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Am J Physiol Heart Circ Physiol 284: H1513-H1520, 2003; doi:10.1152/ajpheart.00529.2001
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Vol. 284, Issue 5, H1513-H1520, May 2003

Effect of PDE5 inhibition on coronary hemodynamics in pacing-induced heart failure

YingJie Chen, Jay H. Traverse, Mingxiao Hou, Yunfang Li, Ruisheng Du, and Robert J. Bache

Department of Medicine, Division of Cardiology, University of Minnesota Medical School, Minneapolis, Minnesota 55455


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Inhibition of phosphodiesterase type 5 (PDE5) can relax systemic and coronary vessels by causing accumulation of cGMP. Both the endothelial dysfunction with decreased nitric oxide production and increased natriuretic peptide levels in congestive heart failure (CHF) have the potential to alter cGMP production, thereby influencing the response to PDE5 inhibition. Consequently, this study examined the effects of PDE5 inhibition with sildenafil in dogs with CHF produced by rapid ventricular pacing. CHF resulted in decreases of left ventricular (LV) systolic pressure, coronary blood flow, and the maximal first time derivative of LV pressure (LV dP/dtmax) at rest and during treadmill exercise compared with normal, whereas resting LV end-diastolic pressure increased from 10 ± 1.4 to 23 ± 1.4 mmHg. Sildenafil (2 and 10 mg/kg per os) caused a 5- to 6-mmHg decrease of aortic pressure (P < 0.05), with no change of heart rate, LV systolic pressure, or LV dP/dtmax. Sildenafil caused no change in coronary flow or myocardial oxygen consumption in animals with CHF at rest or during exercise. In contrast to findings in normal animals, sildenafil did not augment endothelium-dependent coronary vasodilation in response to acetylcholine in animals with CHF. Furthermore, Western blotting showed decreased PDE5 protein expression in myocardium from failing hearts. These findings demonstrate that PDE5 contributes little to regulation of coronary hemodynamics in CHF.

phosphodiesterase; guanosine 3',5'-cyclic monophosphate; blood flow; myocardial O2 consumption


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

ACTIVATION OF GUANYLYL CYCLASE in vascular smooth muscle causes increases in cGMP and cGMP-dependent protein kinases that result in vasodilation through modulation of calcium channels and by decreasing the calcium sensitivity of vascular smooth muscle contractile proteins (18). Although the principal mechanism of cGMP production in the coronary vessels involves activation of soluble guanylyl cyclase by nitric oxide (NO) produced in the vascular endothelium, the natriuretic peptides can also cause coronary vasodilation by interacting with vascular smooth muscle natriuretic peptide receptor type A to cause activation of particulate guanylyl cyclase (14, 29). The vasodilator response to cGMP is terminated by several cyclic nucleotide phosphodiesterases. Phosphodiesterase type 5 (PDE5), which selectively degrades cGMP but does not catabolize cAMP (2, 19, 26), has recently received attention because of the availability of sildenafil, a selective inhibitor of PDE5, for treatment of erectile dysfunction (5). PDE5 is found in high concentrations in the corpus cavernosum; sildenafil increases cGMP levels and causes smooth muscle relaxation (5). PDE5 is also found in vascular smooth muscle of systemic and coronary arteries (26), and thus has the potential to produce effects on arterial pressure and coronary blood flow. In isolated coronary artery segments sildenafil has been reported to cause increased intracellular levels of cGMP and relaxation (26). We recently observed that PDE5 inhibition with sildenafil caused a modest increase of myocardial blood flow at rest and during exercise in normal dogs (24) and augmented the coronary vasodilator response to acetylcholine (8).

The influence of PDE5 on coronary hemodynamics in the setting of congestive heart failure (CHF) has not been reported. This is of importance because CHF is associated with abnormalities that might alter vascular cGMP levels and therefore change the response to PDE5 inhibition. Thus CHF is associated with decreased endothelial NO synthase (eNOS) expression, which would be expected to decrease endothelial cGMP production (27). Conversely, circulating levels of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are increased in the setting of CHF (28). These natriuretic peptides activate particulate guanylyl cyclase, which represents an alternate pathway for increased production of cGMP. If this alternate pathway results in increased cGMP production in the coronary or peripheral vasculature, then PDE5 inhibition might cause greater effects in the setting of CHF. Consequently, the purpose of this study was to examine the effects of PDE5 inhibition with sildenafil on the coronary circulation at rest and during exercise in animals with CHF.


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Studies were performed on 13 adult mongrel dogs (20-26 kg wt) trained to run on a treadmill. All experiments were performed in accordance with the "Guiding Principles in the Care and Use of Laboratory Animals" as approved by the American Physiological Society and with prior approval of the Animal Care Committee of the University of Minnesota.

Surgical preparation. The animals were anesthetized with pentobarbital sodium (30-35 mg/kg), intubated, and ventilated with oxygen-enriched (30%) room air. A left thoracotomy was performed in the fifth intercostal space, and the heart was suspended in a pericardial cradle. A polyvinyl chloride catheter (3.0 mm OD) filled with heparinized saline was inserted into the internal thoracic artery and advanced into the ascending aorta. A similar catheter was introduced into the left ventricle (LV) through the apex and secured in place. A solid-state micromanometer (model P5, Konigsberg Instrument; Pasadena, CA) was also introduced into the LV at the apex. A final catheter was introduced into the right atrial appendage, manipulated into the coronary sinus ostium, and advanced into the great cardiac vein until the tip could be palpitated within 1 cm of the interventricular sulcus to allow selective sampling of coronary venous blood draining the myocardium perfused by the left anterior descending coronary artery (LAD). Approximately 1.5 cm of the proximal LAD was dissected free, and a Doppler velocity probe (Hartley; Houston, TX) was positioned around the artery. Immediately distal to the velocity probe, a hydraulic occluder was placed around the vessel. The pericardium was then loosely closed, the catheters and electrical leads were tunneled subcutaneously to exit at the base of the neck, and the chest was closed in layers. The catheters were flushed daily with heparinized saline. Postoperative analgesia was provided with butorphanol (0.4 mg/kg subcutaneously every 4-6 h).

Exercise responses in normal heart. After 10 to 14 days were allowed for recovery from surgery, the animals were returned to the laboratory for control measurements of systemic hemodynamics and coronary blood flow at rest and during exercise. With the dogs standing quietly on the treadmill, resting hemodynamics and coronary flow were recorded while 2 ml of blood was withdrawn anaerobically from the aortic and coronary venous catheters and maintained on ice until blood gas analysis could be performed (within 30 min after sample collection). Subsequently, a two-stage treadmill exercise protocol was begun (stage 1: 3.2 km/h at 0% grade; stage 2: 6.4 km/h at 0% grade). Each exercise stage was 3 min in duration. Aortic and coronary venous blood samples were withdrawn during the last 30 s of each exercise stage in eight dogs when hemodynamics had reached a steady state.

Production of CHF. CHF was produced by rapid ventricular pacing. The day after completion of the baseline exercise studies, the pacemaker was activated and pacing begun at 220 beats/min; pacing was continued at that rate or adjusted upward to a maximum of 250 beats/min based on the progression of heart failure. Weekly assessments of hemodynamics and coronary blood flow were obtained with the dogs standing quietly in a sling in normal sinus rhythm 1 h after the pacemaker had been deactivated. CHF was deemed to have developed when the resting LV end-diastolic pressure (LVEDP) was >= 20 mmHg or when visual estimation of ejection fraction by two-dimensional echocardiography was <30%.

Exercise responses in CHF. On the day of study, the pacemaker was deactivated, and 2 h later the dogs were placed on the treadmill. Resting hemodynamic measurements were obtained and aortic and coronary venous blood specimens were withdrawn for blood gas determination. Exercise was then begun at 3.2 km/h; 3 min after beginning exercise blood gases were obtained. The treadmill speed was then increased to 6.4 km/h at 0% grade, and hemodynamic blood gas and coronary flow measurements were repeated as described above.

Subsequently, 12 dogs were given sildenafil (2 mg/kg orally). One hour after sildenafil administration, hemodynamic measurements and coronary blood flow were recorded during resting conditions, and the two-stage exercise protocol was then repeated as described above. In eight dogs, aortic and coronary venous blood specimens were withdrawn at rest and during exercise for blood gas analysis. At the conclusion of study, 3 ml of blood were withdrawn from the aortic catheter and immediately centrifuged at 3,000 revolutions/min for 10 min at 4°C. The plasma was frozen at -70°C for determination of sildenafil concentration using high-performance liquid chromatography with mass spectrometric detection (25). Because this dose of sildenafil had little effect on coronary blood flow, in five additional dogs with CHF, measurements were obtained at rest and with exercise during control conditions as described above, and sildenafil was then administered in a dose of 10 mg/kg orally. One hour after sildenafil administration, all measurements were repeated at rest and during exercise.

Endothelium-dependent and -independent vasodilation. The effect of high-dose sildenafil on endothelium-dependent and endothelium-independent vasodilation was examined in four dogs with CHF. Hemodynamic and coronary blood flow measurements were obtained with the dog standing quietly in a sling. The increases in coronary flow produced by intracoronary infusion of the endothelium-dependent vasodilator acetylcholine (3.75 to 75 µg/min) and the endothelium-independent vasodilator sodium nitroprusside (3.75 to 75 µg/min) were observed. After these measurements were completed, sildenafil was administered orally in a dose of 10 mg/kg. Thirty minutes after drug administration, coronary flow responses to acetylcholine and sodium nitroprusside were repeated.

Hemodynamic measurements. Pressures were measured with transducers positioned at midchest level. LV pressure was measured with the micromanometer calibrated with the fluid-filled LV catheter. The first time derivative of LV pressure (dP/dt) was obtained via electrical differentiation. Coronary blood velocity was measured with a Doppler flowmeter system (Hartley). Data were recorded on an eight-channel direct-writing oscillograph (Coulbourn Instruments; Lehigh Valley, PA).

Myocardial oxygen consumption. Measurements of PO2, PCO2, and pH were performed with a blood gas analyzer (model 113, Instrumentation Laboratory; Lexington, MA). Hemoglobin content was determined by the cyanmethemoglobin method. Hemoglobin oxygen saturation was calculated from the blood PO2, pH, and temperature by use of the oxygen dissociation curve for canine blood. Blood oxygen content was computed as (hemoglobin × 1.34 × %O2 saturation) + (0.0031 × PO2). Oxygen consumption in the region of myocardium perfused by the LAD was calculated as the product of blood flow measured with the Doppler flow probe and the difference in oxygen content between aortic and coronary venous blood.

Western blotting. Tissue homogenates of LV myocardium (80 µg) were separated on 12% SDS-PAGE, transferred onto nitrocellulose membrane (Amersham Life Science; Arlington Heights, IL), followed by routine Western blotting. Monoclonal antibody against PDE5 was purchased from BD Transduction Laboratories (Lexington, KY). The strength of the signal was analyzed with the use of densitometry and the results expressed as arbitrary units.

Data analysis. Heart rate, LV and aortic pressures, and coronary velocity were measured from the chart recordings. Coronary flow was computed from the Doppler shift using the equation Q = 2.5 × f × D2, where Q is coronary blood flow (ml/min), f is the Doppler shift (in kHz), and D is the internal diameter of the coronary artery within the velocity probe (3). Coronary perfusion pressure was calculated from the difference of mean aortic pressure and LVEDP. Coronary vascular resistance was calculated as coronary perfusion pressure divided by coronary blood flow. Data were analyzed using two-way (exercise level and treatment) ANOVA for repeated measures. When a significant effect was observed, comparisons within groups were made using one-way ANOVA, followed by Scheffé's post hoc test. Comparisons between groups were made using Student's t-test with the Bonferroni correction. The effects of treatment on the relationship between two variables were analyzed by ANCOVA. Statistical significance was accepted at P < 0.05. All data are presented as means ± SE.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Systemic hemodynamics. Hemodynamic data at rest and during exercise in 8 normal dogs and 12 CHF dogs are shown in Table 1. Compared with normal dogs, the resting heart rate during sinus rhythm was significantly faster after the development of CHF, whereas mean aortic pressure, LV systolic pressure, and LV dP/dt were significantly less (each P < 0.05). LVEDP at rest increased from 7.9 ± 2.1 mmHg in normal animals to 24 ± 1.4 mmHg after the development of CHF (P < 0.05). Heart rates during exercise were not significantly different before and after the development of CHF. Mean aortic pressure, LV systolic pressure, rate pressure product, and LV dP/dt increased significantly in response to exercise, but remained less than before the development of CHF (P < 0.05).

                              
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  		Table 1.   Effects of exercise on normal and CHF dogs and CHF dogs after sildenafil administration

In animals with CHF sildenafil (2 mg/kg) caused no significant change of heart rate, LV systolic pressure or LV dP/dtmax. Sildenafil caused a 6 mmHg average decrease in mean aortic pressure at rest and during exercise (P < 0.05; Table 1). Two-way ANOVA, considering rest and both exercise stages simultaneously, demonstrated that LVEDP was slightly but significantly decreased after sildenafil, although this difference did not achieve significance when rest and each exercise stage were tested individually (Table 1). After sildenafil, exercise caused significant increases in heart rate, LV systolic pressure, LV dP/dtmax, and rate pressure product that were not different from those before administration of sildenafil.

Coronary hemodynamics. Coronary blood flow responses to exercise during control conditions are shown in Table 1 and Fig. 1. Before development of CHF, mean coronary blood flow at rest was 61 ± 4.6 ml/min and increased linearly with respect to the rate pressure product during exercise to a maximum of 97 ± 8.7 ml/min. Coronary blood flow was significantly (P < 0.05) less than normal after the development of CHF at rest and during each exercise stage (Table 2 and Fig. 1). Sildenafil (2 mg/kg) did not significantly alter coronary blood flow either at rest or during exercise stages 1 or 2. Two-way ANOVA considering rest and the two exercise stages simultaneously demonstrated a slight but significant decrease in coronary vascular resistance after sildenafil, although this did not achieve statistical significance when rest and each exercise stage were treated individually. A parallel downward shift in the relationship between coronary blood flow and rate pressure product was noted after the development of CHF (Fig. 1), but sildenafil did not change this relationship in the failing hearts (Fig. 1).


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  		Fig. 1.   Relationship between coronary blood flow and rate pressure product at rest and during graded treadmill exercise. Data were obtained before pacing (normal), after the development of congestive heart failure (CHF), and CHF after sildenafil administration (2 mg/kg orally). CHF resulted in a downward shift in the relationship between coronary blood flow and rate pressure product CHF (P < 0.05), but sildenafil caused no change of this relationship.


                              
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  		Table 2.   Effects of high-dose sildenafil on hemodynamics in CHF dogs

Myocardial oxygen consumption. Myocardial oxygen consumption (MVO2) measurements are shown in Table 1 and Fig. 2. Before the development of CHF, MVO2 increased linearly as a function of the rate pressure product as the result of both an increase in coronary flow as well as an increase in myocardial oxygen extraction, with a decrease in coronary venous PO2 from 21 ± 1.8 mmHg at rest to 17 ± 1.9 mmHg during exercise stage 2 (P < 0.05). After the development of CHF, MVO2 was significantly reduced compared with normal dogs at rest and during both levels of exercise (each P < 0.05) (Table 1) with a downward shift in the relationship between MVO2 and rate pressure product (Fig. 2). Sildenafil had no effect on this relationship.


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  		Fig. 2.   Relationship between myocardial O2 consumption and rate pressure product at rest and during graded treadmill exercise. Data were obtained before pacing (normal), after the development of CHF, and CHF after sildenafil administration (2 mg/kg orally). CHF resulted in a downward shift in the relationship between myocardial oxygen consumption and rate pressure product (P < 0.05), but sildenafil caused no change of this relationship.

During resting conditions coronary venous PO2 was not different from normal after the development of CHF, despite a significant decrease of MVO2 (Table 1 and Fig. 3). Exercise resulted in significant decreases of coronary venous PO2 both during control conditions and after the development of CHF (Fig. 3). However, at similar levels of MVO2, coronary venous PO2 was significantly lower than normal after the development of CHF (Fig. 3). Sildenafil had no effect on coronary venous PO2 or on the relationship between MVO2 and coronary venous PO2 (Fig. 3). After a dose of 2 mg/kg, plasma sildenafil levels ranged from 267 to 1,293 nM (mean = 861 ± 217 nM). Because sildenafil is 84% bound to plasma protein in the dog (34), this represents a mean plasma-free sildenafil concentration of 138 ± 35 nM.


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  		Fig. 3.   Relationship between coronary venous O2 tension and myocardial O2 consumption at rest and during graded treadmill exercise. Data were obtained before pacing (normal), after development of CHF, and with CHF after sildenafil administration (2 mg/kg orally). Coronary venous oxygen tension was not different from normal during resting conditions after the development of CHF. However, at comparable levels of myocardial oxygen consumption, coronary venous PO2 was less than normal after the development of CHF. Sildenafil had no effect on coronary venous PO2 either at rest or during exercise.

High-dose sildenafil. Data from five animals with CHF that received sildenafil in a dose of 10 mg/kg are shown in Table 2. Sildenafil caused decreases of aortic pressure at rest and during exercise that were similar to low-dose sildenafil. Coronary blood flow tended to be increased after sildenafil, but this was not significant. Two-way ANOVA considering rest and the two exercise stages simultaneously demonstrated a slight but significant decrease in coronary vascular resistance after sildenafil, although this did not achieve statistical significance when rest and each exercise stage were treated individually.

Endothelium-dependent and -independent vasodilation. Responses to intracoronary infusion of acetylcholine, 3.75 to 75 µg/min in four CHF dogs are shown in Fig. 4. Acetylcholine had no significant effect on mean aortic pressure, LV pressure, or heart rate, but caused dose-related increases of coronary blood flow. Sildenafil (10 mg/kg) caused a significant decrease of mean aortic pressure from 96 ± 5.2 to 88 ± 3.9 mmHg (P < 0.05) with no change in heart rate (137 ± 10 beats/min). Sildenafil caused no change in the response of coronary blood flow to acetylcholine. Intracoronary infusion of the endothelium independent vasodilator sodium nitroprusside, 3.75 to 75 µg/min in four CHF dogs caused significant dose-related increases of coronary flow. Sildenafil (10 mg/kg) did not alter the response of coronary blood flow to sodium nitroprusside (data not shown).


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  		Fig. 4.   Dose-response curves demonstrating the increase of left anterior descending coronary artery blood flow produced by graded intracoronary infusions of acetylcholine during control conditions and after sildenafil (10 mg/kg orally) in four dogs with CHF.

Expression of PDE5. To determine whether PDE5 protein levels were altered in CHF hearts, ventricular homogenates were subjected to Western analysis in five CHF and five normal dogs (Fig. 5). Compared with normal hearts, PDE5 (~95 kDa) was significantly decreased in the failing hearts (0.64 ± 0.08 vs. 1.0 ± 0.09, P < 0.01).


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  		Fig. 5.   Western blot analysis of phosphodiesterase type 5 protein levels in myocardial homogenates from five normal dogs and five dogs with pacing-induced heart failure.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

In the present study, sildenafil caused a modest decrease in aortic pressure at rest and during exercise, with no change of heart rate, LV systolic pressure, or LV dP/dtmax. Despite the decrease of aortic pressure, coronary flow was unchanged after sildenafil as the result of a modest decrease in coronary vascular resistance. This appeared to result from an autoregulatory response of the coronary resistance vessels rather than a direct vasodilator effect of sildenafil, inasmuch as myocardial oxygen extraction and coronary venous oxygen tension were unchanged, indicating that sildenafil did not perturb the coupling between myocardial oxygen needs and coronary blood flow. The implications of these findings are discussed below.

Effects of PDE5 inhibition on coronary vasomotion. NO and the natriuretic peptides produce vasodilation by causing activation of guanylyl cyclase in vascular smooth muscle cells to produce cGMP. An important consideration is the degree to which cGMP mediated resistance vessel dilation contributes to regulation of coronary blood flow. cGMP-mediated vasodilation occurs preferentially in arterial vessels with less effect at the level of the coronary arterioles (9). Selective inhibition of PDE5 with sildenafil (29), E4021 (1, 20), or zaprinast (17) significantly increased cGMP and caused relaxation of isolated normal epicardial coronary arteries. In normal animals, stimulation of endogenous coronary NO production with acetylcholine (13), intraarterial infusion of NO donors (7, 11), or intraarterial infusions of ANP or BNP resulted in resistance vessel dilation with an increase of coronary blood flow (14, 29). In normal hearts sildenafil augmented the vasodilator response to acetylcholine, in agreement with the concept that inhibition of PDE5 causes accumulation of cGMP during agonist-mediated NO production (8). In the failing hearts in the present study, sildenafil caused no change in resting coronary blood flow and did not alter the increase in coronary flow that occurred in response to the increased myocardial oxygen demands during exercise. Furthermore, sildenafil caused no change of coronary venous oxygen tension, indicating that this agent did not alter the coupling between coronary blood flow and myocardial oxygen utilization. Finally, even at a dose five times greater than that previously demonstrated to augment the coronary vasodilator response to acetylcholine in normal animals, sildenafil had no effect on the coronary flow responses in animals with heart failure.

The vascular cGMP concentration depends on the balance between the rate of cGMP production and the rate of degradation by cGMP phosphodiesterases (mainly PDE5 and PDE1). In the normal heart, cGMP is produced principally in response to NO generated by eNOS. However, in the failing heart, endothelial NO bioavailability is decreased (27). Particulate guanylyl cyclase in vascular smooth muscle is an alternate source of cGMP. Natriuretic peptides, including ANP and BNP, can engage specific endothelial receptors to activate particulate guanylyl cyclase, thereby increasing cGMP production (9, 14, 29). Previous studies (10) have demonstrated increased plasma levels of ANP and BNP in CHF. However, isolated coronary arteries from dogs with pacing-induced CHF showed a dramatic decrease in relaxation in response to ANP or BNP, although relaxation to NO was maintained, suggesting downregulation of natriuretic peptide receptors (16). In that study the relaxation to 8-bromo-cGMP was maintained in coronary artery segments from failing hearts, suggesting that the decreased response to PDE5 inhibition likely was not due to decreased sensitivity of the vascular smooth muscle to cGMP. These previous findings suggest that decreased endothelial NO bioavailability in association with downregulation of natriuretic peptide receptor activity could have contributed to the decreased response to PDE5 inhibition observed in the present study. In agreement with the present findings, Senzaki et al. (21) recently reported that cGMP modulation of sympathetic responsiveness is diminished in pacing-induced CHF in canine hearts. Thus PDE5 inhibition depressed the dobutamine-induced inotropic response in normal hearts, but did not alter the response to catecholamine stimulation in failing hearts. In that report, as in the present study, PDE5 protein expression was decreased in unfractionated myocardial tissue. Interestingly, however, Senzaki et al. (21) observed that PDE5 protein content and activity were similar in disaggregated myocytes from normal and failing hearts, indicating that the decrease in PDE5 occurred in nonmyocyte tissue. The present findings suggest that the decreased PDE5 content likely includes coronary microvessels and are in agreement with previous data demonstrating that vascular cGMP levels are decreased in rats with heart failure (24), although cGMP levels in coronary resistance vessels have not been reported.

Myocardial oxygen consumption. Several investigators have suggested that NO can contribute to regulation of MVO2, so that blockade of NO production (3, 4, 21) led to significant increases of MVO2 while stimulating endogenous NO production or administering an NO donor decreased MVO2 (12, 15). In the present study sildenafil had no effect on MVO2 or on the increase in oxygen consumption that occurred in response to exercise. The lack of effect of sildenafil on MVO2 is not surprising, inasmuch as the effect of NO on mitochondrial respiration is not mediated by cGMP, but appears to represent a direct effect of NO at cytochrome oxidase (6). Inhibition of PDE3 or PDE4 could cause an increase of MVO2 by inhibiting cAMP degradation, thereby augmenting contractility. However, sildenafil had no effect on LV dP/dtmax, in agreement with a previous report that sildenafil had no effect on contractility of isolated canine right ventricular trabecular muscle from normal dogs (26). These results suggest that sildenafil had negligible effects on the PDE isoenzymes that catabolize cAMP in myocardial myocytes.

In normal dogs, sildenafil caused a modest significant increase of coronary blood flow at rest and during exercise, but this was associated with a significant decrease of hemoglobin, so that the relationship between MVO2 and oxygen delivery to the heart was unchanged (8). Unlike the normal dogs, in the dogs with CHF in the present study inhibition of PDE5 with sildenafil caused no change in hemoglobin or in coronary blood flow. Thus sildenafil did not interfere with the metabolic vasoregulatory mechanisms by which oxygen delivery is maintained proportionate to myocardial oxygen needs.

Limitations. Because sildenafil did not significantly increase coronary blood flow in animals with CHF, even at a drug dose five times higher than a dose that did increase coronary flow in normal animals, and did augment the response to acetylcholine, one must consider whether alternate pathways for degradation of cGMP might have concealed the effect of sildenafil. Vascular smooth muscle cGMP-hydrolyzing activity is mainly due to PDE1 and PDE5, whereas cAMP-hydrolyzing activity is mainly due to PDE4 and PDE3 (2, 26). There are no reports of changes in vascular or myocardial PDE1 expression in CHF. The IC50 for inhibition of PDE1, PDE2, PDE3, PDE4, and PDE5 by sildenafil was 280 nM, 6,800 nM, 16,200 nM, 7,200 nM, and 3.5 nM, respectively, indicating a high selectivity for PDE5 (26). The mean plasma-free drug concentration of 138 ± 35 nM after low-dose sildenafil in the present study should have provided a high degree of blockade of PDE5 with some degree of inhibition of PDE1. In an earlier study, we observed that the lower dose of sildenafil used in the present study significantly augmented the coronary vasodilator response to acetylcholine in normal animals (8). Thus the measured blood levels, the inclusion of a group of animals that received a fivefold greater dose of sildenafil in the present study, and the biological response to the low dose of sildenafil previously observed in normal animals, all support the adequacy of the drug dose used in the present study.

In conclusion, PDE5 activity contributes little to regulation of coronary hemodynamics in animals with CHF. These findings are in agreement with previous studies demonstrating a markedly decreased effect of PDE5 in modulating the myocardial response to adrenergic stimulation in the failing heart (21), as well as reports that vascular cGMP content is low in the setting of CHF (23), likely as the result of decreased NO bioavailability as well as downregulation of vascular smooth muscle natriuretic peptide receptors.


    ACKNOWLEDGEMENTS

This study was supported by National Heart, Lung, and Blood Institute Grants HL-20598 and HL-21872. J. H. Traverse is the recipient of a Scientist Development Award from the American Heart Association.


    FOOTNOTES

We acknowledge the expert technical assistance provided by Melanie Crampton, Paul Lindstrom, and Shauna Voss, and the secretarial assistance of Carol Quirt. Sildenafil was provided by Pfizer (Groton, CT). The plasma sildenafil assays were performed by Dr. Rachel Halliday, Pfizer Central Research (Sandwich, UK).

Address for reprint requests and other correspondence: R. J. Bache, Div. of Cardiology, Dept. of Medicine, Univ. of Minnesota Medical School, Mayo Mail Code 508 UMHC, 420 Delaware St. SE, Minneapolis, MN 55455 (E-mail: bache001{at}tc.umn.edu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

10.1152/ajpheart.00529.2001

Received 19 June 2001; accepted in final form 2 January 2003.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

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Am J Physiol Heart Circ Physiol 284(5):H1513-H1520
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