Coronary vascular responsiveness to adenosine is impaired additively by blockade of nitric oxide synthesis and a sulfonylurea

Impaired Vascular Redox Signaling in the Vascular Complications of Obesity and Diabetes Mellitus

Significance: Oxidative stress, a crucial regulator of vascular disease pathogenesis, may be involved in the vascular complications of obesity, systemic insulin resistance (IR), and diabetes mellitus (DM). Recent Advances: Excessive production of reactive oxygen species in the vascular wall has been linked with vascular disease pathogenesis. Recent evidence has revealed that vascular redox state is dysregulated in cases of obesity, systemic IR, and DM, potentially participating in the well-known vascular complications of these disease entities. Critical Issues: The detrimental effects of obesity and the metabolic syndrome on vascular biology have been extensively described at a clinical level. Further, vascular oxidative stress has often been associated with the presence of obesity and IR as well as with a variety of detrimental vascular phenotypes. However, the mechanisms of vascular redox state regulation under conditions of obesity and systemic IR, as well as their clinical relevance, are not adequately explored. In addition, the notion of vascular IR, and its relationship with systemic parameters of obesity and systemic IR, is not fully understood. In this review, we present all the important components of vascular redox state and the evidence linking oxidative stress with obesity and IR. Future Directions: Future studies are required to describe the cellular effects and the translational potential of vascular redox state in the context of vascular disease. In addition, further elucidation of the direct vascular effects of obesity and IR is required for better management of the vascular complications of DM.

Regulation of Coronary Blood Flow

The heart is uniquely responsible for providing its own blood supply through the coronary circulation. Regulation of coronary blood flow is quite complex and, after over 100 years of dedicated research, is understood to be dictated through multiple mechanisms that include extravascular compressive forces (tissue pressure), coronary perfusion pressure, myogenic, local metabolic, endothelial as well as neural and hormonal influences. While each of these determinants can have profound influence over myocardial perfusion, largely through effects on end-effector ion channels, these mechanisms collectively modulate coronary vascular resistance and act to ensure that the myocardial requirements for oxygen and substrates are adequately provided by the coronary circulation. The purpose of this series of Comprehensive Physiology is to highlight current knowledge regarding the physiologic regulation of coronary blood flow, with emphasis on functional anatomy and the interplay between the physical and biological determinants of myocardial oxygen delivery. © 2017 American Physiological Society. Compr Physiol 7:321-382, 2017.

Fractional flow reserve: physiological basis, advantages and limitations, and potential gender differences

Fractional flow reserve (FFR) is a physiological index of the severity of a stenosis in an epicardial coronary artery, based on the pressure differential across the stenosis. Clinicians are increasingly relying on this method because it is independent of baseline flow, relatively simple, and cost effective. The accurate measurement of FFR is predicated on maximal hyperemia being achieved by pharmacological dilation of the downstream resistance vessels (arterioles). When the stenosis causes FFR to be impaired by > 20%, it is considered to be significant and to justify revascularization. A diminished hyperemic response due to microvascular dysfunction can lead to a false normal FFR value, and a misguided clinical decision. The blunted vasodilation could be the result of defects in the signaling pathways modulated (activated or inhibited) by the drug. This might involve a downregulation or reduced number of vascular receptors, endothelial impairment, or an increased activity of an opposing vasoconstricting mechanism, such as the coronary sympathetic nerves or endothelin. There are data to suggest that microvascular dysfunction is more prevalent in post-menopausal women, perhaps due to reduced estrogen levels. The current review discusses the historical background and physiological basis for FFR, its advantages and limitations, and the phenomenon of microvascular dysfunction and its impact on FFR measurements. The question of whether it is warranted to apply gender-specific guidelines in interpreting FFR measurements is addressed.

Contribution of adenosine A(2A) and A(2B) receptors to ischemic coronary dilation: role of K(V) and K(ATP) channels

This study was designed to elucidate the contribution of adenosine A(2A) and A(2B) receptors to coronary reactive hyperemia and downstream K(+) channels involved. Coronary blood flow was measured in open-chest anesthetized dogs. Adenosine dose-dependently increased coronary flow from 0.72 ± 0.1 to 2.6 ± 0.5 mL/minute/g under control conditions. Inhibition of A(2A) receptors with SCH58261 (1 μm) attenuated adenosine-induced dilation by ∼50%, while combined administration with the A(2B) receptor antagonist alloxazine (3 μm) produced no additional effect. SCH58261 significantly reduced reactive hyperemia in response to a transient 15 second occlusion; debt/repayment ratio decreased from 343 ± 63 to 232 ± 44%. Alloxazine alone attenuated adenosine-induced increases in coronary blood flow by ∼30% but failed to alter reactive hyperemia. A(2A) receptor agonist CGS21680 (10 μg bolus) increased coronary blood flow by 3.08 ± 0.31 mL/minute/g. This dilator response was attenuated to 0.76 ± 0.14 mL/minute/g by inhibition of K(V) channels with 4-aminopyridine (0.3mm) and to 0.11 ± 0.31 mL/minute/g by inhibition of K(ATP) channels with glibenclamide (3 mg/kg). Combined administration abolished vasodilation to CGS21680. These data indicate that A(2A) receptors contribute to coronary vasodilation in response to cardiac ischemia via activation of K(V) and K(ATP) channels.

Coronary Blood Flow Responses are Impaired Independent of NO and Endothelial Function in Conscious Dogs with Dilated Cardiomyopathy

BACKGROUND

Dilated cardiomyopathy (DCM) is characterized by nitric oxide (NO) deficiency and endothelial dysfunction. Whether endothelium-independent vasodilation is preserved, particularly in the coronary circulation, remains controversial.

METHODS AND RESULTS

We studied systemic and coronary flow responses to the endothelium-dependent agonist, acetylcholine, the cGMP-dependent NO-donor, nitroglycerin, the predominantly endothelium-independent agonist, adenosine, the beta-adrenergic cAMP-dependent agonist, isoproterenol, and the calcium channel antagonist, nicardipine, in conscious dogs with pacing-induced DCM. Systemic blood flow response was impaired to acetylcholine but preserved to other vasodilators in DCM. In contrast, coronary blood flow response was significantly ( P < .05) depressed to all agonists. (Peak coronary blood flow response, control versus DCM: acetylcholine: 221 +/- 14% versus 156 +/- 11%; nitroglycerin: 220 +/- 17% versus 138 +/- 9%; adenosine: 635 +/- 65% versus 376 +/- 56%; nicardipine: 338 +/- 59% versus 115 +/- 23%; isoproterenol: 219 +/- 18% versus 86 +/- 20%). The attenuation was independent of systemic hemodynamic differences.

CONCLUSION

In contrast to systemic responses, coronary blood flow responses in DCM are impaired dependent or independent of NO or second messenger mechanisms, implying either distal signaling defects or structural abnormalities in the coronary vasculature.

Matching coronary blood flow to myocardial oxygen consumption

At rest the myocardium extracts approximately 75% of the oxygen delivered by coronary blood flow. Thus there is little extraction reserve when myocardial oxygen consumption is augmented severalfold during exercise. There are local metabolic feedback and sympathetic feedforward control mechanisms that match coronary blood flow to myocardial oxygen consumption. Despite intensive research the local feedback control mechanism remains unknown. Physiological local metabolic control is not due to adenosine, ATP-dependent K(+) channels, nitric oxide, prostaglandins, or inhibition of endothelin. Adenosine and ATP-dependent K(+) channels are involved in pathophysiological ischemic or hypoxic coronary dilation and myocardial protection during ischemia. Sympathetic beta-adrenoceptor-mediated feedforward arteriolar vasodilation contributes approximately 25% of the increase in coronary blood flow during exercise. Sympathetic alpha-adrenoceptor-mediated vasoconstriction in medium and large coronary arteries during exercise helps maintain blood flow to the vulnerable subendocardium when cardiac contractility, heart rate, and myocardial oxygen consumption are high. In conclusion, several potential mediators of local metabolic control of the coronary circulation have been evaluated without success. More research is needed.

Effect of ATP-sensitive potassium channel inhibition on coronary metabolic vasodilation in humans

OBJECTIVE

Experimental evidence indicates that ATP-sensitive potassium (K(ATP)) channels regulate coronary blood flow (CBF). However, their contribution to human coronary metabolic vasodilation is unknown.

METHODS AND RESULTS

Seventeen patients (12 male, age 58+/-10 years) were studied. Coronary hemodynamics were assessed before and after K(ATP) channel inhibition with subselective intracoronary glibenclamide infused at 40 microg/min in an angiographically smooth coronary artery after successful percutaneous coronary intervention to another vessel. Metabolic vasodilation was induced by 2 minutes of rapid right ventricular pacing. Coronary blood velocity was measured with a Doppler guidewire and CBF calculated. The time course of hyperemia was recorded for 2 minutes after pacing, and hyperemic volume was estimated from the area under the flow-versus-time curve (AUC). Compared with vehicle infusion (0.9% saline), glibenclamide reduced resting CBF by 9% (P=0.04) and increased resting coronary vascular resistance (CVR) by 15% (P=0.03). Glibenclamide reduced pacing-induced peak CBF (50.8+/-6.8 versus 42.0+/-5.4 mL/min, P=0.001), peak CBF corrected for baseline flow (25.1+/-4.6 versus 17.6+/-3.1 mL/min, P=0.01), and increased minimum CVR (2.6+/-0.3 versus 3.1+/-0.3 mm Hg/mL per minute, P=0.002). Compared with vehicle, glibenclamide reduced total AUC at 2 minutes (3535+/-397 versus 3027+/-326 mL, P=0.001).

CONCLUSIONS

Vascular K(ATP) channels appear to be involved in functional coronary hyperemia after metabolic stimulation.

Plasma asymmetric dimethylarginine and hyperemic myocardial blood flow in young subjects with borderline hypertension or familial hypercholesterolemia

OBJECTIVE

The goal of this study was to examine the relationship between plasma asymmetric dimethylarginine (ADMA) level and hyperemic myocardial blood flow (MBF) in subjects with borderline hypertension (BHT) and familial hypercholesterolemia (FH).

METHODS

Asymmetric dimethylarginine is an endogenous competitive inhibitor of nitric oxide synthase that may modulate vascular function. We measured plasma ADMA levels and myocardial flow in 77 young men (mean age 35 +/- 5 years), including 47 healthy controls, 16 men with BHT, and 14 men with FH. Basal and dipyridamole-induced myocardial flow was measured using positron emission tomography. Plasma ADMA levels were measured using high-pressure liquid chromatography.

RESULTS

Asymmetric dimethylarginine levels were significantly elevated in the BHT group compared with controls (0.59 +/- 0.13 micromol/l vs. 0.43 +/- 0.12 micromol/l, p < 0.001), and they had significantly lower dipyridamole flow (2.85 +/- 1.20 ml/min/g vs. 3.69 +/- 1.68 ml/min/g, p < 0.05). In a multivariate regression model adjusted for the study group, dipyridamole flow was inversely associated with ADMA (p < 0.05), age (p < 0.05), and apolipoprotein B concentration (p < 0.05).

CONCLUSIONS

We conclude that plasma ADMA concentration is related to dipyridamole-induced vasodilatory function in young men, independently of blood pressure elevation and hypercholesterolemia. Subjects with BHT have significantly increased plasma ADMA levels, which may partly explain the impaired hyperemic MBF in this condition.

Regulation of flow and wall shear stress in arteriolar networks of the hamster cheek pouch

Our purpose was to define arteriolar network hemodynamics during moderate increases in interstitial adenosine or nitric oxide in the hamster (n = 34, pentobarbital sodium 70 mg/kg) cheek pouch tissue. The network consists of a feed arteriole (approximately 12-microm diameter, approximately 800-microm length) with three to six branches. Observations of diameter, red blood cell flux, and velocity were obtained at the feed before the branch and within the branch. A comparison of baseline with suffused adenosine or sodium nitroprusside (SNP) 10(-9) to 10(-5) M showed the following. First, diameter change was heterogeneous by agonist, did not reflect the expected dilatory response, and was related to location within the network. With adenosine, upstream branch points constricted and those downstream dilated, even at 10(-5) M. With SNP, upstream branch points dilated, whereas those downstream constricted. Second, with adenosine, changes in diameter, flux, and velocity together resulted in no change in wall shear stress until 10(-5) M. Wall shear stress was not maintained at a constant level with Nomega-nitro-L-arginine (10(-5) M), suggesting a role for flow-dependent diameter changes with adenosine. With SNP, diameter change correlated with the baseline (before SNP) shear stress conditions.

Control of coronary blood flow during exercise

Under normal physiological conditions, coronary blood flow is closely matched with the rate of myocardial oxygen consumption. This matching of flow and metabolism is physiologically important due to the limited oxygen extraction reserve of the heart. Thus, when myocardial oxygen consumption is increased, as during exercise, coronary vasodilation and increased oxygen delivery are critical to preventing myocardial underperfusion and ischemia. Exercise coronary vasodilation is thought to be mediated primarily by the production of local metabolic vasodilators released from cardiomyocytes secondary to an increase in myocardial oxygen consumption. However, despite various investigations into this mechanism, the mediator(s) of metabolic coronary vasodilation remain unknown. As will be seen in this review, the adenosine, K(+)(ATP) channel and nitric oxide hypotheses have been found to be inadequate, either alone or in combination as multiple redundant compensatory mechanisms. Prostaglandins and potassium are also not important in steady-state coronary flow regulation. Other factors such as ATP and endothelium-derived hyperpolarizing factors have been proposed as potential local metabolic factors, but have not been examined during exercise coronary vasodilation. In contrast, norepinephrine released from sympathetic nerve endings mediates a feed-forward betaadrenoceptor coronary vasodilation that accounts for approximately 25% of coronary vasodilation observed during exercise. There is also a feed-forward alpha-adrenoceptor-mediated vasoconstriction that helps maintain blood flow to the vulnerable subendocardium when heart rate, myocardial contractility, and oxygen consumption are elevated during exercise. Control of coronary blood flow during pathophysiological conditions such as hypertension, diabetes mellitus, and heart failure is also addressed.

Effect of ATP-sensitive potassium channel inhibition on resting coronary vascular responses in humans

Experimental data suggest that vascular ATP-sensitive potassium (K(ATP)) channels regulate coronary blood flow (CBF), but their role in regulating human CBF is unclear. We sought to determine the contribution of K(ATP) channels to resting conduit vessel and microvascular function in the human coronary circulation. Twenty-five patients (19 male/6 female, aged 56 +/- 12 years) were recruited. Systemic and coronary hemodynamics were assessed in 20 patients before and after K(ATP) channel inhibition with graded intracoronary glibenclamide infusions (4, 16, and 40 microg/min), in an angiographically smooth or mildly stenosed coronary artery following successful elective percutaneous coronary intervention to another vessel. Coronary blood velocity was measured with a Doppler guidewire and CBF calculated. Adenosine-induced hyperemia was determined following bolus intracoronary adenosine injection (24 microg). Time control studies were undertaken in 5 patients. Compared with vehicle infusion (0.9% saline), glibenclamide reduced resting conduit vessel diameter from 2.5 +/- 0.1 to 2.3 +/- 0.1 mm (P<0.01), resting CBF by 17% (P=0.05), and resting CBF corrected for rate pressure-product by 18% (P=0.01) in a dose-dependent manner. A corresponding 24% increase in coronary vascular resistance was noted at the highest dose (P<0.01). No alteration to resting CBF was noted in the time control studies. Glibenclamide reduced peak adenosine-induced hyperemia (P=0.01) but did not alter coronary flow reserve. Plasma insulin increased from 5.6 +/- 1.2 to 7.6 +/- 1.3 mU/L (P=0.02); however, plasma glucose was unchanged. Vascular K(ATP) channels are involved in the maintenance of basal coronary tone but may not be essential to adenosine-induced coronary hyperemia in humans.

Nitric oxide modulates right ventricular flow and oxygen consumption during norepinephrine infusion

This study was designed to test if nitric oxide (NO) contributes to norepinephrine-induced right coronary vasodilation and if NO blunts the norepinephrine-induced increase in myocardial oxygen consumption (MVO(2)) in the right ventricle. In five anesthetized, open-chest dogs, mean aortic pressure, heart rate, right ventricular rate of pressure development over time (dP/dt), right coronary blood flow, and right ventricular MVO(2) were measured before and during graded intracoronary infusions of norepinephrine in the absence and presence of a NO synthase blocker, N(omega)-nitro-L-arginine methyl ester (L-NAME; 150 microg/min i.c.). During both conditions, right coronary blood flow and right ventricular MVO(2) significantly increased with graded infusions of norepinephrine. L-NAME significantly blunted the coronary hyperemic response to norepinephrine, although L-NAME did not alter the relationship between right ventricular MVO(2) and norepinephrine dose. However, when right ventricular function was indexed by heart rate x right ventricular maximum dP/dt x peak right ventricular systolic pressure, L-NAME significantly increased the oxygen cost of right ventricular function. These results indicate that NO contributes to norepinephrine-induced right coronary vasodilation and improves right ventricular oxygen utilization efficiency.

Early impairment of coronary flow reserve is not associated with Chlamydia pneumoniae antibodies

BACKGROUND

Chlamydia pneumoniae infection has been associated with atherosclerosis by sero-epidemiological, histopathological and interventional studies, and animal experiments. We hypothesized that if chlamydial infection is causative of atherosclerosis, the occurrence of antibodies against C. pneumoniae should be associated with coronary vasomotor dysfunction - an early sign of atherosclerosis.

AIM

To study the association between C. pneumoniae infection and coronary vasomotor function in young men without signs of ischemic heart disease.

METHODS

Serum IgG and IgA antibody concentrations against C. pneumoniae were determined in 125 clinically healthy subjects undergoing positron emission tomography (PET) studies. Myocardial blood flow was measured at rest and during pharmacologically induced hyperemia using [15O]H2O Coronary flow reserve was calculated as the ratio of hyperemic blood flow to resting blood flow.

RESULTS

No association was found between serum C. pneumoniae antibody concentrations and myocardial blood flow parameters. In contrast, more conventional risk factors for coronary artery disease, such as total cholesterol and apolipoprotein B, were inversely associated with hyperemic flow and flow reserve.

CONCLUSIONS

We found no association between C. pneumoniae antibodies and coronary vasomotor function in subjects without ischemic heart disease. Thus, these results do not support the role of C. pneumoniae infection as an early phase risk factor for coronary artery disease.

Influence of nitric oxide synthase and adrenergic inhibition on adenosine-induced myocardial hyperemia

BACKGROUND

Myocardial perfusion during adenosine-induced hyperemia is used both in clinical diagnosis of coronary heart disease and for scientific investigations of the myocardial microcirculation. The objective of this study was to clarify whether adenosine-induced hyperemia is dependent on endothelial NO production or is influenced by adrenergic mechanisms.

METHODS AND RESULTS

In 12 healthy men, myocardial perfusion was measured with PET in 2 protocols performed in random order, each including 3 perfusion measurements. First, perfusion was measured at rest. Second, either saline or the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 4 mg/kg) was infused, and perfusion during adenosine-induced hyperemia was determined. Last, in both protocols, the alpha-receptor blocker phentolamine was infused, and perfusion during adenosine-induced hyperemia was determined again. Resting perfusion was similar in the 2 protocols (0.69+/-0.14 and 0.66+/-0.18 mL. min(-1). g(-1)). L-NAME increased mean arterial blood pressure by 12+/-7 mm Hg (P<0.01) and reduced heart rate by 16+/-7 bpm (P<0.01). Adenosine-induced hyperemia (1.90+/-0.33 mL. min(-1). g(-1)) was attenuated by L-NAME (1.50+/-0.55 mL. min(-1). g(-1), P<0.01). The addition of phentolamine had no effect on the adenosine-induced hyperemia (2.10+/-0.34 mL. min(-1). g(-1), P=NS). In the presence of L-NAME, however, when the adenosine response was attenuated, phentolamine was able to increase hyperemic perfusion (2.05+/-0.44 mL. min(-1). g(-1), P<0.05).

CONCLUSIONS

Inhibition of endogenous NO synthesis attenuates myocardial perfusion during adenosine-induced hyperemia, indicating that coronary vasodilation by adenosine is partly endothelium dependent. alpha-Adrenergic blockade has no effect on adenosine-induced hyperemia unless NO synthesis is inhibited.

eNOS 894T allele and coronary blood flow at rest and during adenosine-induced hyperemia

The 894T allele of a G894T polymorphism in the endothelial nitric oxide synthase (eNOS) gene is associated with decreased eNOS activity, cleavage of the protein, and endothelial dysfunction. The present study evaluated the association with coronary blood flow (CBF) at rest and during adenosine (ADO)-induced hyperemia. CBF was determined by Doppler flow wire and angiography in 97 left anterior descending arteries of individuals without coronary artery disease. At rest, average peak velocity (APV) was lower and coronary vascular resistance (CVR) was higher in homozygous carriers of the 894T allele than in heterozygotes and individuals without the 894T allele. CBF tended to be lower in eNOS 894T allele carriers. During ADO-induced hyperemia (18 microg ic), APV, CVR, and CBF were not statistically different between the genotypes. The reduced APV at rest in conjunction with an increased CVR indicates a vasomotor dysfunction related to an increased microvascular resting tone in eNOS 894T allele carriers.

Endogenous nitric oxide modulates myocardial oxygen consumption in canine right ventricle

The role of endogenous nitric oxide (NO) in modulating myocardial oxygen consumption (MVO2) is unclear, in part because of systemic and coronary hemodynamic effects of blocking NO release. This study evaluated the effect of NO on right ventricular MVO2 under controlled hemodynamic conditions. In 12 open-chest dogs, N(omega)-nitro-L-arginine methyl ester (L-NAME, 150 microg/min), a NO synthase (NOS) blocker, was infused into the right coronary artery. Heart rate and mean aortic pressure were constant. Right coronary blood flow and right ventricular MVO2 were measured at normal and elevated right coronary perfusion pressures (RCP) before and after L-NAME. To avoid effects of NO synthesis blockade on right coronary blood flow, which might have altered right ventricular MVO2, experiments, were conducted during adenosine-induced maximal coronary vasodilation. L-NAME did not affect right coronary blood flow (P = 0.51). However, L-NAME significantly increased right ventricular MVO2 (6% at RCP 100 mmHg, and 21% at RCP 180 mmHg). Right coronary blood flow varied with perfusion pressure (P < 0.02), and the elevation of MVO2 produced by L-NAME increased at higher flows (P < 0.04), consistent with the greater shear stress-mediated release of NO. These findings indicate that endogenous NO limits right ventricular MVO2.

Role of nitric oxide and adenosine in control of coronary blood flow in exercising dogs

BACKGROUND

Inhibition of nitric oxide (NO) synthesis results in very little change in coronary blood flow, but this is thought to be because cardiac adenosine concentration increases to compensate for the loss of NO vasodilation. Accordingly, in the present study, adenosine measurements were made before and during NO synthesis inhibition during exercise.

METHODS AND RESULTS

Experiments were performed in chronically instrumented dogs at rest and during graded treadmill exercise before and during inhibition of NO synthesis with N(omega)-nitro-L-arginine (L-NNA, 35 mg/kg IV). Before inhibition of NO synthesis, myocardial oxygen consumption increased approximately 3.7-fold, and coronary blood flow increased approximately 3.2-fold from rest to the highest level of exercise, and this was not changed by NO synthesis inhibition. Coronary venous oxygen tension was modestly reduced by L-NNA at all levels of myocardial oxygen consumption. However, the slope of the relationship between myocardial oxygen consumption and coronary venous oxygen tension was not altered by L-NNA. Inhibition of NO synthesis did not increase coronary venous plasma or estimated interstitial adenosine concentration. During exercise, estimated interstitial adenosine remained well below the threshold concentration necessary for coronary vasodilation before or after L-NNA.

CONCLUSIONS

NO causes a modest coronary vasodilation at rest and during exercise but does not act as a local metabolic vasodilator. Adenosine does not mediate a compensatory local metabolic coronary vasodilation when NO synthesis is inhibited.

Role of nitric oxide in vascular tone and in reactivity to isoproterenol and adenosine in the goat coronary circulation

The present study examined the role of nitric oxide in coronary vascular tone and in the coronary vasodilatation in response to beta-adrenoceptor stimulation and adenosine. In anesthetized goats, the effects of intracoronary and i.v. administration of the inhibitor of nitric oxide synthesis, N(w)-nitro-L-arginine methyl ester (L-NAME), and those of isoproterenol, adenosine and acetylcholine on coronary blood flow, measured electromagnetically in the left circumflex coronary artery, were recorded. Intracoronary infusion of L-NAME (30-40 microg kg(-1) min(-1), four goats) reduced resting coronary blood flow by 14+/-3% (P<0.05) without changing arterial pressure and heart rate. L-NAME (40 mg kg(-1), eight goats) i.v. reduced resting coronary blood flow by 19+/-4% (P<0.05), increased mean systemic arterial pressure by 22+/-3% (P<0.01) and decreased heart rate by 10+/-2% (P<0.05). These effects of L-NAME were partially, but significantly reversed by L-arginine (six goats). Isoproterenol (10-100 ng, eight goats), adenosine (0.3-10 microg, seven goats) and acetylcholine (3-100 ng, five goats), injected intracoronarily, increased coronary conductance in a dose-dependent way and, under control conditions, these increases for isoproterenol, ranged from 32+/-5% to 82+/-12%; for adenosine, 6+/-2% to 174+/-22%; and for acetylcholine, 39+/-5% to 145+/-15%. During i.v. L-NAME the increases in coronary conductance induced by isoproterenol and acetylcholine were significantly reduced by about 50 and 60% (P<0.05), respectively, whereas those induced by adenosine were significantly increased further (about 30-100%, P<0. 05). During L-NAME plus L-arginine, the effects of isoproterenol, acetylcholine and adenosine on coronary conductance were not significantly different from those under control conditions. Therefore, it is suggested that in the coronary circulation: (a) nitric oxide may produce a basal vasodilator tone under normal conditions; (b) nitric oxide may be an intermediate in the vasodilatation due to beta-adrenoceptor stimulation and acetylcholine, and (c) the vasodilatation due to adenosine is potentiated during reduction of nitric oxide production.

cAMP-independent dilation of coronary arterioles to adenosine : role of nitric oxide, G proteins, and K(ATP) channels

Adenosine is known to play an important role in the regulation of coronary blood flow during metabolic stress. However, there is sparse information on the mechanism of adenosine-induced dilation at the microcirculatory levels. In the present study, we examined the role of endothelial nitric oxide (NO), G proteins, cyclic nucleotides, and potassium channels in coronary arteriolar dilation to adenosine. Pig subepicardial coronary arterioles (50 to 100 microm in diameter) were isolated, cannulated, and pressurized to 60 cm H(2)O without flow for in vitro study. The arterioles developed basal tone and dilated dose dependently to adenosine. Disruption of endothelium, blocking of endothelial ATP-sensitive potassium (K(ATP)) channels by glibenclamide, and inhibition of NO synthase by N(G)-nitro-L-arginine methyl ester and of soluble guanylyl cyclase by 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one produced identical attenuation of vasodilation to adenosine. Combined administration of these inhibitors did not further attenuate the vasodilatory response. Production of NO from coronary arterioles was significantly increased by adenosine. Pertussis toxin, but not cholera toxin, significantly inhibited vasodilation to adenosine, and this inhibitory effect was only evident in vessels with an intact endothelium. Tetraethylammonium, glibenclamide, and a high concentration of extraluminal KCl abolished vasodilation of denuded vessels to adenosine; however, inhibition of calcium-activated potassium channels by iberiotoxin had no effect on this dilation. Rp-8-Br-cAMPS, a cAMP antagonist, inhibited vasodilation to cAMP analog 8-Br-cAMP but failed to block adenosine-induced dilation. Furthermore, vasodilations to 8-Br-cAMP and sodium nitroprusside were not inhibited by glibenclamide, indicating that cAMP- and cGMP-induced dilations are not mediated by the activation of K(ATP) channels. These results suggest that adenosine activates both endothelial and smooth muscle pathways to exert its vasodilatory function. On one hand, adenosine opens endothelial K(ATP) channels through activation of pertussis toxin-sensitive G proteins. This signaling leads to the production and release of NO, which subsequently activates smooth muscle soluble guanylyl cyclase for vasodilation. On the other hand, adenosine activates smooth muscle K(ATP) channels and leads to vasodilation through hyperpolarization. It appears that the latter vasodilatory process is independent of G proteins and of cAMP/cGMP pathways.

Improvement in coronary flow reserve determined by positron emission tomography after 6 months of cholesterol-lowering therapy in patients with early stages of coronary atherosclerosis

BACKGROUND

Early stages of coronary atherosclerosis are characterized by a mainly functional impairment of coronary vasodilator capacity under the impact of such risk factors as hypercholesterolemia. The goal of this study was to determine whether 6-month cholesterol-lowering therapy improves coronary flow reserve in patients with angina, reduced flow reserve despite minimally diseased coronary vessels or even normal angiogram, and mild to moderately elevated LDL levels on average.

METHODS AND RESULTS

We noninvasively investigated 23 consecutive patients (18 men, 5 women; mean age, 56+/-7.6 years) with a mean LDL level of 165+/-34 mg/dL at baseline by PET for myocardial blood flow measurement with [13N]ammonia at rest and under dipyridamole stress (0.56 mg/kg) before and after lipid-lowering therapy with simvastatin for 6 months. Between baseline and the 6-month follow-up, total cholesterol concentration fell from 241+/-44 to 168+/-34 mg/dL, and the LDL level decreased from 165+/-34 to 95+/-26 mg/dL (P<0.001). Overall, coronary flow reserve increased from 2.2+/-0.6 to 2.64+/-0.6 (P<0.01). Maximal coronary flow increased significantly from 182+/-36 to 238+/-58 mL/minx100 g (P<0.001) at follow-up. Minimum coronary resistance declined significantly from 0. 51+/-0.12 to 0.40+/-0.14 mm Hg. mL-1. minx100 g (P<0.001). Concomitantly, a regression of anginal symptoms was observed in most patients.

CONCLUSIONS

Our results suggest that cholesterol-lowering therapy with simvastatin may improve overall coronary vasodilator capacity assessed noninvasively by PET in patients with mild to moderate hypercholesterolemia. Consequently, intensive lipid-lowering therapy is considered a vasoprotective treatment for selected patients in very early stages of coronary atherosclerosis with the potential of preventing further disease progression.