perfused rat heart

Many vasoactive substances are involved in the regulation of vasomotor tone and some of them, like nitric oxide (NO), are derived from the endothelium. Nitric oxide is able to relax preconstricted coronary resistance vessels almost completely. However, it is not clear what the contribution of NO is to vasomotor tone in the intact blood perfused heart. The aim of the present study was to evaluate the contribution of NO to coronary pressure-flow relations. We used isovolumically beating, donor supported, blood perfused isolated rat hearts. We measured pressure-flow relations under control conditions, after blocking endothelial NO production with NG-nitro-L-Arginine (LNNA) and after administration of L-Arginine (L-Arg) in order to overrule the blocking effect. Administration of LNNA at a perfusion pressure of 105 mm Hg resulted, after about 40 min, in a significant (Wilcoxon's signed-rank test, (n = 8) p < 0.05) reduction of coronary flow to 47 +/- 5% (mean +/- SEM) of control and a reduction of developed isovolumic left-ventricular pressure to 62 +/- 4% of control. L-Arg returned flow to 60 +/- 7% of control which is a significant increase with respect to LNNA (p < 0.05). L-Arg did not increase the left-ventricular pressure. The entire perfusion pressure-flow relation (pressure range 65-125 mm Hg) was significantly shifted downwards after LNNA with respect to control. Pressure-flow relations after L-Arg were in between those during control and after block of NO production. L-Arg alone was found to have no effect on flow and left-ventricular pressure (n = 2) and both LNNA and L-Arg were found to have no effect on contractility of isolated trabeculae (n = 6), thus, coronary blood flow reduction after LNNA administration is mainly the result of inhibition of endothelial NO production. At a perfusion pressure of 105 mm Hg reactive hyperemia is still present after LNNA and subsequent L-Arg administration, indicating that endothelial NO is not the only factor involved in flow regulation. We conclude that endothelium-derived NO is involved in the control of coronary flow in the blood perfused rat heart.

Endothelial vasomotor regulation in health and disease

The purpose of this review is to provide the anaesthetists with a comprehensive update on the endothelial-cell control of local blood flow. This single cell layer was originally thought to represent only a passive barrier. It is now evident that it plays an active role in a broad variety of biological functions. Since the discovery of the endothelial-derived relaxing factor (EDRF), it has been the subject of a considerable amount of research. It is established that EDRF is secreted continuously at a basal state and that many physical stimuli as well as vasoactive substances can modulate its secretion. Evidence presented indicates that the endogenous vasodilatation produced by EDRF is similar to that of the exogenous nitrovasodilator nitroglycerin and nitroprusside (i.e., nitric oxide). Aside from EDRF, the endothelium produces other vasodilating as well as vasoconstricting factors. A review of the physiology of the endothelium regarding the local control of blood flow is provided along with its influence upon several pathophysiological states. Also included is an overview of the influence of anaesthetic agents on endothelial function. These findings linking vasomotor control to endothelial function will help to explain pathophysiological process and may lead to new therapeutic modalities.

Nitrates in different vascular beds, nitrate tolerance, and interactions with endothelial function

The favorable anti-ischemic effect of nitrates is based on the unique distribution pattern of vascular relaxation that they evoke in different vascular sections. Nitrovasodilators reduce cardiac preload and wall tension, and thus myocardial oxygen consumption. They increase precollateral coronary perfusion pressure, thereby augmenting oxygen delivery to ischemic sections, especially to the subendocardial layers. These vasodilator actions are caused by the nitric oxide (NO)-induced activation of soluble guanylyl cyclase, which augments vascular cyclic guanosine monophosphate (cGMP) levels to suppress intracellular Ca2+ concentrations. After some metabolic steps NO is finally cleaved from all nitrovasodilators and is probably identical with, or very closely related to, endothelium-derived relaxing factor (EDRF). A dinitrosyl-iron complex may serve under biologic conditions to stabilize the NO- radical, which has an extremely short half-life. NO derived from nitrovasodilators is used therapeutically to substitute for a deficient endothelium-mediated vascular control and autacoid production.

Mechanisms of action and efficacy of nitrates in heart failure

Nitrates exert hemodynamic and possibly nonhemodynamic effects that result in reduced cardiac filling pressures, increased cardiac output, reduced pulmonary vascular pressures, and improvement in symptoms and exercise tolerance in patients with heart failure. Combined with hydralazine, chronic oral administration of isosorbide dinitrate has been demonstrated to improve survival when added to digoxin and diuretic therapy. A long-term improvement in left ventricular ejection fraction in these clinical studies has raised the possibility that the nitrates may be acting at least in part by inhibiting left ventricular remodeling. The precise role of nitrates and the optimal dosing regimen in the therapeutic armamentarium for heart failure, particularly in patients already treated with a converting enzyme inhibitor, remains to be established.

Clinical relevance of endothelium-derived relaxing factor (EDRF)

1. In addition to metabolic and neurohumoral factors endothelium-derived autacoids like the nitric oxide radical NO and prostacyclin are effective regulators of vascular tone and thus tissue perfusion. NO is produced in endothelial cells from L-arginine by a Ca2+/calmodulin-dependent enzyme NO synthase. In addition, the NO radical is ultimately cleaved from all nitrovasodilators and resembles their vasoactive and antiaggregatory principle, which is used under pathological conditions as substitution therapy for impaired endothelial function and autacoid production. Impaired endothelium-dependent vasomotor control has been documented in hypercholesterolaemia, atheromatosis, diabetes, hypertension, and in reperfusion damage. L-arginine supplementation is effective in a few instances.

The relationship between regional blood flow and contractile function in normal, ischemic, and reperfused myocardium

The prevailing paradigm of coronary physiology and pathophysiology is that a balance between blood flow (i.e., supply) and function (i.e., demand) exists under normal conditions and that an imbalance between supply and demand occurs during ischemia. However, this paradigm is derived largely from studies relating changes in total coronary inflow to global ventricular function. The present article examines the relationship between myocardial blood flow and function on a regional level and proposes that a change may be needed in the current paradigm of coronary pathophysiology. In normal myocardium, considerable heterogeneity of regional blood flow exists, indicating either similar heterogeneity of metabolic demand and function or questioning the precision of metabolic coupling between flow and function. After the onset of ischemia, a transient imbalance between the reduced blood flow and function may exist. However, myocardial function rapidly declines and during early steady-state ischemia regional myocardial blood flow and function are once again evenly matched. Such supply-demand balance may persist over prolonged periods of ischemia enabling the myocardium to remain viable through reduction of energy expenditure for contractile function, i.e., to "hibernate". Whereas in "hibernating" ischemic myocardium, regional myocardial blood flow and function are both reduced but appropriately matched to one another, flow and function appear to be largely uncoupled in reperfused "stunned" myocardium. The clinical identification of viable but ischemic (hibernating) and postischemic (stunned) myocardium is of utmost importance in patients undergoing reperfusion procedures. A new paradigm of coronary and myocardial pathophysiology, encompassing a regional as well as a global view of perfusion and function, will have to include explanations for phenomena such as myocardial hibernation and myocardial stunning.

Endothelium-mediated regulation of coronary tone

To what extent endothelial autacoids like endothelium-derived relaxant factor/nitric oxide (EDRF/NO), in addition to neural-humoral factors, are involved in the regulation of myocardial perfusion, is presently not known. Therefore, we investigated in conscious, chronically instrumented dogs the effect of stereospecific inhibitors (NG-monomethyl-L-arginine (L-NMMA), NG-nitro-L-arginine (L-NNA), NG-monomethylester-L-arginine (L-NAME] of nitric oxide-synthesis and -release on epicardial coronary tone (and coronary diameter) and myocardial perfusion. A hydraulic coronary cuff was used, to produce reactive hyperemia and to keep the myocardial perfusion constant over short periods. 40 mg/kg L-NNA i.v. caused a long-lasting increase in mean arterial blood pressure from 94 +/- 8 to 129 +/- 11 mmHg and a simultaneous decrease in coronary diameter by 2.8 +/- 0.3%. Heart rate dropped from 87 to 58 min-1, but the double product of heart rate and blood pressure dropped by only 8 +/- 2% (p = 0.05). The maximal coronary conductance during peak reactive hyperemia (after 20 s ischemia) indicating complete coronary dilation was diminished by 48% after L-NNA. The severe drop in resting myocardial perfusion and O2-supply, and nearly unchanged rate pressure product and thus myocardial metabolic rate following the inhibition of nitric oxide formation demonstrate a substantial contribution of EDRF/NO to the regulation of myocardial perfusion.

ACE inhibitors for the treatment of myocardial ischemia?

Apart from their established use in the treatment of hypertension and heart failure, ACE inhibitors have been suggested to exert anti-ischemic effects. This article reviews the mechanisms of systemic and intracardiac angiotensin formation, as well as its interaction with the bradykinin, the prostaglandin, and the sympathetic nervous system. While high doses of angiotensin can precipitate myocardial ischemia. experimental data on a potential beneficial effect of ACE inhibitors on ischemic myocardial blood flow and function are inconsistent and controversial. Pooling the few available clinical data, several ACE inhibitors may attenuate myocardial ischemia at rest and during exercise. However, a significant fraction of patients does not benefit or even deteriorates. Recent experimental studies suggest a beneficial role of ACE inhibitors in attenuating reperfusion arrhythmias and postinfarction left ventricular remodeling. Unless the mechanisms and determinants of potential anti-ischemic actions of ACE inhibitors can be better defined, their use for treatment of myocardial ischemia cannot be recommended at present.