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Circulation. 1996;94:2542-2550

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*EPINEPHRINE
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*Heart Transplantation

(Circulation. 1996;94:2542-2550.)
© 1996 American Heart Association, Inc.

Articles

Sympathetic Stimulation Overrides Flow-Mediated Endothelium-Dependent Epicardial Coronary Vasodilation in Transplant Patients

Eduardo Aptecar, MD; Patrick Dupouy, MD; Christophe Benvenuti, MD; Jean-Philippe Mazzucotelli, MD; Emmanuel Teiger, MD; Herbert Geschwind, MD; Alain Castaigne, MD; Daniel Loisance, MD; Jean-Luc Dubois-Rande, MD

the Federation de Cardiologie et Institut National de la Sante et de la Recherche Medicale U400, Unite d'Hemodynamique et de Cardiologie Interventionnelle, Service des Explorations Fonctionnelles, and Service de Chirurgie Thoracique et Cardio-vasculaire et Centre National de la Recherche Scientifique URA 1431, Hopital Henri Mondor (E.A., P.D., C.B., P.M., E.T., H.G., A.C., D.L., J.-L.D.-R.); and the Service de Readaptation Cardiaque, Hopital Albert Chenevier (C.B.), Creteil, France.

Correspondence to E. Aptecar, MD, Federation de Cardiologie, Service du Pr A Castaigne, Hopital Henri Mondor, 51 Av du Marechal de Lattre de Tassigny, 94010 Creteil, France.


*    Abstract
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 *Abstract
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 down arrowResults
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Background Abnormal coronary vasomotor responses have been described in transplant patients. The aim of this study was to evaluate the graft epicardial vasomotor responses to different stimuli that increase coronary blood flow.

Methods and Results Twelve heart transplant recipients with angiographically normal epicardial coronary arteries were compared 2.7±1.2 months after surgery with 6 control subjects. Coronary flow velocity was measured with a guidewire Doppler. Coronary diameter changes of the proximal and midportion of the left anterior descending coronary artery were assessed by quantitative coronary angiography during rapid atrial pacing, cold pressor test, supine exercise, and subselective infusion of papaverine and after intracoronary injection of linsidomine (SIN-1). Catecholamine plasmatic levels were determined at the different stages of the protocol. In 6 other transplant patients, a cold pressor test was performed before and after intracoronary infusion of phentolamine (10 µg·kg-1·min-1). Coronary flow velocity increased significantly in both groups during each phase of the protocol. In control subjects, dilation was observed in response to atrial pacing (8.7±7.6%; P<.05), CPT (8.8±2.3%; P<.01), exercise (14.5±9.4%; P<.001), and papaverine infusion (14.2±6.1%; P<.001) and after injection of SIN-1 (26.8±11.9%; P<.001). In transplant patients, similar dilation was observed during atrial pacing (8.2±8.3%; P<.05) and papaverine infusion (14.6±7.8%; P<.001) and after SIN-1 (25.8±10.8%; P<.001). CPT and exercise caused slight constriction (-3.5±4.5% and -2.7±10.5%, respectively; both P<.001 versus control subjects). Norepinephrine plasmatic levels increased in both groups during CPT and exercise. Slight constriction during the cold pressor test (-4.5±9.6%) changed to dilation (6.8±7.0%) after {alpha}-blockade with phentolamine (P<.001).

Conclusions These results show that flow-mediated, endothelium-dependent vasodilation is preserved early after transplantation. Sympathetic stimulation, which overrides the endothelium-dependent mechanism, can be related to hypersensitivity to catecholamines due to denervation.


Key Words: transplantation • blood flow • vasodilation


*    Introduction
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 up arrowAbstract
 *Introduction
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 down arrowResults
 down arrowDiscussion
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Abnormal coronary vasomotor responses have been observed early after surgery in heart transplant patients before angiographic evidence of graft coronary disease.1 2 3 Because the endothelial lining of the coronary circulation is the anatomic barrier between the graft and the host circulating antibodies and immunoactive cells and is believed to be the site where the immunologic process leading to the development of graft atherosclerosis begins, endothelial dysfunction is thought to be the cause of the abnormal epicardial coronary vasomotor responses that have been observed with pharmacological tests.4

Normal epicardial arteries dilate when coronary blood flow increases.5 Vasodilation of conductance vessels induced by increase in flow is mediated by the endothelium, mainly through the release of endothelium-derived relaxing factor (EDRF),6 7 identified as nitric oxide,8 although other substances (such as prostacyclin or hyperpolarizing factors) may also contribute to it.9 10 Increases in myocardial flow can be obtained with stimuli such as pacing, exercise, and cold pressor testing (CPT), by mental stress, or by pharmacological dilation of coronary resistance vessels with agents such as papaverine or adenosine. These stimuli differ in that some of them, namely, CPT, exercise, and mental stress, induce a sympathetic stimulation and increase in circulating catecholamines.11 12 In humans, all of these stimuli have been shown to dilate normal small and large coronary arteries.13 14 15 In addition, it is noteworthy that the presence of atherosclerosis alters the so-called flow-mediated vasodilating response of coronary arteries.16 17 18 19

In transplant patients, vasodilation of angiographically normal coronary arteries has been described in response to atrial pacing20 and to papaverine.21 22 Conversely, we reported23 abnormal vasomotor response to CPT, suggesting an imbalance between the flow-mediated, endothelium-dependent and the vascular adrenergic receptor–mediated regulation of vasomotor tone in heart transplants.

The aim of the present study was to assess the coronary vasomotor responses to atrial pacing, CPT, exercise, and papaverine infusion early after surgery in a group of transplant patients with normal epicardial coronary arteries. The results were compared with those of a group of young control patients.


*    Methods
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 *Methods
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Patient Selection
Control Subjects
Six young patients were selected as control subjects. All had angiographically normal coronary arteries without luminal irregularities. They were referred for diagnostic coronary arteriography because of repetitive atypical chest-pain syndromes, equivocal bicycle exercise test, or thallium scan scintigraphy. All subjects were normotensive, and none had diabetes mellitus or hypercholesterolemia. All had normal left ventricular systolic function assessed by two-dimensional and M-mode echocardiography (Table 1Down), and none had cardiac hypertrophy on echocardiography, according to the criteria of the American Society of Echocardiography.24 At the time of the investigations, none of these patients had received any cardiovascular therapy.


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  		Table 1. Study Population Characteristics

Transplanted Patients
Among 37 consecutive patients who had undergone cardiac transplantation, 18 were included in the study (HTX). Reasons for excluding the other 19 patients were postoperative complications that precluded early catheterization (8 patients), failure to obtain consent (4), early mortality (3), abnormal coronary arteries found at catheterization (2), and impossibility of obtaining a stable Doppler signal (2). The time of testing was 2.7±1.2 months after surgery, at which time selected patients were able to do an adequate hand-bicycle supine exercise. All of them had angiographically normal coronary arteries, without luminal stenoses or irregularities, as judged by two experienced observers. All had normal left ventricular two-dimensional and M-mode echocardiography parameters (Table 1Up). Posttransplantation immunosuppressive therapy included prednisone and cyclosporine for all patients and azathioprine in 13 of 18. Cyclosporine was titrated to maintain serum levels between 150 and 200 ng/mL, as measured by nonspecific radioimmunoassay (Sandoz). Right ventricular endomyocardial biopsy was performed the day of the investigation. Three patients had histological signs of rejection justifying augmented immunosuppressive therapy. Potential risk factors analyzed for graft vasculopathy included recipient age, donor age, number of previous rejection episodes, incidence of hypertension and diabetes, lipid profile, and cytomegalovirus infection.

The study protocol was approved by the local Ethical Committee. Informed consent was obtained from each patient.

Study Design
All patients fasted for at least 12 hours before the investigation. Vasoactive therapy including calcium channel blockers, ß-adrenergic receptor blockers, {alpha}-receptor antagonists, and ACE inhibitors was discontinued 24 hours before catheterization. No premedication was administered. One percent lidocaine was used for local anesthesia, and 5000 U of heparin was administered intravenously at the moment of percutaneous femoral cannulation. After completion of diagnostic right and left heart catheterization, an additional 5000 U of heparin was given. A 7F guide catheter was positioned into the left main coronary artery. A 6F bipolar pacing wire was placed in the right atrium through the femoral vein.

Assessment of Coronary Blood Flow Velocity
A 0.018-inch-tip guidewire Doppler (12 MHz, Cardiometrics, Inc) was advanced through the guiding catheter into the proximal segment of the left anterior descending coronary artery distal to any large branch. The Doppler guidewire position was not changed thereafter. Throughout the experimental protocol, spectral analysis of the Doppler frequency was recorded. Time-average peak velocity was retained for analysis as a measurement of mean coronary blood flow velocity.25 After an appropriate spectral Doppler signal was obtained, the following interventions were made consecutively, with a 5- to 10-minute period observed between the different tests to allow hemodynamic recovery (Fig 1Down):



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  		Figure 1. Schematic of study protocol (see text for explanations). Sin-1 indicates linsidomine.

  1. Atrial pacing—atrial pacing was used to increase the heart rate by at least 25% of the initial resting sinus rate. Special care was taken to avoid pacing at high rates that might induce significant fall of the aortic pressure. Pacing was maintained for 2 minutes.
  2. CPT—CPT was performed by immersion of the patient's hands and forearms in ice water for 90 seconds.
  3. Exercise—a supine exercise test was done with the use of a hand bicycle (Etablissement Gauthier), which allowed subjects to perform a stepwise exercise for 3 to 4 minutes. Exercise started at 30 W and progressed by increments of 30 W (each for 1 minute) up to 90 or 120 W, depending on the patient's ability.
  4. Papaverine infusion—Subselective infusion of a bolus of 7 mg papaverine in 0.9% saline was performed into the distal portion of the left anterior descending coronary artery through a 3F wire-guide extension catheter (Cook Co). The catheter was placed distal to the coronary artery segments analyzed by angiography to assess the direct effect of increase in flow on the proximal segments rather than the effect of papaverine itself. The effect of the papaverine-induced increase in blood flow was assessed at the peak of the increase in flow, which typically occurred between 30 and 60 seconds after papaverine injection.

After completion of this part of the protocol, a bolus of 1 mg of linsidomine was injected through the guiding catheter to assess the vasodilatory response of the coronary arteries to a non–endothelium-dependent-mediated vasodilator.26

Serial hand injections of nonionic contrast media (Iopamidol, Schering Laboratories) into the left main stem were performed at baseline, immediately before the end of each step of the protocol, at the end of recovery periods after each test, and 3 minutes after injection of linsidomine.

Systolic, diastolic, and mean aortic pressures, heart rate, and the ECG were monitored continuously throughout the protocol.

Effects of Coronary {alpha}-Blockade
To further assess the role of coronary {alpha}-receptors in the vasomotor responses of epicardial coronary arteries in HTX, six additional patients (four men, two women, 48±6 years old, 2.6±1.2 months after transplantation) underwent a different protocol. After completion of diagnostic right and left heart catheterization, a 7F guide catheter was positioned into the left main coronary artery. A 0.018-inch-tip guidewire Doppler was placed into the proximal segment of the left anterior descending coronary artery distal to any large branch. After 3 minutes of a 0.9% saline infusion (0.8 mL/min) performed through the guiding catheter, a CPT was done, as already described. After a 5- to 10-minute period of recovery, an infusion of phentolamine (Regitin IC, Ciba-Geigy) was started through the guiding catheter at a dose of 10 µg·kg-1·min-1 and at a rate of 0.8 mL/min. The intracoronary infusion was used to minimize the systemic effects of the drug. After 3 minutes, the CPT was repeated, without interruption of the phentolamine infusion. Coronary blood flow velocity and epicardial artery diameter of two segments of the left anterior descending artery were assessed at baseline, immediately before the end of each step, and at the end of the recovery periods. Heart rate, aortic pressure, and ECG were monitored continuously.

The adequacy of the {alpha}-blockade dose used in this study (phentolamine, 10 µg·kg-1·min-1) was previously addressed in four other transplant patients. The diameter of two segments of the left anterior descending artery was measured at baseline and 3 minutes after subselective infusion of the {alpha}-agonist phenylephrine (Neosynephrine, Boeringher Ingelheim France) into the proximal left anterior descending artery. Phenylephrine infusion dose and rate were 1.6 µg/min and 0.8 mL/min, respectively, yielding an estimated blood concentration of 10-7 mol/L. After recovery of the basal diameter (10±3 min), phentolamine was infused into the left coronary artery. Three minutes later, a second infusion of phenylephrine was started, and measurements were repeated immediately before and 3 minutes thereafter. The {alpha}-blockade obtained with this dose of phentolamine was considered to be adequate, because vasoconstriction induced with phenylephrine was diminished by >80% after phentolamine infusion.

Quantitative Coronary Angiography
A General Electric-CGR x-ray machine connected to an ADAC DPS PLUS system (ADAC Laboratories) was used in all studies. The size of the image intensifier was 6 inches. The digital system and software analysis program (ARTREK, ADAC Laboratories) used in this study, which allow fully automatic edge detection and quantitation of coronary segments, have been validated previously.27 For each patient, a transverse view was chosen to allow performance of the supine exercise. The proximal and middle portions of the left anterior descending coronary artery were positioned near the isocenter. With this view, pincushion distortion was found to be small because measurements made at the periphery of the zone of interest varied by 5% from those made at the center.28 Thus, because the same position was maintained constantly and relations between focal spot, patient, and height of image intensifier were not modified throughout the study, no correction for distortion was introduced. Digitally acquired images coupled to an ECG monitor were recorded at the rate of 25 frames/s. End-diastolic frames were analyzed by use of 4x magnification to help provide adequate pixel resolution for small vessels and catheters. The distal end of the injection catheter served for calibration purposes with automatic tracking for edge determination. Fixed anatomic coordinates were used to reproduce the same regions of interest to assess serial changes. Two defined regions of the proximal and middle left anterior descending coronary artery were selected in both groups of patients. Coronary segments analyzed were {approx}10 mm in length (mean, 9.6±2.5 mm). Automatic vessel segment contour detection was performed, and an average of the diameter of each segment was obtained directly from the computer. Additionally, the measurements were made by two operators who were blinded to the conditions being studied. The interobserver (SEE for repeated measurements stated as a percent of the mean vessel diameter) and intraobserver variabilities using this system were found to be low: 4.3% and 4.7%, respectively.

Responses of the coronary arteries to the different stimuli were expressed as percent change versus control value.

Catecholamine Dosing
Plasmatic norepinephrine and epinephrine concentrations were determined by use of the radioenzymatic method29 in blood samples drawn from the femoral vein at baseline, immediately before the end of atrial pacing, CPT, and exercise, and at the end of recovery periods that succeeded these tests.

Statistical Analysis
All data are presented as mean±SD. When applicable, comparisons between groups were performed with unpaired t tests. Serial changes in luminal diameter or hemodynamic variables were compared by two-way ANOVA for repeated measures, followed by the Fisher protected least significant difference test or Dunnett's test for comparisons with control values. Statistical difference was assumed if the null hypothesis could be rejected at the 0.05 probability level.


*    Results
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 up arrowMethods
 *Results
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Study Population Characteristics
The age of donors of the transplanted hearts was similar to the age of control subjects. The two groups were similar with respect to left ventricular dimensions, fractional shortening, and left ventricular mass. Lipid profile was normal and similar in the two groups, except for apolipoprotein A-1, which was slightly but significantly lower in the control group (P<.05). (See Table 1Up.)

Systemic Hemodynamic Responses
All patients were normotensive at the time of investigation. Heart rate, systolic aortic pressure, and the rate-pressure product at baseline were similar in control subjects and HTX. Atrial pacing and CPT induced a similar increase in the rate-pressure product in the two groups. During exercise, the increase in the rate-pressure product was significantly higher in control subjects than in HTX: 124±34% versus 53±23% (P<.05). Hemodynamic parameters did not change in either group during papaverine infusion. Linsidomine induced a significant fall in systolic aortic pressure (-10±12%, P<.0001) and in the rate-pressure product (-10±15%, P<.05) in HTX but not in control subjects. (See Table 2Down.)


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  		Table 2. Systemic Hemodynamics

Effect on Coronary Blood Flow Velocity
Table 3Down shows that coronary blood flow velocity increased significantly in both groups during each phase of the study protocol. This increase was similar for atrial pacing (57±32% and 33±28%) and CPT (35±22% and 32±11%) in control subjects and in HTX, respectively, whereas exercise induced a significantly greater increase of blood flow velocity in control subjects (90±26%) than in HTX (34±17%; P<.001). The greatest increase in blood flow velocity was elicited by subselective papaverine infusion (158±50% and 171±75%, control subjects and HTX, respectively). Finally, linsidomine infusion induced a slight decrease in blood flow velocity in both groups compared with the basal measurement that was not significant (-13±14% and -16±35%).


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  		Table 3. Coronary Blood Flow Velocity

Vasomotor Response of Epicardial Coronary Arteries
The baseline mean diameters of the proximal and middle left anterior descending artery segments were similar in the two groups: 3.3±0.6 and 2.5±0.5 mm in the control subjects, and 3.4±0.7 and 2.6±0.5 mm in HTX.

Normal Control Arteries
In control subjects, the epicardial coronary vasomotor response to all stimuli was a significant dilation: 8.7±7.6% during atrial pacing (P<.05); 8.8±2.3% during CPT (P<.01); 14.5±9.4% during exercise (P<.001); and 14.2±6.1% during papaverine infusion (P<.001). Intracoronary linsidomine increased diameter by 26.8±11.9% (P<.001). (See Fig 2Down.)



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  		Figure 2. Changes in coronary segment diameters expressed as percent of baseline (mean±SD) in response to different stimuli. Base indicates baseline; Atr Pa, atrial pacing; CPT, cold pressor test; Exerc, supine exercise with a hand bicycle; Papav, papaverine infusion; Sin-1, linsidomine infusion; and HTX, transplant patients. *P<.05, **P<.01, ***P<.001 versus baseline (intragroup comparisons).

Angiographically Normal Arteries in HTX
In HTX, atrial pacing and subselective infusion of papaverine caused a significant dilation of studied epicardial coronary artery segments similar to that observed in the control patients (Fig 2Up): 8.2±8.3% during atrial pacing (P<.05) and 14.6±7.8% during papaverine infusion (P<.001). On the other hand, the response of the same studied segments to CPT and to exercise was a small but not statistically significant constriction: CPT caused a reduction in diameter of 3.5±4.5%, and exercise caused a reduction in diameter of 2.7±10.5%. Comparison between the two groups of patients showed that the vasomotor response to CPT and to exercise was highly significantly different (both P<.001, respectively) (Figs 2 and 3UpDown). Finally, linsidomine dilated graft coronary arteries by 25.8±10.8% (P<.001).



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  		Figure 3. The percent changes in plasma norepinephrine (noradrenaline) concentration and in coronary artery diameter during atrial pacing, cold pressor test (CPT), and exercise are displayed. During atrial pacing, norepinephrine did not increase, and coronary dilation was observed in both control subjects and transplant patients (HTX). CPT and exercise elicited an increase in norepinephrine concentrations in both groups, and opposite vasomotor responses (dilation in control subjects and slight constriction in HTX) were observed during both tests (both P<.001 between groups).

Neurohormonal Responses
At baseline, epinephrine and norepinephrine plasmatic concentrations were similar in both groups: 122±65 and 452±242 ng/L, respectively, in control subjects, and 148±87 and 404±181 ng/L in HTX. Fig 4Down shows that circulating norepinephrine increased both during the CPT and during exercise, whereas it did not change during atrial pacing either in control subjects or in HTX. Norepinephrine levels were similar and higher in both groups during exercise than during CPT (both P<.001).



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  		Figure 4. Plasmatic norepinephrine concentrations (ng/L) during atrial pacing (Atr Pa), cold pressor test (CPT), and exercise (Exerc). HTX indicates transplant patients. *P<.05, **P<.01, ***P<.001 versus the corresponding baseline values. There were no significant differences between groups.

Epinephrine levels did not change during pacing and did not increase significantly during CPT or exercise (from 121±57 to 134±57 ng/L and from 135±63 to 171±85 ng/L in control subjects; from 123±82 to 164±141 ng/L and from 137±102 to 184±167 ng/L in HTX, respectively).

Effects of {alpha}-Blockade
As shown in Table 4Down, aortic pressure and the rate-pressure product at baseline decreased significantly after {alpha}-blockade (-9±6% and -12±6%, respectively, both P<.05), and coronary blood flow velocity fell by 8±12% (P=NS). Baseline mean diameters of proximal and middle left anterior descending artery segments were not significantly different before or after {alpha}-blockade (Fig 5Down): 3.1±0.4 and 3.2±0.4 mm (proximal), respectively, and 2.1±0.4 and 2.2±0.5 mm (middle), respectively. The epicardial vasomotor response induced by the CPT was changed from slight vasoconstriction (-4.5±9.6%) before {alpha}-blockade to vasodilation after {alpha}-blockade (6.8±7.0%). Compared with their baselines, the two responses were not significantly different, but compared to each other, the difference was highly significant (P<.001) (Fig 5Down).


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  		Table 4. Systemic Hemodynamics and Coronary Blood Flow Velocity Before and After {alpha}-Blockade With Intracoronary Phentolamine in Six Transplant Patients



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  		Figure 5. Effect of {alpha}-blockade on the response to cold pressor test (CPT) in transplant patients (n=6). Before {alpha}-blockade, CPT induced slight epicardial coronary vasoconstriction, and after {alpha}-blockade, vasodilation was observed when CPT was repeated. Although these responses were not significant versus respective baselines, comparison between them shows a highly significant difference. LAD indicates left anterior descending coronary artery.


*    Discussion
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 up arrowAbstract
 up arrowIntroduction
 up arrowMethods
 up arrowResults
 *Discussion
down arrowReferences
 
The present study shows that the normal epicardial coronary vasodilating response to tachycardia and to subselective infusion of papaverine is preserved early after transplantation. On the other hand, exercise and CPT, which cause adrenergic stimulation, fail to elicit epicardial coronary vasodilation in the same patients. These results suggest that the flow-mediated, endothelium-dependent vasomotor response is preserved early after transplantation, but sympathetic stimulation overrides this endothelium-dependent mechanism, leading to an abnormal vasomotor response.

Coronary Vasomotor Responses to Increase in Flow in Control Subjects
As shown in this study, flow-mediated, endothelium-dependent vasodilation of the coronary bed can be elicited by different stimuli, both physical and pharmacological. All these stimuli increased coronary blood flow, and all of them induced significant epicardial vasodilation in control subjects, despite the fact that CPT and physical exertion were associated with adrenergic stimulation resulting in increased levels of circulating norepinephrine (Fig 4Up). Although the stimulation of {alpha}1- and {alpha}2-adrenergic receptors on vascular smooth muscle causes direct epicardial vasoconstriction,30 normal endothelium counteracts the constrictor effects of catecholamines by several mechanisms, including the release of EDRF in response to stimulation of {alpha}2-adrenergic receptors on endothelial cells.31 32

In control subjects as well as in HTX, epicardial coronary arteries dilated in response to intracoronary infusion of linsidomine, indicating the ability of vascular smooth muscle to relax. Linsidomine, the active metabolite of molsidomine, is used frequently during coronary arteriography. It belongs to the group of exogenous nitrates, but it possesses a distinctive mode of action because it is a direct nitric oxide donor with an endothelium- and cysteine-independent action.26

Coronary Vasomotor Responses to Increase in Flow in HTX
Responses to Atrial Pacing and to Papaverine Infusion
Atrial pacing and subselective infusion of papaverine elicited a significant dilation of normal epicardial coronary arteries in HTX (Fig 2Up), indicating that flow-mediated, endothelium-dependent vasodilation was preserved early after transplantation. The results of the present study agree with previous observations20 21 showing that flow-mediated coronary vasomotion is normal in transplant patients with angiographically normal coronary arteries.

Responses to CPT and to Exercise
Epicardial coronary arteries of HTX failed to dilate during the CPT and during physical exertion, as opposed to normal subjects (Fig 2Up). This was observed despite normal systemic hemodynamic responses. During exercise, HTX, who reached a lower peak effort, showed a significantly lower rise in the rate-pressure product and in blood flow velocity. However, blood flow velocity increase during exercise was similar to that observed during CPT and during atrial pacing. Norepinephrine circulating levels, similar at the basal state in HTX and control subjects, increased to the same extent during CPT and exercise in both groups (Fig 4Up).

Thus, stimuli that do not induce adrenergic system activation (eg, atrial pacing or papaverine infusion) elicit a similar vasodilating response in transplant patients and control subjects, whereas stimuli that induce an activation of the adrenergic system (eg, CPT or exercise) elicit a paradoxical vasomotor response in transplant patients (Fig 4Up). These results suggest an imbalance between the flow-mediated, endothelium-dependent and the vascular adrenergic-mediated regulation of vasomotor tone in transplanted hearts that could be related to an endothelial dysfunction, to an excessive vasoconstrictor effect of the released catecholamines, or both.

Impairment of Endothelium-Mediated Vasomotor Response of Large Coronary Arteries
Endothelial dysfunction could explain the abnormal vasomotor responses observed during CPT and during exercise, because functional or anatomic alterations of the endothelium can unmask the constrictive effects of catecholamines on smooth muscle cells. This has been shown in animal models33 34 35 as well as in humans.36 Abnormal coronary vasomotor responses during CPT13 18 37 and during exercise14 18 have been described in human atherosclerotic vessels.

Endothelial dysfunction has been reported previously in cardiac transplant patients as early as 1 month after surgery.1 2 3 Several reasons may explain endothelial dysfunction early after transplantation, even before the development of anatomic cardiac allograft vasculopathy demonstrated by normal intravascular ultrasound or coronary angiography.38 Endothelial damage can occur during the harvesting of the donor heart with the current methods of cardiac preservation.39 Graft ischemic time during the transplantation procedure could be another contributing factor, because hypoxia and anoxia inhibit the release of EDRF and also stimulate the release of endothelium-derived constricting factor.40 Immune-mediated endothelial alterations could result in loss of release of EDRF. Atherosclerosis of the donor hearts should be considered,41 but the mean age of the donors was low and similar to that of the control subjects (Table 1Up). Hypercholesterolemia or high levels of LDL and hypertension, which can impair endothelial dilator activity,42 43 were not present in the transplant patients in this study (Table 1Up). Finally, cyclosporine has been shown to elicit cell membrane disruption, diminish prostacyclin synthesis by vascular endothelial cells, and enhance thromboxane A2 release.44 45 Also, impaired release of EDRF due to cyclosporine has been described in epicardial and resistance canine coronary arteries.46

Although endothelial dysfunction is likely to explain paradoxical vasoconstriction of normal coronary arteries observed with acetylcholine in transplant patients,1 2 3 studies using substance P47 failed to confirm the "endothelial hypothesis." The results of our study oppose the theory of endothelial dysfunction in transplant patients, because flow-dependent vasodilation requires the presence of an anatomically and functionally normal endothelium.48 49 Although slight endothelial dysfunction cannot be excluded, other mechanisms should be considered to explain the abnormal vasomotor responses observed during the CPT and during exercise.

Increased Sensitivity to Catecholamines
Because hypersensitivity of myocardial ß-adrenoceptors has been evidenced at the presynaptic level in cardiac transplant patients,50 51 a similar increased sensitivity of vascular smooth muscle cell {alpha}-adrenoceptors related to denervation could be responsible for the abnormal vasomotor responses we observed during CPT and exercise in the present study. This "adrenergic hypothesis" is strengthened by the fact that the constrictive response observed during the CPT was reversed by the intracoronary infusion of phentolamine, a nonselective {alpha}-adrenergic blocking agent (Fig 5Up).

The effect of adrenergic blockade on the coronary circulation in the denervated human heart has been addressed previously by Hodgson et al.52 These authors found little effect of {alpha}- or ß-blockade on epicardial artery dimensions under resting conditions in normally innervated and in denervated transplant patients. Our data confirm the finding that under resting conditions, the {alpha}-adrenergic influence on epicardial vessels diameter is negligible in transplanted hearts, because the coronary diameter does not vary before and after {alpha}-blockade. On the other hand, during cold exposure, phentolamine could cause epicardial vasodilation by two mechanisms: by inducing a greater increase of coronary blood flow, thereby eliciting a flow-dependent vasodilation, or by releasing the adrenergic vasoconstrictor tone. In chronically instrumented dogs, greater increases in coronary blood flow have been shown to occur during exercise after nonselective {alpha}-blockade than during control conditions.53 54 However, in another study, Baran et al55 found in dogs that intracoronary prazosin, a selective {alpha}1-blocker, caused greater epicardial vasodilation during exercise without a simultaneous significant increase in coronary blood flow. Similarly, we did not find a further increase in coronary blood flow during cold exposure after intracoronary nonselective {alpha}-blockade, probably because of the systemic effects of phentolamine, which significantly reduced aortic pressure and the rate-pressure product. Thus, it appears that the epicardial vasodilation we observed in transplant patients during CPT after nonselective {alpha}-blockade is caused by the release of the adrenergic vasoconstrictor tone that overrides the flow-mediated, vasodilating mechanisms.

Finally, the potential vasoconstrictor role of cyclosporine has to be considered. Cyclosporin A is known to stimulate endothelin synthesis56 and to increase endothelin bindings sites in cardiac smooth muscle cells.57 In addition, endothelin may increase the response to sympathetic stimulation.58

Limitations of the Study
Although we selected patients with angiographically normal coronary arteries, this approach has limitations. Intravascular ultrasound has a greater yield for detecting atherosclerosis in angiographically normal coronary arteries. It is possible that some of the HTX had already developed some degree of intimal thickening that was angiographically silent. However, graft endothelial dysfunction has been suggested by the paradoxical response to acetylcholine in segments of coronary arteries without endothelial thickening as evaluated by intravascular ultrasound.38 Thus, even with the use of intravascular ultrasound, endothelial dysfunction could not be excluded as a contributing factor to explain the abnormal vasomotor responses observed in HTX.

Conclusion
The results of this study strongly suggest that hypersensitivity to catecholamines secondary to denervation is present at the epicardial coronary level in the transplanted heart. This notion should be considered when coronary vasomotion is addressed in transplant patients.

Received April 16, 1996; revision received July 1, 1996; accepted July 11, 1996.


*    References
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