Decrease in beta 1- and increase in beta 2-adrenoceptors in long-term follow-up after orthotopic cardiac transplantation

Effect of stress on the chronotropic and inotropic responses to β-adrenergic agonists in isolated atria of KOβ2 mice

Altered sensitivity to the chronotropic and inotropic effects of catecholamines and reduction in β₁/β₂-adrenoceptor (β₁/β₂-AR) ratio were reported in failing and in senescent human heart, as well as in isolated atria and ventricle of rats submitted to stress. This was due to downregulation of β₁-AR with or without up-regulation of β₂-AR.

AIMS

To investigate the stress-induced behavior of β₁-AR in the heart of mice expressing a non-functional β₂-AR subtype. The guiding hypothesis is that the absence of β₂-AR signaling will not affect the behavior of β₁-AR during stress and that those are independent processes.

MATERIALS AND METHODS

The chronotropic and inotropic responses to β-AR agonists in isolated atria of stressed mice expressing a non-functional β₂-AR were analyzed. The mRNA and protein expressions of β₁- and β₂-AR were also determined.

KEY FINDINGS

No deaths were observed in mice under stress protocol. Atria of stressed mice displayed reduced sensitivity to isoprenaline compared to the controls, an effect that was abolished by the β₂- and β₁-AR antagonists 50 nM ICI118,551 and 300 nM CGP20712A, respectively. Sensitivity and maximum response to the β-agonists dobutamine and salbutamol were not altered by stress or ICI118,551. The responses to dobutamine and salbutamol were prevented by CGP20712A. The expression of β₁-AR was reduced at protein levels.

SIGNIFICANCE

Collectively, our data provide evidence that the cardiac β₂-AR is not essential for survival in a stressful situation and that the stress-induced reduction of β₁-AR expression was independent of the β₂-AR presence.

Coronary Artery Vasospasm Requiring Cardiac Autotransplantation Yet Controlled With Tobacco

Coronary artery vasospasm is typically managed through avoidance of triggers and with symptomatic treatments with calcium channel blockers and long-acting nitrates. Here, we report a rare case of medically refractory coronary artery vasospasm associated with genetic predispositions that initially required cardiac autotransplantation followed paradoxically by nicotine for long-term symptomatic control. (Level of Difficulty: Intermediate.)

Myocardial and peripheral lymphocytic transcriptomic dissociation of beta-adrenoceptors and G protein-coupled receptor kinases in heart transplantation

BACKGROUND

The genetic expression of adrenergic receptors plays an important pathophysiologic role in heart failure. G protein-coupled receptor kinases (GRKs) desensitize the beta-receptor to catecholaminergic stimulation. It has been suggested that their mRNA expression in peripheral lymphocytes could mirror the changes in their myocardial expression in the failing heart, but this relationship between the myocyte and lymphocyte has not been studied in heart transplantation (HT). The objective of this study was to analyze adrenergic receptor and GRK mRNA expression in myocardium and lymphocytes and their correlation.

METHODS

Twenty-three HT patients without evidence of acute rejection or echocardiographic dysfunction were assessed. Myocardial biopsy samples and peripheral blood lymphocytes were obtained, and alpha(1)- and beta-adrenoceptor subtype and GRK subtype mRNA was analyzed using reverse transcript-polymerase chain reaction (RT-PCR).

RESULTS

Mean age was 45 +/- 15 years, with a median of time since HT of 205 (351) days. In biopsies, the beta(1)/beta(2)-adrenoceptor ratio was 57%/42%, and GRK5 was the most commonly expressed, followed by GRK2. In lymphocytes, the beta(1)/beta(2) ratio was 3%/96%, whereas GRK2 mRNA expression was greater than that of other subtypes. There was no correlation between myocardial and lymphocyte parameters. There were no correlations with clinical variables, but lymphocyte beta(2) and GRK2 were increased with time since HT.

CONCLUSIONS

In the transplanted heart, there is no correlation between mRNA expression of adrenoceptors and GRKs in myocardium and peripheral lymphocytes. With time since transplant, mRNA expression of lymphocyte but not myocardial beta(2)-adrenoceptor and GRK2 increases. Therefore, this dissociation between myocardial and lymphocyte mRNA expression limits the potential use of peripheral blood samples for diagnosis of graft dysfunction.

Coronary Vasomotor Response to Phenylephrine in Heart Transplant Patients

BACKGROUND

Coronary vasomotor responses to sympathetic stimulation vary with endothelial-layer integrity or presence of atherosclerosis. Our study objective was to assess the effects of phenylephrine-induced alpha-adrenergic stimulation on coronary vasomotion in heart transplant recipients with and without graft atherosclerosis.

METHODS

Intracoronary phenylephrine (alpha(1)-selective agonist) was injected in 6 control subjects, 9 recipients with angiographically normal coronary arteries and 8 recipients with mild or moderate atherosclerosis. Coronary flow velocity was measured using a Doppler guide-wire. The diameters of 3 epicardial segments of the left coronary artery and coronary blood flow and resistance were assessed at baseline, after infusion of increasing acetylcholine doses (10(-7) and 10(-6) mol/liter) and after phenylephrine (150- to 200-microg bolus). Systemic and coronary hemodynamic parameters were measured immediately after acetylcholine and 1, 3, 5, 7, 10 and 15 minutes after phenylephrine.

RESULTS

Phenylephrine induced similar significant increases in rate pressure product in the 3 groups. Acetylcholine induced epicardial vasodilation in controls and vasoconstriction in transplant recipients. Phenylephrine induced epicardial vasodilation in controls and in angiographically normal recipients; subsequent vasoconstriction occurred in this last group. In the recipients with angiographic abnormalities, sustained vasoconstriction occurred. At peak phenylephrine effect, coronary blood flow (CBF) increased significantly (p < 0.001 vs baseline) in all 3 groups. Coronary resistance decreased in the 3 groups but the decrease was smaller in the recipients with angiographic abnormalities (p < 0.05 vs controls).

CONCLUSIONS

In heart transplant patients, graft atherosclerosis unmasks the direct coronary vasoconstricting effects of pharmacologic alpha-adrenergic stimulation.

Impaired cardiac contraction and relaxation and decreased expression of sarcoplasmic Ca2+-ATPase in mice lacking the CREM gene

Congestive heart failure is the common endpoint of various cardiac diseases representing a leading cause of cardiovascular mortality in Western countries. Characteristic functional alterations of the failing heart are explained by expressional changes of myocardial regulatory proteins; however, little is known about underlying mechanisms regulating cardiac gene expression in the failing heart. Here, we address the specific role of transcription factor CREM for cardiac function in CREM mutant mice with complete inactivation of the CREM gene. We show that CREM mutant mice display distinct alterations of cardiac function resembling characteristic functional defects of the failing heart. Left ventricular hemodynamic assessment of CREM mutant mice revealed impairment of both cardiac contraction and relaxation in basal state, as well as a decreased responsiveness to beta-adrenergic stimulation. The diminished cardiac contractile performance was associated with a selective down-regulation of beta1-adrenergic receptors and a decreased ventricular expression of SERCA, the Ca2+-ATPase of the sarcoplasmic reticulum. The cardiac phenotype of CREM mutant mice provides the first evidence that CREM represents an important key regulator of cardiac gene expression, which is essential for normal left ventricular contractile performance and response to beta-adrenoreceptor stimulation.

Real-time RT-PCR for the detection of beta-adrenoceptor messenger RNAs in small human endomyocardial biopsies

Quantification of mRNAs from extremely small human samples remains a challenge. Requiring minimal amounts of tissue and no post-reaction manipulation, real-time reverse transcriptase-polymerase chain reaction (RT-PCR) is an attractive method to quantitatively assess the expression of rare mRNAs. We evaluated the applicability of the technique on RNA extracted from human endomyocardial biopsies and isolated cardiomyocytes, and compared the technique to the RT-competitive PCR approach. Primers and probes were designed to amplify the three subtypes of human beta -adrenoceptors (beta1-, beta2- and beta3 AR), as well as reference genes such as glyceraldehyde-3-phosphate dehydrogenase (GAPDH), Hypoxanthine-guanine phosphoribosyltransferase (HPRT), and the oncogene ABL by real-time RT-PCR. Specific primers and a deleted competitor were synthetized to compare the quantitation of the beta 3 AR mRNA expression by RT-competitive PCR. We validated the technique on human cardiomyocytes either freshly isolated or selectively excised from fixed sections of human myocardium by Laser Capture Microdissection. The standard curves obtained for the cDNA's analysed showed mean slopes comprised between -3.3 and -3.7. Inter- and intra-assay variability of gene quantitation was reflected by mean values of the variance coefficients of Ct of 4.84+/-1.13% and 2.73+/-0.39% or 3.32+/-1.03% and 2.21+/-0.24% (corresponding to percent variances of copy numbers of 83.07+/-12.72% and 34.45+/-9.03% or 47.40+/-8.59% and 23.83+/-3.16%) for human beta3 AR and GAPDH genes, respectively. The expression of GAPDH, HPRT and ABL mRNA was characterized by a very low dispersion of individual values across cardiac pathologies, suggesting that these genes may be used as reference genes in quantitative PCR studies. Finally, we applied the technique to detect rare mRNAs, such as beta -AR mRNAs, from small human endomyocardial biopsies and even isolated cardiomyocytes. Real-time RT-PCR is appropriate to quantitate rare messenger RNAs, including in extremely small human tissue samples. This method appears very promising for futures studies of gene expression in several pathophysiological conditions, including heart failure.

Exaggerated chronotropic and energetic response to dobutamine after orthotopic cardiac transplantation

BACKGROUND

After heart transplantation, the transplanted denervated heart displays both an exaggerated chronotropic and an exaggerated inotropic response to circulating catecholamines. This study assessed whether denervated transplanted hearts also display an exaggerated energetic response when challenged with dobutamine.

METHODS AND RESULTS

A total of 18 heart transplant recipients and 14 normal volunteers underwent measurements of myocardial oxygen consumption (MVO2), external work (EW), and pressure-volume area (PVA), at rest and during infusion of dobutamine. At rest, calculated myocardial (PVA/MVO2) and mechanical (EW/MVO2) efficiencies were similar among transplant recipients and normal volunteers. During low-dose dobutamine infusion (8 microg/kg/min), transplant recipients exhibited a larger increase in heart rate (to 126 +/- 14 vs 87 +/- 26 beats/min, p < 0.001) and MVO2 (to 269 +/- 43 vs 233 +/- 19 J/min/100g, p < 0.05) and a smaller increase in EW (64 +/- 18 vs 72 +/- 13 J/min/100g, p < 0.05) and PVA (70 +/- 16 vs 81 +/- 13 J/min/100g, p < 0.05) than did normal volunteers. As a result, both myocardial (26 +/- 4 vs 35 +/- 4%, p < 0.05) and mechanical (23 +/- 4 vs 30 +/- 4%, p < 0.001) efficiencies were lower during dobutamine infusion in transplant recipients than in normal volunteers. During the infusion of a higher dose of dobutamine (19 microg/kg/min), the chronotropic and inotropic responses of heart transplant recipients were even more exaggerated. The fall in myocardial efficiency induced by dobutamine correlated with the increase in heart rate (r = -0.58) and could be reproduced in normal volunteers by coadministration of atropine.

CONCLUSIONS

Transplant recipients exhibit a larger fall in contractile efficiency and a larger oxygen-wasting effect during dobutamine infusion than do normal volunteers. Because normal volunteers pre-medicated with atropine presented with a similar increase in heart rate and a similar fall in efficiency, the exaggerated energetic response of transplanted hearts to dobutamine likely resulted from the same mechanisms as their chronotropic supersensitivity, i.e., the loss of inhibitory parasympathetic innervation.

Depletion of endogenous dopamine stores and shift in beta-adrenoceptor subtypes in cardiac tissue following five weeks of chronic denervation

Surgical ablation of extrinsic cardiac nerve fibers results in a chronically denervated state of the left ventricle of the heart. The present study was performed to elucidate the effect of a period of 5 weeks of chronic denervation on cardiac catecholamine levels in general and dopamine in particular. Moreover, the possible effect on cardiac beta-adrenoceptor subtypes was investigated. Experiments were performed on adult dogs. In addition to adrenaline and noradrenaline the tissue levels of dopamine were found to be severely depressed. A significant shift from beta1- to beta2-adrenoceptor subtype was observed, while the total beta-adrenoceptor density remained unaffected. The present findings indicate that catecholamine synthesis in chronically denervated hearts is impaired upstream of dopamine and that a shift in beta-adrenoceptor subtype occurs already within a relatively short period of five weeks of denervation, and suggest that the lack of endogenous catecholamines influence the relative expression levels of the two subtypes of beta-adrenoceptors present in cardiac tissue.

Functional role of beta2-adrenoceptors in the transplanted human heart

In the transplanted human heart, beta-adrenoceptor subtypes change with time after transplantation: beta1-adrenoceptors tend to decline, whereas beta2-adrenoceptors are upregulated. The aim of this study was to determine whether, in the transplanted human heart, stimulation of beta2-adrenoceptors can induce heart-rate increases. For this purpose, we assessed in eight heart-transplant recipients (mean posttransplant time: 932 days) the effects of infusion of graded doses of isoprenaline (3.5-35 ng/kg/min) 120 min after pretreatment with the beta1-adrenoceptor antagonist bisoprolol (10 mg p.o.; beta1-adrenoceptor occupancy approximately 80%; beta2-adrenoceptor occupancy <5%) on heart rate in the recipient's native (innervated) and transplanted (denervated) sinus nodes. Isoprenaline, acting under these conditions predominantly at beta2-adrenoceptors, increased heart rate both in the recipient's transplanted and native sinus nodes in a dose-dependent manner; at each dose, increases were significantly higher in the transplanted than in the native sinus node. ED20 values (dose to increase heart rate by 20 beats/min) in the transplanted sinus node were 22.2 +/- 1.8 ng/kg/min, and in the native, >35 ng/kg/min (p < 0.01). We conclude that in the transplanted human heart, beta2-adrenoceptor stimulation does evoke increases in heart rate. The enhanced response to isoprenaline in the transplanted sinus node could be caused by the upregulated beta2-adrenoceptors or by the fact that during isoprenaline infusion, vagal activity increases, thus blunting the response in the native (innervated) but not in the transplanted (denervated) sinus node.

Gas exchange and cardiovascular kinetics with different exercise protocols in heart transplant recipients

Metabolic and cardiovascular adjustments to various submaximal exercises were evaluated in 82 heart transplant recipients (HTR) and in 35 control subjects (C). HTR were tested 21.5 +/- 25.3 (SD) mo (range 1.0-137.1 mo) posttransplantation. Three protocols were used: protocol A consisted of 5 min of rectangular 50-W load repeated twice, 5 min apart [5 min rest, 5 min 50 W (Ex 1), 5 min recovery, 5 min 50 W (Ex 2)]; protocol B consisted of 5 min of rectangular load at 25, 50, or 75 W; protocol C consisted of 15 min of rectangular load at 25 W. Breath-by-breath pulmonary ventilation (VE), O2 uptake (VO2), and CO2 output (VCO2) were determined. During protocol A, beat-by-beat cardiac output (Q) was estimated by impedance cardiography. The half times (t1/2) of the on- and off-kinetics of the variables were calculated. In all protocols, t1/2 values for VO2 on-, VE on-, and VCO2 on-kinetics were higher (i.e., the kinetics were slower) in HTR than in C, independently of workload and of the time post-transplantation. Also, t1/2 Q on- was higher in HTR than in C. In protocol A, no significant difference of t1/2 VO2 on- was observed in HTR between Ex 1 (48 +/- 9 s) and Ex 2 (46 +/- 8 s), whereas t1/2 Q on- was higher during Ex 1 (55 +/- 24 s) than during Ex 2 (47 +/- 15 s). In all protocols and for all variables, the t1/2 off-values were higher in HTR than in C, In protocol C, no differences of steady-state VE, VO2, and VCO2 were observed in both groups between 5, 10, and 15 min of exercise. We conclude that 1) in HTR, a "priming" exercise, while effective in speeding up the adjustment of convective O2 flow to muscle fibers during a second on-transition, did not affect the VO2 on-kinetics, suggesting that the slower VO2 on- in HTR was attributable to peripheral (muscular) factors; 2) the dissociation between Q on- and VO2 on-kinetics in HTR indicates that an inertia of muscle metabolic machinery is the main factor dictating the VO2 on-kinetics; and 3) the VO2 off-kinetics was slower in HTR than in C, indicating a greater alactic O2 deficit in HTR and, therefore, a sluggish muscle VO2 adjustment.

Influence of post-surgery time after cardiac transplantation on exercise responses

Influence of post-surgery time after cardiac transplantation on exercise responses. Med. Sci. Sports Exerc., Vol. 28, No. 2, pp. 171-175, 1996. To test the hypothesis that exercise response changes with time after cardiac transplantation, we investigated the cardiorespiratory responses of nine orthotopic heart transplant patients (52.4 +/- 2 yr) during graded exercise tests (30 W.3 min-1) done at 1, 3, 6, 9 and 12 months post-surgery. At peak exercise, 1) oxygen uptake per kg of body weight (VO2), minute ventilation (VE) and oxygen pulse (O2 pulse) did not change significantly between 1 and 12 months postsurgery; 2) transplanted heart rate (HRt) and delta heart rate (peak exercise heart rate--resting heart rate) increased significantly over time (P < 0.01; P < 0.05) with a marked increase between 1 and 3 months (P < 0.05); and (3) a significant negative correlation existed between O2 pulse and HRt (r = -0.36, P < 0.05), whereas no correlation was found between delta heart rate and delta VO2 (peak exercise VO2- resting VO2, l.min-1). During submaximal exercise, HRt increased significantly over time (P < 0.001); VO2, VE, and O2 pulse showed no significant change; and the VO2-HRt relationship shifted toward higher values of HRt. We conclude that, in the absence of formal physical training, the exercise response of denervated transplanted heart increases in relation to post-surgery time but does not affect oxygen uptake at submaximal and peak levels of exercise.

Selective downregulation of rat cardiac beta 1-adrenoceptors by cyclosporine A: prevention by diltiazem or angiotensin-converting enzyme inhibitors

OBJECTIVES

This study attempted to determine whether long-term treatment with cyclosporine A in rats affects cardiac beta 1-adrenoceptors and whether this can be prevented by angiotensin-converting enzyme inhibitors or calcium-entry blocking agents.

BACKGROUND

In the transplanted human heart the density of beta 1-adrenoceptors decreases with time after transplantation, whereas that of beta 2-adrenoceptors does not. Because heart transplant recipients are treated with cyclosporine A, we studied whether administration of cyclosporine A in rats might cause this beta 1-adrenoceptor downregulation.

METHODS

We performed two studies. First, we treated groups of 10 male normotensive Wistar rats orally with 30 mg/kg body weight per day of cyclosporine A, 10 mg/kg per day of enalapril and 60 mg/kg per day of diltiazem, alone or in combination, for 6 weeks each. Second, we treated groups of 15 male normotensive Wistar rats orally with 15 mg/kg per day of cyclosporine A and 10 mg/kg per day of lisinopril, alone or in combination, for 6 weeks each. At the end of each treatment regimen, cardiac beta-adrenoceptor density and subtype distribution were assessed by (-)-[125I]iodocyanopindolol binding.

RESULTS

Both doses of cyclosporine A caused a significant decrease in cardiac beta 1-adrenoceptor density without affecting beta 2-adrenoceptor density. Although diltiazem and the angiotensin-converting enzyme inhibitors alone did not affect cardiac beta-adrenoceptors, they prevented the cyclosporine A-induced downregulation of beta 1-adrenoceptors.

CONCLUSIONS

In normotensive Wistar rats, cyclosporine A causes a significant decrease in cardiac beta 1-adrenoceptors without affecting beta 2-adrenoceptors. This can be prevented by diltiazem or angiotensin-converting enzyme inhibitors. In heart transplant recipients, who undergo long-term treatment with cyclosporine A, there is a very similar beta 1-adrenoceptor down-regulation with time after transplantation. Thus, administration of cyclosporine A may cause these beta-adrenoceptor subtype alterations.

Effect of chemical sympathectomy on (-)-isoprenaline-induced changes in cardiac beta-adrenoceptor subtypes in the guinea-pig and rat

1. Quantitative autoradiography was used to determine beta-adrenoceptor densities in cardiac regions of guinea-pigs and rats after chemical sympathectomy with 6-hydroxydopamine, and to examine how chemical sympathectomy affected beta-adrenoceptor changes following infusion of (-)-isoprenaline (400 micrograms kg-1 hr-1, 7 days). 2. Seven days after 6-hydroxydopamine (100 mg kg-1, i.v.), cardiac tissue levels of noradrenaline were reduced by 94.0 +/- 3.5% (guinea-pig) and 86.0 +/- 7.0% (rat). The blood pressure increase in rats to tyramine (0.5 mg, i.v.) was reduced from 118 mmHg in controls to 4.4 mmHg in 6-hydroxydopamine-treated animals. 3. There were no changes 7 and 14 days after 6-hydroxydopamine treatment in total, beta 1-and beta 2-adrenoceptor density in the atrioventricular conducting system and atrial and ventricular myocardium in both species. 4. In control animals, (-)-isoprenaline infusion produced selective reductions in beta 2-adrenoceptor density, whilst beta 1-adrenoceptor density remained unchanged. 5. In 6-hydroxydopamine treated guinea-pigs or rats, (-)-isoprenaline infusion caused no change in beta 1-adrenoceptors except in the right bundle branch whilst beta 2-adrenoceptors were reduced in the atrioventricular conducting system (atrioventricular node, bundle of His, right and left bundle branches) and myocardium (interventricular septum and atria). 6. The differential effect of (-)-isoprenaline on beta 1- and beta 2-adrenoceptors is not therefore due to the occupation of beta 1-adrenoceptors by noradrenaline or to prior down-regulation of beta 1-adrenoceptors by noradrenaline, since it persists after depletion of noradrenaline.

Cardiac beta-adrenoceptors and adenylyl cyclase activity in human left ventricular hypertrophy due to pressure overload

The effect of left ventricular hypertrophy (LVH) due to chronic pressure overload on right atrial (RA) and left ventricular (LV) myocardial beta-adrenergic receptor (beta-AR) density and subtypes, adenylyl cyclase (AC) activity and ADP-pertussis toxin ribosylated proteins was investigated in humans with LVH due to aortic stenosis and in patients without LVH undergoing heart surgery for mitral stenosis or coronary artery disease taken as controls. Both groups presented normal systolic function or plasma catecholamine levels. In LVH and controls, beta-AR density was similar in RA (62 +/- 6 vs 77 +/- 12 fmol.mg-1 protein) and LV (39 +/- 7 vs 32 +/- 2 fmol.mg-1 protein). In LVH, beta 1-AR percentage was < than in controls in LV (35 +/- 11 vs 73 +/- 5%, P < 0.05) but not in RA (79 +/- 5 vs 73 +/- 8%). Basal AC activity in RA (19 +/- 4 vs 21 +/- 6 pmol.mg-1 protein) and LV (22 +/- 5 vs 27 +/- 3 pmol.mg-1 protein) was similar in LVH and in controls. Isoprenaline-induced stimulation of AC in RA was similar in LVH and in controls (51 +/- 18 vs 36 +/- 18%) but < in LV of LVH (7 +/- 6 vs 45 +/- 6%, P < 0.05). In the presence of ICI-118,551 (a beta 2-adrenoceptor antagonist), isoprenaline failed to induce any increase in cAMP in LVH. The quantification of ADP-pertussis toxin ribosylated proteins indicated a lower concentration of substrates in LV myocardial membranes from LVH. These data indicate that in LVH due to pressure overload, there is a down-regulation of beta 1-AR and an increase in beta 2-AR density. This is associated with alterations of the transmembrane signalling marked by a decreased capacity of isoprenaline to stimulate AC and an impaired expression of Gi proteins.

Beta-adrenoceptors in cardiac disease

The human heart contains both beta 1 and beta 2-adrenoceptors; both mediate positive inotropic and chronotropic effects. In chronic heart failure, beta-adrenoceptor number is reduced, presumably, by down-regulation by endogenous noradrenaline which is elevated due to increased sympathetic activity. Since the human heart contains only a few spare receptors for beta-adrenoceptor-mediated positive inotropic effects and the amount of spare receptors declines in chronic heart failure, it is not surprising that the reduced beta-adrenoceptor number is accompanied by decreased contractile responses to beta-adrenoceptor agonists (including endogenous catecholamines), and the extent of decrease in maximal inotropic response is more pronounced as the disease becomes more advanced. Moreover, in chronic heart failure myocardial G(i)-protein, which inhibits cAMP formation, is increased, which might further contribute to the reduction in beta-adrenoceptor-mediated effects. It appears that, at present, the best therapy for severe heart failure is a successful heart transplant, since in the transplanted heart beta-adrenoceptor number and function seems to be normalized. Moreover, the data currently available do not suggest any development of super- or subsensitivity of postsynaptic cardiac beta-adrenoceptors in the transplanted human heart.