Allopurinol Enhances the Contractile Response to Dobutamine and Exercise in Dogs With Pacing-Induced Heart Failure
Background—Superoxide (O2−) generated by enhanced xanthine oxidase (XO) activity may contribute to the increased myocardial oxidative stress in heart failure (CHF). Because blocking XO with allopurinol augments myofilament Ca2+ sensitivity in reperfusion injury and CHF, we hypothesized that it may improve adrenergic inotropic responsiveness in CHF.
Methods and Results—We studied the effect of allopurinol on the contractile response to dobutamine and exercise in 7 chronically instrumented conscious dogs before and after producing CHF by rapid pacing. Left ventricular (LV) contractile performance was measured by the slopes of the LV end-systolic pressure-volume relation (EES) and stroke work–end-diastolic volume relation (MSW). Before CHF, allopurinol produced no change in LV contractile performance and did not alter the response to dobutamine or exercise. After CHF, allopurinol produced significant (P<0.05) increases in EES (5.0±0.6 versus 3.3±0.6 mm Hg/mL) and MSW. Dobutamine and allopurinol produced greater increases in EES (5.4±0.6 versus 7.4±0.6 mm Hg/mL) and MSW (60.1±7.4 versus 73.7±4.4 mm Hg) than did dobutamine alone. After allopurinol, dP/dtmax, stroke volume, and MSW were higher during CHF exercise. LV diastolic pressures were lower during CHF exercise after allopurinol.
Conclusions—Allopurinol has no discernable effects on LV contractile function or adrenergic responsiveness in normal, conscious animals. In pacing-induced CHF, however, allopurinol improves LV systolic function at rest and during adrenergic stimulation and exercise.
Myocardial oxidative stress due to increased production of reactive oxygen free radicals may play an important role in the development and progression of myocardial dysfunction in heart failure (CHF).1 2 3 4 5 Neurohormonal and inflammatory factors in CHF contribute to the enhanced oxidative stress. In addition, there is increased production of the reactive oxygen radical superoxide (O2−) by myocardial mitochondria in CHF.6 Xanthine oxidase (XO) may also contribute to myocardial oxidative stress in CHF. XO forms O2− as it catalyzes the terminal steps in the breakdown of purines to uric acid. Ekelund et al7 recently demonstrated a 4-fold increase in myocardial XO in dogs with pacing-induced CHF. Patients with decompensated heart failure have elevated serum uric acid8 consistent with increased XO activity in CHF.
O2− generated by enhanced XO activity plays an important role in reperfusion injury by damaging or functionally modifying contractile proteins, resulting in reduced calcium sensitivity.9 10 11 Blocking XO with allopurinol enhances calcium sensitivity in rat stunned trabeculae.12 Similarly, blocking the increased myocardial XO activity in animals with pacing-induced CHF enhances baseline left ventricular (LV) contractile performance while decreasing myocardial oxygen consumption.7 These observations suggest that blocking XO may enhance myocardial calcium sensitivity in CHF.
β-Adrenergic stimulation produced pharmacologically or during exercise increases myocardial contractile performance. This is partially due to an enhancement of the calcium transient. The response to β-adrenergic stimulation is reduced in CHF because of receptor downregulation and uncoupling.13 The reduced β-adrenergic responsiveness may play an important role in an abnormal response to exercise in CHF.14
We hypothesize that if blocking XO increases myocardial calcium sensitivity in CHF, it should enhance the contractile response to pharmacologically produced β-adrenergic stimulation as well as to the endogenous β-adrenergic stimulation that occurs during exercise. Accordingly, we studied the contractile response to dobutamine and exercise in dogs before and after producing CHF by tachycardia pacing. Our results provide insight into a mechanism of contractile dysfunction in CHF and suggest a potential method of enhancing the β-adrenergic responsiveness of the failing myocardium.
Seven healthy, adult, heartworm-negative mongrel dogs (25 to 35 kg) were instrumented to measure 3 LV internal dimensions and LV and left atrial (LA) pressures. Hydraulic occluders were placed around the venae cavae by the technique that we described previously.15 16
Allopurinol (300 mg, Sigma) was dissolved in 100 mL of normal saline after slight heating and alkalization with NaOH.
Studies Before CHF at Rest
Studies were begun after full recovery from the instrumentation. To obtain baseline values, data were initially recorded with the unsedated dogs standing quietly on a motorized treadmill. Three sets of variably loaded LV pressure-volume (P-V) loops were generated by sudden, transient occlusion of the cavae as we described previously.14 The effects of the following interventions were studied on separate days in random order with the animals allowed to equilibrate for 2 days between studies.
1. Infusion of an inhibitor of XO, allopurinol (0.2 mg · kg−1 · min−1 IV), for 40 minutes.
2. Infusion of dobutamine (8 μg · kg−1 · min−1 IV).
3. Infusion of allopurinol for 40 minutes, followed by an infusion of dobutamine.
4. The effect of treadmill exercise. We collected the data at 3 and 4 mph and at the maximum tolerated level of steady-state exercise as described.14 After a 30-minute rest period, allopurinol was administered. Forty minutes after allopurinol administration, the exercise protocol was repeated.
Induction of CHF
After completion of the initial studies, rapid right ventricular pacing was initiated. The pacing rate was adjusted with an external magnetic control unit to 200 to 240 bpm as described.14 After pacing for 3 to 4 weeks, when the LV end-diastolic pressure during the nonpaced period had increased by >15 mm Hg over the prepacing control level, CHF data were obtained.
Studies After CHF
During the stable CHF period, we conducted the same studies as before CHF. Before each study, the pacemaker was turned off, and the animal was allowed to equilibrate for 1 hour. After the study, the rapid pacing was resumed.
Data Processing and Analysis
As previously described,16 LV volume was calculated as a modified general ellipsoid. The rate of LV relaxation was analyzed by determining the exponential time constant (τ) of the isovolumic fall of LV pressure.
Analyses of LV P-V Loop During Caval Occlusion
As previously described,17 we determined the LV end-systolic pressure (PES)–end-systolic volume (VES) relation, the stroke work (SW)–end-diastolic volume (VED) relation, and the dP/dtmax-VED relation from the variably loaded beats produced by transient caval occlusion. Because we could not perform caval occlusions during exercise, we assessed LV contractile performance during exercise by calculating MSW assuming that the volume axis intercept was unchanged.18
Data are expressed as mean±SD. Multiple comparisons were performed by ANOVA. When a significant overall effect was present, intergroup comparisons were performed with a Bonferroni correction for multiple comparisons. The level of significance was P<0.05. We evaluated the interaction of allopurinol with dobutamine and exercise by ANOVA of 2 factors with replication.
Effects of Allopurinol Administered Before and After CHF at Rest
Before CHF, allopurinol produced no changes in steady-state hemodynamic parameters (Table 1⇓) or in any of the LV P-V measures of LV contractile performance (Table 2⇓, Figure 1⇓). After CHF, allopurinol caused increases in stroke volume and dP/dtmax and reductions in LV diastolic pressure, LA pressure, dP/dtmin, and τ (Table 1⇓). In addition, allopurinol increased the slopes of the 3 P-V relations (Table 2⇓ and Figure 1⇓), indicating enhanced LV contractile performance.
Effects of Combination of Allopurinol and Dobutamine
Before CHF, allopurinol had no discernable effect on the response of LV contractile performance to dobutamine (Table 2⇑, Figure 2⇓). After CHF, the response to dobutamine alone was blunted. After allopurinol, however, dobutamine produced greater increases in stroke volume and cardiac output (Table 1⇑). The contractile response to dobutamine was also enhanced by pretreatment with allopurinol (Table 2⇑, Figure 2⇓). Two-factor ANOVA demonstrated no significant interaction between allopurinol and dobutamine.
Effects of Exercise After Allopurinol
Before CHF, allopurinol did not significantly alter the response to exercise. The response of LV systolic performance to exercise was reduced after CHF, and minimum LV pressure and τ increased (rather than the normal decrease during exercise) during CHF exercise. After allopurinol, dP/dtmax, dV/dtmax, and MSW were greater during exercise, and LV diastolic pressure and τ were lower than during control CHF exercise (Table 3⇓, Figures 3⇓ and 4⇓). There was no significant interaction between exercise and allopurinol.
We found that blocking XO with allopurinol had no discernable effect on normal LV function or β-adrenergic responsiveness. After pacing-induced CHF, however, allopurinol produced a modest improvement in resting LV contractile performance and enhanced the contractile response to both pharmacological β-adrenergic stimulation and exercise.
O2− production after reperfusion of ischemic myocardium contributes to the reduced myocardial calcium sensitivity that is an important part of myocardial stunning. Blocking XO with allopurinol enhances calcium sensitivity in stunned rat trabeculae.12 Ekelund et al7 recently demonstrated a 4-fold increase in myocardial XO in dogs with pacing-induced CHF. They found that blocking XO activity in these animals with allopurinol enhanced baseline LV contractile performance while decreasing myocardial oxygen consumption. Our results confirm their observations and extend them by demonstrating an enhanced response to β-adrenergic stimulation and exercise. This indicates that an XO-mediated effect may contribute to baseline contractile dysfunction and to reduced β-adrenergic responsiveness in CHF.
Our finding that a XO inhibitor had no discernable effect on normal LV function or β-adrenergic responsiveness, although it augmented LV performance and β-adrenergic responsiveness in CHF, is similar to previous observations that blocking the generation of nitric oxide (NO) partially reverses the decreased myocardial β-adrenergic responsiveness in CHF.19 20 How do NO and O2− produce such similar effects on β-adrenergic responsiveness in CHF? It is possible that these effects are mediated through different mechanisms. Another possibility is that NO and O2− act synergistically. Specifically, NO reacts with O2− to form peroxynitrite (ONOO−).21 22 ONOO− depresses myocardial contraction and relaxation, probably by causing protein nitration.23 24 Thus, it is possible that enhanced production of both NO and O2− in CHF contributes to the generation of ONOO−, which results in altered contractile properties that contribute to reduced β-adrenergic responsiveness.
Despite an increased risk of death, dobutamine infusions are used in patients with decompensated CHF.25 Our results suggest that allopurinol may allow use of lower doses of dobutamine by increasing β-adrenergic responsiveness. This might allow achievement of similar hemodynamic improvement with less risk, especially because allopurinol decreases myocardial oxygen requirements.7 It is also possible that allopurinol, by increasing the total inotropic response, might increase the risk of dobutamine. In addition, our observation that allopurinol enhanced LV systolic and diastolic performance during exercise suggests that it might improve exercise tolerance in CHF.
We observed that after CHF, allopurinol produced slight arterial vasodilatation, increased the rate of LV isovolumic pressure fall, and lowered LV minimal pressure. In addition, the vasodilatory and lusitropic effects of dobutamine and exercise were greater after allopurinol. These effects are different from those observed by Ekelund et al,7 who found no lusitropic or vasodilatory effect of allopurinol after CHF.
Several limitations of our study should be considered. Although we studied an animal model of CHF (pacing tachycardia) that reproduces many of the functional and neurohormonal features of clinical CHF, we cannot be certain that our results apply to CHF of other causes. In addition, we studied the acute effects of allopurinol. We do not know the effect of prolonged treatment with allopurinol.
We studied the effect of allopurinol on the response to 1 dose of dobutamine. Thus, we did not define the adrenergic dose-response curve. Although LV contractile performance was significantly greater when dobutamine was combined with allopurinol, we observed no significant interaction between dobutamine and allopurinol. This suggests that allopurinol may produce a parallel upward shift of the adrenergic dose-response curve without a change in slope. The response to graded exercise (Figure 4⇑) is also consistent with a parallel shift, indicating that the increment in contractile function produced by allopurinol at rest also occurs during adrenergic stimulation and exercise.
Finally, our study does not define the mechanism of action of allopurinol in CHF. It is possible that its actions are mediated by effects other than reducing the generation of O2−.
In conclusion, we found that allopurinol has no discernable effects on normal LV contractile function or β-adrenergic responsiveness in conscious animals. In pacing-induced CHF, however, allopurinol decreases LV contractile dysfunction, enhances β-adrenergic responsiveness, and improves the LV systolic and diastolic response to exercise.
This study was supported in part by the National Institutes of Health (R01-HL-53541) and the American Heart Association (9640189N). Dr Cheng is an Established Investigator of the American Heart Association. We gratefully acknowledge the computer programming of Ping Tan (Spectrum), the technical assistance of Michael Cross, and the secretarial assistance of Amanda Burnette.
Reprint requests to Che-Ping Cheng, MD, PhD, Cardiology Section, Wake Forest University School of Medicine, Medical Center Boulevard, Winston-Salem, NC 27157-1045.
- Received May 24, 2000.
- Revision received August 3, 2000.
- Accepted August 9, 2000.
- Copyright © 2001 by American Heart Association
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