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European Heart Journal (1996) 17 {Supplement B), 8-16

Cardiac adrenergic receptor effects of carvedilol

T. Yoshikawa, J. D. Port, K. Asano, P. Chidiak, M. Bouvier, D. Dutcher,R. L. Roden, W. Minobe, K. D. Tremmel and M. R. Bristow

Division of Cardiology, The Temple Hoyne Buell Heart Research Laboratories, University of Colorado HealthSciences Center, U.S.A.

Carvedilol is an adrenoceptor antagonist which modulatesthe activity not only of P, and P2 but also of a, adrenergicreceptors present on the cell surface membrane of thehuman cardiac myocyte. In the heart, carvedilol has ap-proximately 7 times higher potency for p, and p^ adreno-ceptors, but in the doses 50-100 mg . day" ' used in clinicalpractice, it is essentially non-selective. In human myocardialpreparations and in cultured heart cells, carvedilol has nointrinsic sympathomimetic activity but is able to identifyhigh affinity agonist-binding receptors whose pharmaco-logical signature is reduction in binding by incubation withguanine nucleotides (guanine nucleotide-modulatable bind-ing). This property is more prominent for the human P2

than for the P, adrenoceptor. The property of gauninenucleotide-modulatable binding for carvedilol and struc-turally related bucindolol correlates with their ability

to directly down-regulate P,-like receptors present in cul-tured chick myocytes, and with a lack of reversal ofdown-regulation of cardiac P-receptors in patients withheart failure. Carvedilol does not exhibit high levels ofinverse agonist activity, which may contribute to its goodtolerability in subjects with heart failure.

These data indicate that carvedilol produces a high degreeof adrenergic receptor blockade in the failing humanheart, and does not re-sensitize the P-receptor pathway tostimulation by adrenergic agonists.(Eur Heart J (1996) 17 (Suppl B): 8-16)

Key Words: Cardiac adrenoceptors, ^-blockade,a-blockade, carvedilol, heart failure.

Introduction

The contractile function of human cardiac myocytes isdependent on two mechanisms (Fig. 1). Intrinsic con-tractile function, expressed by the Frank-Starling rela-tionship, accounts for the ability of the cardiac myocyteto respond to increased stretch by increased power ofcontraction and is utilized in the normal heart to main-tain pump performance at rest. In addition, the heartpossesses the ability to increase or decrease its functionsubstantially and rapidly. In the normal heart, cardiacoutput can be increased by 2-10 fold within seconds tomeet the circulatory demands of increased activity111.These changes in function are accomplished by mech-anisms which may be categorized as those subservingmodulated cardiac function12^. Under normal physio-logical conditions the role of these supportive mech-anisms is to allow cardiac pumping performance to meetthe circulatory demands of increased activity.

When the heart begins to fail, the modulatedfunction mechanisms are utilized to increase output bothby increasing heart rate and contractility. The most

Correspondence: Michael R. Bristow, MD, PhD, University ofColorado, Health Sciences Center, Cardiology, B-139, 4200 EastNinth Avenue, Denver, Colorado 80262, U.S A.

important of these mechanisms responsible for thestimulation of cardiac function are the adrenergicpathways. There are two ^-adrenergic receptorsubtypes—fii and fi2 — coupled by the stimulatorygaunine nucleotide-binding protein (Gs) to the effector

Cardiac insult

Intrinsic(resting)function

Modulated(stimulation)

function

Activation ofcompensatorymechanisms

* 1. Prank-Sterling mechanism2. Adrenergic stimulation3. Hypertrophy

Figure 1 Hypothetical model of the development ofmyocardial failure.

0195-668X/96/0BOO08 + O9 $18.00/0 1996 The European Society of Cardiology

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Cardiac receptors and carvedilol 9

I Ang-II

fay U ATX

Ang-II

ATP cAMP PK-C

Figure 2 Schematic representation of four seven-membrane spanning G-proteincoupled receptors and their relationship to adrenergic neurons.

enzyme adenylyl cyclase (AC) on the cell surface mem-brane of human myocardial cells (Fig. 2). When anagonist binds to /?, or /?2-receptors, the a subunit ofG, (aG,) increases its binding affinity for GTP, whichthen binds GTP preferentially to GDP. LigandedaG5 (aGs • GTP) is a powerful stimulus for the activa-tion of AC, which generates cyclic AMP from ATP.Cyclic AMP exerts positive inotropic and chronotropicactivity by increasing the flux of calcium through sarco-lemmal slow Ca2+ channels and increasing Ca2+ uptakeand release by the cytoplasmic reticulum. In addition,/?, -adrenergic receptors are coupled through G5 to slowCa2+ channel influx by cyclic AMP-independent path-ways'31. When the heart begins to fail, these mechanismsare stimulated by increased cardiac adrenergic activity.This occurs as a consequence of increased sympatheticnerve activity, presynaptic facilitation of norepinephrinerelease and later by decreased neuronal norepinephrinereuptake. Increased circulating epinephrine also partici-pates in stimulation of cardiac /?-adrenergic receptors,particularly in the initial phases of heart failure141.Norepinephrine is 60 times more selective for humancardiac /?, than ft2 adrenoceptors, but epinephrine isnonselective15"71. This and other observations have led tothe concept that the /?, adrenoceptor subtype is theneurotransmitter (norepinephrine) receptor, while the /?2

subtype is the hormone (epinephrine) receptor.

Immediate stimulation of pump performance by/?-adrenergic mechanisms is subsequently aided by twoadditional means of stabilizing or increasing cardiacfunction, namely increased plasma volume producingan increase in preload, and hypertrophy of the cardiacmyocyte resulting in more contractile elements. Plasmavolume expansion results from endocrine and intrarenalmechanisms. Cardiac hypertrophy is produced by acombination of increased myocyte stretch, increasedneurotransmitter release, and a variety of autocrine,paracrine and hormonal activities which togetherenhance cardiac myocyte growth. The specialized

subcellular mechanisms mediating the induction andmaintenance of hypertrophy belong to the modulatedmechanistic influences shown in Fig. 1 and are themeans by which the myocyte can increase its contractilestate. These specialized growth-promoting mechanismsinclude but are not confined to the a, and ^-adrenergicreceptor pathways, the angiotensin IT (AT,) receptorpathway and the endothelin 1 (ET,) receptor pathway.The a,, AT, and ET, receptors are all coupled throughthe effector enzyme phospholipase C (PLC), as well asthrough other effector enzymes. The second messengersfor hypertrophy include diacyl glycerol-protein kinaseC, cyclic AMP-protein kinase A, Ca2+ and a variety ofkinase cascades which terminate in the production oftranscriptions factors.

Signalling of the three major means of increasingcardiac contractile function (/?-adrenergic stimulation,increased preload, and cardiac myocyte hypertrophy) islargely accomplished by simultaneous and co-regulatedactivation or induction of the adrenergic and renin-angiotensin systems (Fig. 3). The /?,, /?2 and a, adrenergicand the angiotensin II AT, receptors are all 7membrane-spanning proteins which form bindingpockets to trap agonists on the cell surface, and haveintra-membrane and intracellular portions to interactwith G proteins and various regulatory kinases. Thedensities of the four receptors varies greatly in humancardiac membranes, ranging in non-failing myocytesfrom 50-80 fmol. mg~' for the /?,-adrenergic to3-6 fmol .mg" 1 for the angiotensin II AT, receptor, ina rank order of /?, g>/?2>a ,>AT, (Fig. 4). The adenylylcyclase coupled receptors are relatively high densitywhile phospholipase C-coupled receptors are low den-sity, so that their detection in high yield, crude mem-brane fractions is technically difficult. Each of thesemodulated function receptors (MFRs) undergoes regu-latory changes in chronic myocardial failure, changesthat are indicative of exposure to elevated levels ofcognate agonist.

Eur Heart J, Vol. 17 Suppl B 1996



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10 T. Yoshikawa et al.

Adrenergic "•A \

-•• Renin-angiotensin

Directcardiotoxicity

'asoconstriction

Increasedheart rate and

contractility

IncreasedMVO2

Volumeoverloaded

. Increasedwall stress

Myocyte- • damage Hypertrophy •<-

Decreased contractility

Figure 3 Critical role of the co-activated/induced adrenergic andrenin-angiotensin systems in producing myocardial damage and de-creased intrinsic myocardial function in chronic heart failure.

(r\ = 2R NT 34 = 12 NF. 11 F)

AT,

'n = 16 NF, 27 F)

Figure 4 Receptor densities for four key seven-membrane spanning G-protein coupledreceptors in crude membrane preparations from non-failing ( • ) and failing (D) human leftventricles. Failing left ventricles were taken from Class ITI-IV heart failure patients withidiopathic dilated cardiomyopathy who were not being supported by intravenous inotropesor mechanical assist devices. The mean age in non-failing hearts was 36-5 ± 3-2 years, andin failing hearts 371 ±2-5 years (/>=NS). P<005 vs non-failing.

Regulatory changes in modulatedfunction receptors in failing human

ventricular myocardiumIn the failing ventricular myocardium, the /?,-adrenergic18'91 and angiotensin II AT, receptors1'01 bothexhibit down-regulation or loss of receptor protein fromall identifiable cellular pools (Fig. 4; Table 1). For boththe /?, adrenergic and AT, angiotensin II receptors, thisappears to be due to a reduction in the steady-state

abundance of mRNA[12]. In ischaemic cardiomyopathy,/?, receptors may also be partially uncoupled frompharmacological response. p2 receptors are not down-regulated in the failing human heart but are weaklyuncoupled from pharmacological response'141. In thefailing ventricle, at adrenergic receptors are only slightlyup-regulated16'15'161, and are partially uncoupled frompharmacological response1'71.

The variety of additional changes have beendescribed in G proteins, regulatory kinases and adenylyl

Eur Heart J, Vol. 17 Suppl B 19%



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Table 1 Adrenergic and angiotensin II signal transduction changes in failing humanventricular myocardium

ConstituentDegree of change 0-3 +

IDC (LV, RV) ISC (LV, RV) PPH (RV only)

1. j ! , AR density2. p2 AR coupling3. /?, AR coupling4. G, function5. AC catalytic unit

6. jJARK.,7. Ang II AT, R density

11I

NSC

TLV, NSC

|RV

TU

IU1T

NSC

—

cyclase. Several laboratories have identified an up-regulation in the functional activity118"201 or amount'21221

of the inhibitory protein G,, which transduces the signalsof the M2 muscarinic, A, adenosine or somatostatinreceptor pathways for inhibition of adenylyl cyclaseactivity. No changes have been identified in the asubunit of G5 in the failing heart, although its activity isdecreased by age1231. The activity and amount of /?ARK,an agonist-activated receptor kinase, is increased in thefailing heart'11]. Finally, the activity and the amount ofadenylyl cyclase is decreased in pressure overload fail-ure, but not in the volume overloaded left ventricle120-241.These changes in MFR withdraw the cardiac myocytefrom chronic stimulation by the adrenergic and renin-angiotensin systems and are the equivalent of producingincomplete adrenergic blockade with a partial agonist.Since regulatory changes only account for loss of 50-60% of total /^-receptor pathway activity in advancedheart failure, the cardiac myocyte remains exposed tosome adrenergic stimulation. Chronic stimulation con-tinues through these same pathways due to increasedagonist exposure so that these regulatory changes resultin the compromise of modulated/stimulated function ofthe same systems. Thus the prime functional activity ofthese systems is compromised, while the adverse effectsremain.

An important point to emphasize is the differ-ence in adrenergic receptor distribution in failing ascompared to that in non-failing myocardium. This maybe demonstrated by comparing the relative subtypepercentages of /?,, /?2 and a, receptors in non-failing andfailing ventricular myocardium taken from subjects withidiopathic dilated cardiomyopathy compared to non-failing organ donors. The non-failing myocardium isdominated by the /?, receptor subtype, whereas failingmyocardium exhibits a mixture of receptor subtypeswith the fi2 and ax receptor comprising approximately50% of the total population (Fig. 5). In the failing heart,the /?2 receptor represents 35%-40% of the totalpreceptor population (Fig. 6). These data would suggestthat /?,-selective blocking agents may have inherentlimitations in their ability to inhibit the adverse biologi-cal effects of elevated cardiac adrenergic drive in thefailing human heart.

Figure 5 Adrenergic receptor percentages in non-failing( • ) and failing ( Z ) human heart, same subjects as inFig. 4. *P<0-05 vs non-failing.

B,

Figure 6 ^-adrenergic receptor percentages in non-failing ( • ) and failing (D) human heart, same subjects asin Fig. 4. */J<0-001 vs non-failing.

Pharmacology of carvedilol in humancardiac and model systems

Examination of adrenergic receptor subtypeselectivity

Previous studies in human ventricular myocardial andlymphocyte membranes have suggested that carvedilolhas a relatively small degree of /?, selectivity. Computermodelling of [125I]-ICYP-CGP20712A competitioncurves generated in mixed receptor populations in




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ICYP-metoprolol +/- GPPNHP

ICYP-bucindolol +/- GPPNHP

ICYP-carvedilol +/- GPPNHP

• ..... J J miij A J J

-11 -10 - 9 - 8 - 7 - 6 -5

ICYP-xamoterol +/- GPPNHP

-11 -10 - 9 - 8 - 7Log [ligand]

-11 -10 -9 -8 -7

Log [ligand]

- 5 - 4

Figure 7 Competition binding curves between [I25I|-ICYP and cold /? receptor ligands incrude membrane fractions of human left ventricle, x =in the absence of Gpp(NH)p;O =in the presence o f 3 x l O ~ s M o f Gpp(NH)p.

Table 2 Selectivity for human fi, vs. p2 receptors (Kj or Kg, nM ± SEM), in presence of Gpp(NH)p

Compound

Carvedilol(n = 5-8)

Bucindolol(n = 2-10)

Metoprolol(n = 5)

Bisoprolol(n = 3)

Propranolol(n=l-6)

Xamoterol(n = 5)

CGP20712A(n = 2-21)

ICYP competitioncurves, nonfailing

Ht vs lymph.

ft ft

4-5 ±1-2 49 ± 1 6

2-7 ±1-1 9-7 ±1-7

— —

— —

3-8 ± 1-9 12-6

_— —

— —

Cardiac ICYP competition Recombinantfunction assays curves, 1

Q Q Q

ft ft Pi

6-2 ± 2-2 36 ± 24 •

4-6 ±3-5 4-3 ±1-0 5-2 ±2-0

— — 48 ± 22

— — 70 ± 38

— — 4-4 ±1-5

— — 64 ± 32

— — 2-2 ±0-6

failing heart systems

ft ft ft

* 1-2 ±0-3 2-3 ±1-4

5-2 ±2-0 2-0 ±1-3 1-0 ±0-7

3777 ±1709 — —

7135 ±3383 — —

4-4± 1-5 — —

4412±1816 — —

1398 ±236 2 0 3333

Average

ft

4-0

3-6

50

69

41

64

21

values

ft

291

50

3825

7135

8-5

4412

2366

Selectivityft:ft

7-3

1-4

79

102

21

69

1126

•Competition curve data unreliable with < 10-fold selectivity.

human ventricular myocardium indicates a /?,:/?2 selec-tivity ratio of approximately 3000-fold. However, theinterpretation of these findings is complicated by theagonist binding properties of carvedilol which leads tocomplex binding curves (Fig. 7). There is also a lackof precision of computer modelling when binding sitesare relatively similar in affinity. However, assuming thatthe binding in the presence of guanine nucleotides(3 x 10~5M Gpp(NH)p) represents the true antagonist

binding affinity, a comparison of the binding propertiesof carvedilol in multiple human systems indicates thatthe racemic compound does possess some relative /?,receptor selectivity (Table 2). The degree of selectivityvaries from 11-fold using membranes from non-failingventricles, containing >80% /?, receptors compared tolymphocyte membranes containing 100% fi2 receptors,to 2-fold using recombinant human systems. Averagingthe dissociation constants across all types of assays




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Table 3 ft/ljija, receptor binding profile of carvedilol

Laboratory Species/tissue D , DM D , nM a,KD, nM

Sponer1251 Guinea pig heart, 5-7 371 6-5 — —trachea

Monopoli'261 Human LV, IMA 1-6 — — 2-3 1-4Bristol27 '281 Human LV 40 29 1 7-3 9-4 2-4

-0 .1 i i i inrl

-10 -3- 8 - 7 - 6 - 5 - 4

Logdigand)

Figure 8 Competition binding between |I25I]ICYP (ICYP) and the S and Risomers of carvedilol in human recombinant /?, and f}2 receptors, x =S isomer,p\ receptors; O = R isomer, p\ receptor, + = S isomer, /?, receptor; * = R isomer,/?, receptor. The respective K, (IIM) values are 1.1, 15.3, 0.40, and 26.1.

yields a 7-fold selectivity of carvedilol for /?] comparedto y92 receptors, indicating that the drug can be expectedto be non-selective in standard pharmacological doses(Table 2). This agrees with data generated in animalmodel systems, which indicate a 6-5-fold /?,:/?2 selec-tivity1251 (Table 3). This compares with bucindolol andpropranolol which are non-selective and metoprolol andbisoprolol which are highly y3, selective (Table 3).

Other studies'26"28' indicate that carvedilol is apotent antagonist of human a, receptors, with a flja^blocking ratio of approximately two (Table 3). Thisindicates that carvedilol is a high (nM) affinity competi-tive blocking agent for /?,, /?2 and a, receptors, with adescending rank order of potency of 1:2:7 for/?,, a, and/?2 adrenergic receptors respectively.

Guanine nucleotide modulatable binding

^-adrenergic receptor antagonists are capable of identi-fying a higher affinity binding state that is convertedto lower affinity by incubation with high concentra-tions of non-hydrolyzable guanine nucleotides such as

Gpp(NH)p[29>301. Initially, it was thought that antag-onists were not capable of identifying higher affinityagonist binding sites, namely that binding could not bealtered by incubation with guanine nucleotides129'301, butit is now clear that bucindolol and carvedilol do possess'guanine nucleotide modulatable binding''27'28'3'1. Thismay be observed in competition curves for [I25I]-ICYP;carvedilol, bucindolol and the partial agonist xamoterolare displaced to the right by incubation with Gpp(NH)p,compared to the absence of shift with metoprolol(Fig. 7).

Previous studies in myocardial membranesprepared from human left and right ventricles haveindicated that carvedilol possesses guanine nucleotide-modulatable binding (GNMB), so that the additionof non-hydrolyzable guanine nucleotides such asGpp(NH)p results in a reduction in its binding affinity.However, human myocardial membranes contain both/?, and /?2 receptors and carvedilol possesses a slightamount of /?, selectivity. Therefore, the resolution ofcarvedilol competition curves in human myocardialmembranes is complicated by two classes of receptors,either of which may exhibit GNMB. A further compli-




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10 x R :.Agonists

Antagonists(inverse agonism)

Figure 9 Concept of intrinsic activity of receptors, orthat a small percentage of receptors exists in an activestate (RA*) that is in an equilibrium with an inactive, muchmore abundant conformational state (R,). In this frame-work agonists lead to an increased percentage of receptorsin the activated state, while antagonists lead to an activa-tion of active state receptors. The latter property is calledinverse agonism.

cation is the racemic nature of carvedilol, where the Sisomer has higher affinity for both /?, and fi2 receptorsthan the R isomer. To clarify these issues, the GNMBproperties in cells expressing human recombinant /?, andfii receptors have been studied.

Experiments have been performed with bothisomers of carvedilol using hamster kidney tsAF8cells stably transfected with human ^ and y?2 cDNAs.These transfections result in high-density expression ofreceptors, 0-5-2 pmol. mg~ ' in the transfected cellscompared to <50 fmol. mg~ ' in wild-type cells. Rep-resentative competition curves between [I25I]-ICYPand the highly selective /?, -agonist CGP20712A givesimple (1 site fit) binding curves indicating they arehomogeneously /?, and /?2.

In these recombinant systems competition curvesbetween ICYP and the S and R isomers of carvedilolperformed in the presence of Gpp(NH)p demonstratedifferences in /3-adrenergic receptor binding affinities(Fig. 8). The binding curve for the y?, receptor and the Sisomer is to the left of the y?2-S isomer curve. In contrast,the R isomer of carvedilol demonstrates slight selectivityfor /?2 receptors. As a result of these different affinitiesfor /?, and y?2 receptors, the stereospecificity of the S vs Risomers of carvedilol is greater for /?, vs /?2 receptors,65-fold vs 14-fold. Additionally, in these recombinantsystems agonist binding of carvedilol is found only forthe P2 receptor and only with the S isomer.

Lack of intrinsic sympathomimetic activity

Despite the presence of GNMB no intrinsic sympatho-mimetic activity (ISA) has been found for carvedilol inchick heart cell membranes, human myocardium or inthe intact human heart. However, a small amount ofISA responsible for receptor down-regulation andGNMB may still be possible, even though previousstudies have been conducted in the presence of con-ditions which augment signal transduction, e.g. simul-taneous incubation with forskolin. A further screen forminute amounts of ISA will involve measuring cyclicAMP levels in intact cells using stably transfected CHOcells which contain a high signal-to-noise ratio andwhich generate large amounts of cyclic AMP in responseto yS-agonist.

Inverse agonist properties of carvedilol,bucindolol, metoprolol, propranolol and

xamoterol

Unoccupied adrenergic receptors may possess intrinsicactivity (Fig. 9). Agonists may, therefore, function bystimulating inactivated receptors, and antagonists byinactivating receptors which are in the active state,so-called 'inverse agonism'[32-331. Just as agonists differ intheir ability to activate inactivated receptors, rangingfrom partial to full agonists, antagonists also differ intheir abilities to inactivate active state receptors. The Sf9cell transfected with a baculovirus expression system,which exposes human /?, or y?2 receptors at ultra-highdensity (~ 10 pmol . mg~ ') furnishes a useful method ofscreening for inverse agonism as well as for smallamounts of intrinsic activity. The inhibition of cAMPgeneration in this system is a measure of inverseagonism, and this system has been utilized to comparethe inverse agonist properties of carvedilol, bucindolol,metoprolol, propranolol and xamoterol (Fig. 10a).

Using the maximum degree of inhibition, pro-pranolol and metoprolol have relatively large amountsof inverse agonist activity, compared to carvedilol,bucindolol and the partial agonist xamoterol. Using aconcentration 10 x K, for the /?2 receptor, the rank orderof inverse agonist was metoprolol>propranolol ^carvedilol > xamoterol > bucindolol (10b). Thus it is tobe expected that the degree of inverse agonism of a/?-blocking drug will correlate with its negative inotropicand chronotropic properties when sympathetic activityis low or when receptors are unoccupied[34l

Effects of carvedilol on ft-adrenergic receptordensity and mRNA abundance in cultured

ventricular myocytes

The comparative effects of various /^-blocking drugs on^-receptor density in the chick heart cell membrane havebeen studied (Fig. 11). Carvedilol markedly down-regulated the chick heart cell /9,-like receptors, whichraises the question of whether carvedilol destabilizesreceptor mRNA, as do /?-agonists[351. However, therewas no reduction and possibly a slight increase iny?, -receptor mRNA abundance after exposure tocarvedilol.

Clinical pharmacologic relevance of theadrenergic receptor properties of carvedilol

The increased adrenergic drive in heart failure maymediate adverse myocardial effects through threeseparate signal transduction systems, the /?,, y?2 and a,adrenergic receptor pathways and, in commonly useddoses, carvedilol blocks all three receptors. This is incontrast to metoprolol and bisoprolol, which are highly




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40

^^ f

NE1E-6

Carv1E-6

Carv+ Prop

Buc1E-6

Metop Prop1E-5 1E-6

NoGNMB + GNMB Partialagonist

(b)65

60

0 55<

1 50

1 45

40 h

35

30

T

i

1I!

10"7M 10~7M

Figure 11 The effect of 24 h of incubation on various^-receptor ligands on chick heart cell /?,-like receptors,with receptor density (Bmax) determined in crude mem-brane fractions. NE=norepinephrine, Prop=propranolol,Carv=carvedilol, Buc=bucindolol, and Metop=metoprolol. 1 E - 6 = 1 0 " 6 M .

observations). The lack of a marked degree of inverseagonism indicates that carvedilol will not produce exces-sive myocardial depression; myocardial depressionwhich may be present before the drug is given, canbe expected to be compensated for by its vasodilatoractivity.

In summary, carvedilol produces total adrenergicreceptor blockade in the failing human heart. Unlikewith metoprolol, /?, and /?2 adrenergic receptor pathwaysare not up-regulated or recoupled. In addition, unlikemetoprolol, carvedilol significantly lowers cardiacadrenergic activity, due to p2 receptor blockade1371. Asan anti-adrenergic drug, carvedilol is superior tometoprolol or bisoprolol, which may explain at least inpart the apparent difference in clinical results betweencarvedilol and the latter two /?rselective compounds.

Figure 10 'Inverse agonist' activity of four fi receptorantagonists and the partial agonist xamoterol in a bacu-lovirus (BV) expression system. Data are expressed as thepercent reduction in adenylyl cyclase (AC) activity aseither the maximal reduction (a) or the degree of reductionreferenced against concentrations that are approximately10 x the K, for human /?2 adrenergic receptors (AR) (b). In(a) GNMB = guanine nucleotide modulatable binding. 0 =metoprolol; D = propranolol; @= carvedilol; • = bucindolol;E3 = xamoterol; *P^Q-05 vs metoprolol.

/?,-selective and bucindolol, which is non-selective forP receptors and has no significant a,-blocking activity.In addition, carvedilol does not up-regulate down-regulated /?, receptors'361. This is in contrast to meto-prolol and bisoprolol, and in model systems the effect issimilar to that seen with bucindolol. The moderateamount of inverse agonism of carvedilol means that thedrug may give rise to bradycardia and attendant sideeffects'341, although the number of patients in clinicaltrials withdrawn for these symptoms or requiring apacemaker is relatively low (2%-4%, unpublished

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