Nitric Oxide Spares Myocardial Oxygen Consumption Through Attenuation of Contractile Response to β-Adrenergic Stimulation in Patients With Idiopathic Dilated Cardiomyopathy
Background—The results of recent studies suggest that NO synthase may increase in the failing myocardium and that NO modulates the myocardial contractile response to β-adrenergic stimulation. However, there are few data regarding the physiological role of NO in patients with heart failure. The aim of the present study was to address the role of NO in left ventricular (LV) contractile response to β-adrenergic stimulation and corresponding oxygen expenditure in human heart failure.
Methods and Results—We studied 15 patients with heart failure due to idiopathic dilated cardiomyopathy (mean ejection fraction 0.33). We examined LV contractility (Emax, the slope of end-systolic pressure-volume relation), LV external work (EW), myocardial oxygen consumption (MV̇o2), and mechanical efficiency (measured as EW/MV̇o2) with the use of conductance and coronary sinus thermodilution catheters before and during dobutamine (DOB) infusion via a peripheral vein (4.8±0.3 μg · kg−1 · min−1 IV). Heart rate was kept constant with atrial pacing. We carried out a similar protocol during the intracoronary infusion of the NO synthase inhibitor NG-monomethyl-l-arginine (L-NMMA; 200 μmol). DOB increased Emax, EW, and MV̇o2 (by 77±17%, 39±5%, and 21±5%, respectively), leading to an increase in mechanical efficiency (25.4±3.1% to 29.6±4.1%). L-NMMA alone did not significantly change these variables. Although the concurrent infusion of DOB with L-NMMA increased Emax, EW, and MV̇o2 (by 140±21%, 64±9%, and 35±5%, respectively) more than DOB alone, mechanical efficiency did not increase further (24.3±3.3% to 29.5±4.5%) because EW and MV̇o2 increased in parallel.
Conclusions—These data suggest that in patients with idiopathic dilated cardiomyopathy, endogenous NO spares MV̇o2 through attenuation of LV contractile response to β-adrenergic stimulation while maintaining LV energy-converting efficiency.
Nitric oxide (NO) may play important roles in the heart by influencing myocardial inotropic and chronotropic responses.1 Previous studies have shown that NO attenuates cardiac myocyte contraction2 and mediates vagal inhibition of cardiac inotropic response to β-adrenergic stimulation.3 NO may be produced in human myocardium in heart failure,4 but the pathophysiological role of NO in congestive heart failure remains controversial.
Hare et al5 recently reported that the inhibition of NO synthase (NOS) potentiates the positive inotropic response to β-adrenergic stimulation in patients with dilated cardiomyopathy but not in subjects with normal left ventricular (LV) function. These authors suggested that increased myocardial NOS activity in the failing human heart attenuates β-adrenergic responsiveness.
Investigation of the pathophysiology of congestive heart failure requires a metabolic approach to the assessment of myocardial oxygen consumption (MV̇o2) as well as a functional approach. In addition to its potential to modulate MV̇o2 indirectly via attenuation of the contractile response, NO appears to regulate oxidative phosphorylation in cardiac myocytes, probably by binding to heme moieties and iron-sulfur clusters in proteins involved in mitochondrial respiration and thus possibly regulating MV̇o2. Xie et al6 revealed the role of endothelium-derived NO in the control of cardiac mitochondrial respiration in vitro. Recently, Zhang et al7 demonstrated that ACE inhibitors dramatically reduce MV̇o2 in isolated myocardial muscle slices through an NO-dependent mechanism. Conversely, another study has shown that blockade of NO synthesis reduces MV̇o2 in vivo.8 Nevertheless, the way in which NO modulates MV̇o2 in patients with heart failure has not received much attention.
The purpose of the present study was to evaluate the role of NO in cardiac mechanics and energetics in patients with idiopathic dilated cardiomyopathy (IDC) through the use of the relatively load-independent index of contractility, Emax, and systolic pressure-volume area (PVA), both of which were proposed as major determinants of MV̇o2 by Suga9 and others.10
Studies were performed in 15 patients (10 men and 5 women, mean age 49±8 years) who were undergoing diagnostic cardiac catheterization for the evaluation of heart failure on their first admission to our hospital. All were in sinus rhythm and were diagnosed with IDC through the exclusion of coronary artery disease or other known causes of dilated cardiomyopathy (Table 1⇓). The patients were in New York Heart Association functional class II (n=9) or III (n=6). Cardiovascular medication consisted of diuretics (n=11), digitalis (n=6), and vasodilators (n=4). None of the patients were taking ACE inhibitors at the time of catheterization. Complete informed, written consent was obtained from each patient before the study. No unfavorable complications occurred as a result of this investigation. The study protocol was approved by the Institutional Committee on Human Research at Kobe University Hospital.
Cardiac catheterization was performed via the femoral approach on patients in a fasting condition without medication. After routine right and left heart catheterization, a 7F thermodilution Swan-Ganz catheter (Goodtech Inc) was advanced into the pulmonary artery, and an 8F conductance (volume) catheter (CardioDynamics) was advanced into the LV through the right femoral artery. The conductance catheter was attached to a stimulator/processor (Sigma-5; CardioDynamics), and a 2F Millar Instruments catheter was advanced into the LV through the lumen of the conductance catheter. A 4F Judkins catheter was advanced to the ostium of the left main coronary artery via the left femoral artery for intracoronary drug infusion and was continuously flushed at a rate of 2 mL/min with saline containing heparin. An 8F coronary sinus thermodilution catheter (Cordis Webster, Inc) was then advanced into the coronary sinus through the right jugular vein. The ECG and hemodynamic parameters were recorded on a strip-chart recorder. Each measurement was obtained as the mean value of ≥8 consecutive sinus beats.
Assessment of LV Cardiac Mechanoenergetics
The coronary sinus blood flow (CSF) was measured with the thermodilution method.10 Coronary venous blood was sampled from the distal lumen of the coronary sinus catheter for oximetry. MV̇o2 per minute was calculated as the product of CSF (mL/min) and the arterial-coronary sinus oxygen content difference (vol%) and was divided by heart rate to yield MV̇o2 per beat (mL O2/beat), as in previous studies.10 11
LV volume measurements with a conductance catheter were described in a previous report.11 Pressure-volume loops were recorded for the sequence of beats during the transient decrease in preload through the inflation of a balloon catheter (Baxter) just above the inferior vena cava. LV contractility, Emax is the slope of the linear end-systolic pressure-volume relation (ESPVR) (Figure 1A⇓).11 We determined the effective arterial elastance (Ea), a variable that incorporates the values of Windkessel model elements and heart rate as the ratio of end-systolic pressure to stroke volume (Figure 1B⇓).12 The ratio of effective Ea to ventricular elastance (Ea/Emax) represents ventriculoarterial coupling. We normalized Emax and Ea (mm Hg/mL per m2) for body surface area to permit a comparison among patients in the present study, as described previously.11
External work (EW) was calculated as the area that is bounded by the pressure-volume trajectory of 1 beat. Systolic PVA (mm Hg/mL) was calculated as the area bounded by the ESPVR and end-diastolic pressure-volume relation (EDPVR) and the systolic pressure-volume trajectory of each beat (Figure 1B⇑). Potential energy (PE) was calculated by subtracting EW from PVA. We calculated mechanical efficiency as the ratio of EW (J/beat) to MV̇o2 per beat (J/beat), where 1 mm Hg/ml EW and 1 mL O2 of oxygen consumption correspond to 1.33 · 10−4 and 20 J, respectively.9
Tau was calculated from a plot of −dP/dt versus P (P= P0e−t/T+Pb), where P is LV pressure, t is the time from peak −dP/dt, T is time constant of isovolumic pressure decay, and P0 and Pb are constants determined with the data.13 LV chamber stiffness was addressed with both nonlinear regression and linear regression analyses of the EDPVR values for the same beats as for the ESPVR. LV chamber stiffness was quantified by fitting the exponential function EDP=AeβEDV to EDP and EDV through nonlinear least-squares regression, where EDP is end-diastolic pressure, EDV is end-diastolic volume, A is elastic constant, and β is the change in the logarithmic function of LV pressure relative to the change in volume and represents an index of ventricular chamber stiffness.14 LV chamber stiffness was also assessed with the slope of the linear regression analysis of the EDPVR values [EDP=S(EDV−V0d)], where EDP is the end-diastolic pressure, S is the slope of the EDPVR, EDV is end-diastolic volume, and V0d is the diastolic volume intercept.
After routine heart catheterization, atrial pacing was initiated at 90 bpm or at 15 bpm above the baseline heart rate and continued for the duration of the study (94±2 bpm). After stabilization of the hemodynamics, hemodynamic variables, pressure-volume loops, and CSF were measured and blood gas samples were collected from the coronary sinus and coronary arteries. ESPVR was obtained during inferior vena cava occlusion.
After control measurements were made, dobutamine diluted in saline was infused via a systemic vein and titrated to achieve a stable increase in peak +dP/dt. Dobutamine infusion was begun at a rate of 4 μg · kg−1 · min−1 for 10 minutes, and if peak +dP/dt did not increase by at least 20%, the infusion rate was increased to 5 or 6 μg · kg−1 · min−1 at 5-minute intervals. After steady hemodynamic and contractile states were achieved, we made measurements similar to those in the control study.
L-NMMA (Control) Study
Dobutamine infusion was discontinued and hemodynamic variables were monitored for at least 20 minutes until they returned to control values. Then, the intracoronary infusion of NG-monomethyl-l-arginine (L-NMMA), an NOS inhibitor, was started through a Judkins catheter advanced to the left main coronary artery. The infusion rate was 20 μmol/min for 10 minutes. We repeated the same measurements.
L-NMMA infusion was continued (20 μmol/min for 10 minutes), and dobutamine was infused again for 10 minutes at the same rate as in the first dobutamine infusion. Then, we performed the same measurements.
Results are presented as mean±SEM values, unless otherwise indicated. We obtained ESPVR values through linear regression analysis. The effects of dobutamine, L-NMMA, and the combination were analyzed independently with a paired t test with Bonferroni correction. The effect of the infusion of L-NMMA on the peak +dP/dt and Emax in response to dobutamine was analyzed with 2-way repeated measures ANOVA. Differences were considered significant at P<0.05.
Influence of Dobutamine and L-NMMA on Systemic and Coronary Hemodynamic Variables
Table 2⇓ summarizes the changes in systemic and coronary hemodynamics and in MV̇o2. Before L-NMMA infusion, dobutamine increased LV systolic pressure, peak +dP/dt, and cardiac index (by 20±4%, 48±8%, and 41±8%, respectively), with a small decrease in systemic vascular resistance (SVR) (−15±2%). These changes were accompanied by an increase in CSF and MV̇o2 (33±7% and 26±7%, respectively). The mean dose of dobutamine was 4.8±0.3 μg · kg−1 · min−1, and in 2 of 15 patients, the increase in peak +dP/dt did not reach 20% despite a dobutamine infusion of 6 μg · kg−1 · min−1. After cessation of the dobutamine infusion, L-NMMA alone did not change these hemodynamic variables and MV̇o2 compared with the control values. During an intracoronary infusion of L-NMMA, the same dose of dobutamine increased LV systolic pressure, peak +dP/dt, and cardiac index (by 26±3%, 72±12%, and 38±9%, respectively), with a minimal decrease in SVR (−5±1%). The response of peak +dP/dt to dobutamine was enhanced via L-NMMA (Figure 2A⇓). The combination of L-NMMA and dobutamine demonstrated a 17±4% enhancement of peak +dP/dt compared with dobutamine alone, which was not due to the positive inotropic effects of L-NMMA (Figure 2B⇓); this enhancement caused a further increase in MV̇o2 (an increase of 19±4% compared with the dobutamine study).
Effects of Dobutamine and L-NMMA on Mechanoenergetic Variables
We successfully assessed the LV ESPVR in 10 of 15 patients. In 5 patients, we were not able to evaluate Emax because of frequent premature ventricular contractions during the impeding venous return. Table 3⇓ summarizes the effects of dobutamine and L-NMMA on mechanoenergetic variables. Before the intracoronary infusion of L-NMMA, dobutamine increased Emax by 77±17% without altering Ea, resulting in an improvement in ventriculoarterial coupling. Dobutamine increased EW (39±5%) and MV̇o2 (21±5%), leading to an increase in the EW/PVA ratio (13±2%) and EW/MV̇o2 ratio (4.2±1.4%). During the intracoronary infusion of L-NMMA without dobutamine, the mechanoenergetic variables remained unchanged. Then, the intravenous infusion of dobutamine during the intracoronary infusion of L-NMMA caused an increase in Emax (140±21% versus L-NMMA [control]) that was larger than that after dobutamine alone. Figure 3⇓ shows a family of representative pressure-volume loops during inferior vena cava occlusion in response to dobutamine (left) and dobutamine/L-NMMA (right). This additional increase in Emax in response to the same dose of dobutamine as in the dobutamine study occurred via L-NMMA (Figure 4⇓). Dobutamine did not alter Ea and improved ventriculoarterial coupling to a similar level as that before L-NMMA infusion. During the intracoronary infusion of L-NMMA, dobutamine increased EW (64±9%) and MV̇o2 (35±5%), resulting in an increase in the EW/PVA ratio (18±2%) and the EW/MV̇o2 ratio (5.2±2.3%). The EW/PVA and EW/MV̇o2 ratios, however, did not differ between the dobutamine study and the dobutamine/L-NMMA study.
Table 4⇓ summarizes the changes in diastolic properties. Dobutamine decreased peak −dP/dt and the time constant of LV pressure decay (tau) (by 28±8% and 17±3%, respectively) but did not significantly change chamber stiffness (β) and the slope of EDPVR (S). After the cessation of dobutamine infusion, L-NMMA alone did not change these variables compared with the control value. During the intracoronary infusion of L-NMMA, dobutamine decreased peak −dP/dt and tau (by 26±7% and 15±6%, respectively) and did not significantly change β and S. These variables did not different between the dobutamine study and the dobutamine/L-NMMA study.
In the present study, we examined the effects of NOS inhibition on LV mechanoenergetics under basal conditions and in response to β-adrenergic stimulation via dobutamine in patients with IDC. The principal findings of this study were (1) that the intravenous infusion of dobutamine increased Emax, EW, and MV̇o2, resulting in a slight improvement in mechanical efficiency; (2) that the selective inhibition of NOS in myocardium did not change LV mechanoenergetics under baseline conditions; and (3) that the combination of NOS inhibition with dobutamine enhanced Emax, EW, and MV̇o2 compared with the infusion of dobutamine alone. This, however, did not provide a further improvement in mechanical efficiency because of the proportional increase in MV̇o2 with contractile augmentation.
NO is synthesized from l-arginine by 3 NOS isoforms: the constitutive types, brain NOS and eNOS, and the inducible type, iNOS. Recent studies have shown that iNOS activity is increased in cardiac myocytes and endocardial endothelium of patients with dilated cardiomyopathy15 or in ventricular myocardium of patients with heart failure regardless of the cause.16 One of the potential mechanisms for increased iNOS activity in failing myocardium is provided by the observation that plasma levels of tumor necrosis factor-α and interleukin-6 and the expression of tumor necrosis factor-α in the myocardium are increased in patients with heart failure.4 17 Winlaw et al17 showed a significantly increased systemic concentration of plasma nitrate in patients with heart failure and reported that this concentration correlated with the severity of heart failure. Therefore, it seems rational to suppose that the concentration of NO released by iNOS in human heart failure is increased.
In the present study, the acute administration of L-NMMA did not elicit a positive inotropic effect and did not change MV̇o2 in patients with IDC under basal conditions. The LV contractile response to dobutamine, however, was enhanced during the intracoronary infusion of L-NMMA. There are several mechanisms by which NO inhibited the positive inotropic response to β-adrenergic stimulation. NO activates soluble guanylyl cyclase to produce cGMP, which inhibits cAMP-stimulated slow inward Ca2+ channels through the activation of protein kinase G and activates cGMP-dependent phosphodiesterase.18 NO also has direct S-nitrosation of cellular proteins and formation of peroxynitrite.19 Hare et al5 implicated iNOS in part in the mediation of the myocardial contractile dysfunction in patients with dilated cardiomyopathy. Drexler et al20 recently found that the isoproterenol-induced increase in the force of contraction was inversely related to cardiac iNOS activity in LV tissue from the failing human heart.
One of the mechanisms by which NO mediates negative inotropic effect involves a decrease in the calcium sensitivity of contractile element via cGMP-dependent protein kinase.21 Therefore, we expected that the reversal of NO-induced hyporesponsiveness to β-adrenergic stimulation via L-NMMA might not be accompanied by a significant increase in MV̇o2. In the present findings, however, the enhanced response to dobutamine with L-NMMA was associated with a parallel increase in MV̇o2 to contractile augmentation and did not change mechanical efficiency.
We must consider LV afterload, as well as its contractile state, as a determinant of MV̇o2. In the present study, the intravenous infusion of dobutamine during L-NMMA infusion caused small increases in SVR and Ea compared with dobutamine study that were not statistically significant (Tables 2⇑ and 3⇑). A minimal response of the cardiac index to L-NMMA plus dobutamine dissimilar to the peak +dP/dt response may in part be attributed to the increase in systemic afterload for the failing heart. From the framework of PVA and MV̇o2, however, an afterload-dependent increase in MV̇o2 should be accompanied by an increase in PVA, whereas PVA values were not different for the dobutamine study and the dobutamine/L-NMMA study. Thus, given that the slope of the MV̇o2-PVA relation is not affected by cardiac NOS inhibition as was reported in previous studies,22 it seems reasonable to say that an increase in MV̇o2 with the combination of dobutamine with L-NMMA was due to an increase in PVA-independent MV̇o2 in response to the contractile augmentation rather than to changes in systemic afterload.
There have been no data that document the possible involvement of NO in the direct control of PVA-independent MV̇o2, which relates to basal metabolism and excitation-contraction coupling, such as the ATP consumed for Ca2+ cycling and reuptake by the sarcoplasmic reticulum. Recent investigators have demonstrated in vitro a modulatory action of NO on calcium channels of the sarcoplasmic reticulum,18 and it is possible that NO would imply less energy expenditure at excitation-contraction coupling, in particular at higher levels of contractility, although we could not document this. Further studies were warranted to examine the effect of cardiac NOS inhibition on PVA-independent MV̇o2.
We found that mechanical efficiency and overall energy conversion efficiency did not change with cardiac NOS inhibition during β-adrenergic stimulation without an alteration in LV loading conditions and ventriculoarterial coupling. In consideration of these findings in view of cardiac mechanics and energetics, the oxygen-saving effect of NO attenuation of the contractile response to β-adrenergic stimulation does not appear to be deleterious for the failing human heart due to IDC.
In isolated ejecting guinea pig hearts, reduced endogenous NO via L-NMMA has been reported to decrease LV diastolic compliance.23 Paulus et al24 showed that the intracoronary infusion of sodium nitroprusside, a spontaneous NO donor, into human subjects caused a modest decline in LV systolic performance and increased end-diastolic distensibility. Under pathological conditions, such as IDC, we did not observe alteration of the diastolic properties, including LV relaxation and chamber stiffness, with the intracoronary infusion of NOS inhibitor under baseline conditions and in response to dobutamine.
Elucidation of the cardiac effect of NO may be complicated by the systemic effect of the NOS inhibitor.25 By using the direct intracoronary infusion of L-NMMA and relatively load-independent indices such as Emax, we tried to avoid changes in loading conditions that might confound the interpretation of the data. The intracoronary infusion of L-NMMA at a rate of 20 μmol/min did not significantly change SVR or Ea, but our preliminary study revealed that a 40-μmol/min infusion of L-NMMA significantly increased SVR and decreased cardiac index during 4 μg · kg−1 · min−1 infusion of dobutamine (data not shown).
We must consider potential error in the assumption of the linear ESPVR. Kass et al26 suggested that contractility-dependent curvilinearity of the ESPVR may exist. In the present study, possible curvilinearity of the ESPVR may exist because of the relatively narrow range of altered loading conditions. The estimation of PVA with linear regression analysis of ESPVR, however, has been reported to be a reliable predictor of MV̇o2 under different contractile states in human hearts.11
In summary, NO-dependent hyporesponsiveness to β-adrenergic stimulation does not aggravate LV pump performance and does maintain energy transduction efficiency. In view of the pathophysiology of heart failure characterized as altered oxygen metabolism, the present data support the known therapeutic efficacy of organic nitrates and ACE inhibitors that potentially increase cardiac NO in the treatment of this syndrome.7 27
- Received July 9, 1999.
- Revision received October 27, 1999.
- Accepted November 15, 1999.
- Copyright © 2000 by American Heart Association
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