CURRENT AWARENESS

Acknowledgements

The authors’ research

was suppolted by a

Baehrlnger-lngelhelm

fellowship and grants

from the Osterretchische

Natmnalbank and

Fends mrF~%derung der

Wlssenschaftllchen

Farschung

In mammals, sterol biosynthesis inhibitors impair cell growth pre- sumably because sterols are required for cell division26. More- over, regulators of sterol biosynthe- sis are involved in apoptotic cell death27, which explains the cyto- toxicity and immunosuppression elicited by sigma ligandW9. In cer- tain pathophysiological conditions such effects might be desirable. As briefly outlined, the brain depends on the de nova synthesis of choles- terol. Sigma ligands exert anti- ischaemic effects in animal models of strokeao; it is conceivable that inhi- bition of postsqualene sterol biosyn-

Quirion, R. et al. (1987) Trends Neurosci. 10, 444446 Chavkin, C. (1990) Trends Phurmncol. Sci. 11,213-215 Walker, J. M. ef aI. (1990) Pharttiacol. Rev. 42,355402 Itzhak, Y., ed. (1994) Sigma Receptors, Academic Press Su, T-P., London, E. D. and Jaffe, J. H. (1988) Science 240,219-221 Schwarz, S., Pohl, I’. and Zhou, G-Z. (1989) Science 246,1635-1637 Martin, W. R., Eades, C. E., Thompson, J.A., Huppler, R. E. and Gilbert, I’. E. (1976) J. PhurmncoL Exp. Ther. 197,517-532 Quirion, R. et RI. (1992) Trends Phurmacol. Sci. 13,8%36 de Costa, B. R. et RI. (1989) FEBS Lett. 251, 53-58 Moebius, F. F. et al. (1993) Mol. Dhnrmucof. 44,96&971 Hanner, M. et al. (1996) Proc. Nut/. Amd. Sci. U. S. A. 93,8072-8077 Silve, S. et al. (1996) Mol. Cell. Biol. 16, 2719-2727

729-737

Chemical names

AY99ti 1,4-bis(2-chlorobenzyl- aminomethyl)cyclohexane

SKF10047: N-allylnormetazocine

SR31747: cis-N-cyclohexyl-N-ethyl- 3-(3-chloro-4-cyclohexylphenyl) propen-2-ylarnine hydrochloride

Four f3-adrenoceptor subtypes in the Cardiac pl- and

mammalian heart P,-adrenoceptors

Activation of coexisting &- and l3,-adrenoceptors can modify car-

Albert0 J. Kaumann disc function in both beneficial and harmful ways. In the human heart,

A. J. Kaumann.

Senior Research

Officer.

The Babraham

Institute, Cambridge, UK CB24AT

Since its proposal in 1989, evidence adrenoceptors usually do. This has been accumulating for the exist- contrasts with recent reports for the ence and function of a third p- existence of /3,-adrenoceptors, in adrenoceptor that, besides coexist- the human ventricle, that couple ing with pr- and p,-adrenoceptors, to the Gi protein and mediate mediates cardiostimulation in sev- cardiodepressant effects. Here, the eral species, including man. This pharmacological properties of the atypical cardiac p-adrenoceptor can four P-adrenoceptor subtypes are couple to the G, protein/adenylate compared and their function cyclase pathway, as p,- and &- discussed.

both receptors mediate the increases in contractile force’-3 and sinoatrial rate” that occur, for example, in situ- ations of stress, but can also mediate arrhythmias that can be mimicked experimentallys. In human heart tis- sue& and myocytess, both &- and P,-adrenoceptors also accelerate myocardial relaxation, thereby improving diastolic function. In the heart of other species, however, only

70 TiPS - March 1997 (Vol. 18) 0 1997, Elsevier Science Ltd PII: SO165-6147(96)01033-J

fungicides of the morpholine class inhibit fungal growth by interfering with sterol C,-C, isomerization. The morpholine analogue amorolfine is already licensed for topical treat- ment of onychomycosis in man. Potent systemic antimycotics are urgently needed as substitutes for antifungal chemotherapeutics of the polyene (amphotericin B) and azole class (ketoconazole, fluconazole) as severe side-effects, or fungal resist- ance, limit their use. Selective inhibitors of fungal sterol C,-C, isomerases which do not interfere with human sterol biosynthesis can now be developed, employing sigma-ligand binding assays, in addition to recombinant fungal and human isoenzymes.

ders cholesterol biosynthesis in the

thesis can modulate neuronal or glial proliferation and block neuronal

brain and the growing family of

death programmes. Changes in the lipid composition of cerebral mem- branes are expected to alter neuronal

sigma receptors fascinating targets

excitability. Therefore, the long-term effects of the inhibition of post- squalene sterol biosynthesis remain

for future drug development.

to be investigated. It is conceivable that such effects could be exploited therapeutically to treat mental dis- orders such as psychosis. This ren-

Selected references

23 Janowski, B. A., Willy, I’ J., Devi, T. R., Falck, J. R. and Mangelsdorf, D. J. (1996) Nature 383,728731

13 Moebius, F. F. et ~2. (1996) Biochemistry 51, 16871-16878

14 Parks, L. W., Smith, S. J. and Crowley, J. H. (1995) Lipids 30,227-230

15 Zech, C., Staudinger, R., Muhlbacher, J. and GIossmann, H. (1991) Eur. I. i%rma- col. 208,119-130

16 Moebius, F. F., Burrows, G. G., Striessnig, J. and Glossmann, H. (1993) Mol. Pharmu- col. 43,139-148

17 Moebius, F. F. et al. (1994) /. Biol. Chem. 269, 29314-29320

18 Hanner, M. et ~1. (1995) 1. Biol. Ckmr. 270, 7551-7557

19 Silve, S. et al. (1996) I. Biol. C!irm. 271, 22434-22440

Physioi. 37,275308 20 Benveniste, P. (1986) Annu. Rezl. Phf

21 Lutjohann, D. et al. (1996) Proc. Nat/. Acad. Sci. LJ. S. A. 93,9799-9804

22 Clayton, I’., Mills, K., Keeling, J. and Fitzpatrick, D. (1996) Luncet 348,404

24 Sakai, J. et ul. (1996) Cell 85,1037-1046 25 Byskov, A. G. et al. (1995) Nature 374,

559-562 26 Cypriani, B., Tabacik, C. and Descomps, 8.

(1988) Biochim. Biophys. Act17 972,167-178 27 Wang, X. etul. (1996) EMBO]. 15,1012-1020 28 ViIner, B. J., de Costa, B. R. and Bowen,

W. D. (1995) 1. Neurosci. 15, 117-134 29 Casellas, I’. et n2. (1994) J. Neuroimmunol. 52,

195203 30 Ferris, C. D., Hirsch, D. J., Brooks, B. I’. and

Snyder, S. H. (1991) J. Neurocheni. 57,

CURRENT AWARENESS

Table 1. Comparison of agonist potencies (~0, values) and antagonist affinities (pK, values) for four cardiac p-adrenoceptor subtypes

Drug P*1’ P3‘1 Fourth p ‘? Refs

Agonists

(-)-Noradrenaline 6.7 ha, 6.0 hv -4.0 hv 6.0-5.0 hv ~5.7 ra? 1, 2; 2; 26; 36 (-)-Adrenaline 6.7 ha, 6.0 hv 6.9-7.5 ha, 6.3 hv ? ? 13, 2; 13, 2 (-)-lsoprenaline 7.5 hv 7.5 hv -6.0 hv 1 52: 52; 26 (-)-II0363 8.7 ha ~5.5 ha ? -7.5 ha 23 Zinterol ~5.7 ha 8.3 ha 1 1 1 Procaterol 5 7 ca, cv, ha 8.0-9.2 ca, cv, ha 1 ? 53 BRL37344 -5.3 ha 7.0 ha 8.7 hc ~4.7 ra 28; 28; 26; 36

CL31 6243 inactive ga ? 8.2 hv ~4.7 ra 54; 26; 36

SR58611 A ~4.5 ga ? 8.2 hv ~5.7 ra 55; 26, 36 (-)-CGPI 2177 inactive ca, cv inactive ca, cv 5.2 hv 7.3 ha, hv, ra 44; 44, 26: 35, 45, 36 (-)-Pindolol 9.1 ga 9.1 ga ? 7.0 ga 56 (+)-Cyanopindolol 7 ? ? 7 7-6.8 ra, ha 36; 38 (+)-lodocyanopindolol inactive ca, cv inactive ca, cv 1 ? 44

(+Carazolol Inactive ra 7 ? 6.3 ra 57

Antagonists (-)-Propanolol 8 5 ha 8.9 ha ? ~5.7 ra 1. 1. 36

(-)-Bupranolol 9.0 hv 9.0 hv 6.9 hv 7 3 ha, 6.8 ra 52; 52; 26; 35, 36 (-)-CGPI 2177 9 2-10.1, ca, cv 9.1-10.2 ca, cv ? 1 44 (-)-Timolol 9.4 ha 10.1 ha ? ? 58 (+)-Nadolol 8.5 haa 8.1 haa <4 7 hv 5.8b 26 CGP20712A 9 6 ca 5.4 ca ? 6.3-6.4 ra 9; 9, 36 ICI1 18551 7.1 ga 9.5 ga 7 ~5 5 ra 59; 59; 36 (-)-Pindolol 9.1 ha 9.6 ha ? ? 60 (+)-Cyanopindolol 1 ? ? 68ha 38 ($lodocyanopindoloI 10.8-l 0.1 ca, cv ? ? ? 44 (+)-Carazolol 9.9 ra ? ? ? 57 SR59230A 6.7-7.3 ga ? ? 5.1-5.4 ra 48; 36

T. receptor mediates agonrst-evoked cardtostimulation. 1, receptor mediates agonist-evoked cardiodepression (a) In the atrium, (v) m the ventrrcle of several specres: (hi human, (r) rat, (g) guinea-pig and (cl cat. Unpublished experiments of Tao Wang In the author’s laboratory. bUnpublished expenments of Sargero. Molenaar and Kaumann. There is scope for future experiments as shown by the question marks.

P,-but not P,-adrenoceptors hasten face density and intracellular mRNA relaxation”-12. Human cardiac PI- levels of PI-adrenoceptors are and P,-adrenoceptors are usually reducedr7J8, and P,-adrenoceptors coupled to adenylate cyclase appear to be somewhat uncoupled through the G, protein. Noradrena- from adenylate cyclaser6 without a line and adrenaline can induce detectable change in p,-adrenoceptor matching increases in CAMP levels, mRNA levels17J8. Chronic treatment CAMP-dependent protein kinase of patients with selective P,-adreno- (PKA) activity and contractility ceptor blockers increases human atria1 through both receptor populations, contractile responses mediated despite the lower density of p2- through p2-adrenoqtors13J9j*~, sero- adrenoceptors compared to p,- tonin (5-HTJ receptors21 and both adrenoceptors’“. The importance of histamine H, and H, receptors22; human cardiac p,-adrenoceptors in responses through p,-adrenoceptors the mediation of the cardiostimulant are only marginally increased1”J9”~. effects of catecholamines has been Biochemical responses to 5-HT attributed6 to their tighter coupling (cAMI’)21 and histamine [both CAMP to the GJadenylate cyclase system and protein kinase A (PKA) activitylz compared to that exhibited by PI- are also increased by chronic p- adrenoceptors1,2,6,1~16. In patients with adrenoceptor antagonist treatment. advanced heart failure, both the sur- Because chronic P,-adrenoceptor

antagonist treatment does not modify the contractile responses of atria to di- butyryl CAMP (Ref. 19) or of atria1 myocytes to forskoW1, it appears that potentiation occurs by cross-talk

between G protein-coupled receptors through mod&cation of G protein functionrQ4. In the case of hyperre- sponsiveness through histamine H, receptors, however, a mechanism also appears to exist which prevents CAMP hydrolysis by the inhibition of phos- phodiesterase Ill through cGMP gener- ated by the activation of H, receptors”.

Cardiac P,-adrenoceptors Using nanomolar concentrations

of p,-adrenoceptor-selective ag- onistWh such as BRL37344, CL316243 and SR58611A, it has recently been demonstrated, in

TIPS - March 1997 (Vol 18) 7 1

CURRENT AWARENESS

Q-6

NC H H

pindolol cyanopindolol

Cf-6

NC

carazolol iodocyanopindolol

CGP12177 R0363

Fig. 1. Compounds with agonistic activity at the fourth cardiac P-adrenoceptor. All are non- conventlonal partial agonists (i.e. possessing high affinity and blocking potency at both p,- and P,-adrenoceptors), except R0363, which is a p,-adrenoceptor-selective partial agonist.

human isolated ventricular tissue, B-adrenoceptor, is given in Table 1.

that B,-adrenoceptors mediate The cardiodepressant effects of the

cardiodepressant effects and short- B,-adrenoceptor-selective agonist,

ening of the action potential (Table BRL37344, were attenuated by per-

1). Isoprenaline also causes cardio- tussis toxin, suggesting that, in the

depressant effects when both B1- and human ventricle, B,-adrenoceptors

B,-adrenoceptors are blocked with can couple to the Gi protein26, a

nadoloW6, an antagonist with a low phenomenon previously observed

affinity for B,-adrenoceptorsz7. Mar- in adipocyteW0. B,-Adrenoceptor

ginal cardiodepression was also mRNA was detected in human ven-

observed with micromolar concen- tricular myocytes, consistent with

trations of noradrenaline in the pres- the expression of functional B3-

ence of nadolo126 (Table 1). The adrenoceptors26. (B,-Adrenoceptor

nonconventional partial agonist mRNA has previously been found in

CGP12177 (chemistry in Fig. 1) was a the human ventricle31 and atrium32

weak cardiodepressant (Table 1). but its localization in adipocytes had

As expected from an interaction with not been ruled out.) It has been

the B,-adrenoceptorza, the cardio- proposed that the cardiodepres-

depressant effects of BRL37344 were sant effects mediated through B3-

antagonized by bupranolol but not adrenoceptors may contribute to the

by nadoloP. A comparison of the impaired cardiac function seen in

pharmacological characteristics of patients with heart failure26. The

the cardiac B,-adrenoceptor with proposal is based on the relative lack

cardiac Bi- and B,-adrenoceptors, in of desensitization of B,-adrenocep- addition to an atypical fourth tors33, compared to desensitization

of Bi- and B,-adrenoceptors. The effect of the chronic treatment of patients with conventional B-adreno- ceptor antagonists on B,-adreno- ceptor-mediated cardiodepression is

unknown.

A fourth cardiac P-adrenoceptor

In addition to B1- and B,-adreno- ceptors, evidence is accumulating for a third B-adrenoceptor that also mediates cardiostimulation. Until recently, the name third cardiac B-adrenoceptor was used loosely for this atypical cardiostimulatory

B-adrenoceptor23J4-36. Recent evi- dence indicates that the atypical car-

diostimulatory B-adrenoceptor and the B,-adrenoceptor are distincW9, and to avoid confusion, the former is

now designated the fourth cardiac B-adrenoceptor. The existence of the fourth B-adrenoceptor was based on the cardiostimulant effects of nonconventional partial agonists, observed in vitro in several species”4.

Nonconventional partial agonists are blockers, with a high affinity for

both pi- and B,-adrenoceptors, which cause cardiostimulant effects

at concentrations considerably greater than those that antagonize the effects of catecholamines. Sev- eral, but not all, nonconventional

agonists are chemically related to the indoleamine, pindolol (Fig. 1). The cardiostimulant effects of non- conventional partial agonists are

resistant to antagonism by the

and B,-adrenoceptor blocker, k)-propanolol, but antagonized with moderate potency by (-)- bupranoloP4 which has a slightly higher affinity for B1- and B,-adreno- ceptors than (-)-propranoloP (Table

1). Evidence has also been provided for the existence of a fourth cardiac B-adrenoceptor population in the human atrium35,38,39. Furthermore, nonconventional partial agonists can cause tachycardia through the fourth cardiac B-adrenoceptor in vivo in the raP. In the rat, the maxi- mal sinoatrial tachycardia that can be mediated through this receptor is considerably more marked36 than the small tachycardia mediated

7 2 Tip.5 - March 1997 (Vol. 18)

CZIRRENT AWARENESS

a C

100 - 100 -

80 - 80 -

d

60 -

%

40 - r;'"

20 - 4 Q

I I 0-a -12 -11 -10 -9 -8 -7 -6 -5 -4 -10 -9 -8 -7 -6 -5 -4

Log [(-)-CGPl21771 (M) Log W-d (~1

b d

100 -

80 -

60 -

%

40 -

20 -

O- I -12 -11 -10 -9 -8 -7 -6 -5 -4

100 n rnmm

80

60

2::, -10 -9 -8 -7 -6 -5 -4

Log [(-)-CGPl21771 (M) Log [CL3162431 (M)

Fig. 2. Srmilarrties and differences between colonic p,-adrenoceptors and putative cardiac (3,.adrenoceptors 36 Upper panels heart, lower panels colon. a and b: affimty esttmates of (-)-CGP12177 for four rat (?-adrenoceptor populations. a: p,- and &adrenoceptor occupancy curves were calculated from the binding of [3HI(-)-CGP12177 to ventricular membrane+‘. Cardiostimulant concentration-effect curves of (-)-CGP12177, mediated through the putative P,-adrenoceptor. on spontaneously beating right atria (rate, open circles) and paced left atria (force, closed circles). b: Concentration-effect curve of (WGP12177 as a colonic relaxant (open squares); the (3,-adrenoceptor occupancy curve was calculated from the competitrve inhibition by the partial agonist I-)-CGP12177 of colonic relaxation evoked by &adrenoceptor agonists. c: Lack of cardiac agomst and blocking effects of the (3,-adrenoceptor- selective agonist CL316243 (closed squares); positive inotropic effects of (-)-CGP12177 in the absence (closed crrcles) and presence of 60 PM CL316243 (triangles). d: Colon relaxation by CL316243.

in vitro through &adrenoceptorsJr, (-)-CGP12177 and (t)-cyanopin- amounting to 60% and 15%, respec- dolol on the rat left atrium36 (Fig. 2, tively, of the maximum tachycardia Tables 1 and 2); (+)-hydroxybenzyl- mediated through p,-adrenoceptors. pindoloW@, (k)-iodocyanopin- Pindolol can cause tachycardia in doloP and (?)-CGP12177 (Ref. 44) on man42 and it has been suggested that feline right ventricular and left atria1 this may be mediated, at least in part, myocardium; and (-)-CGP12177 (Refs through the fourth cardiac P-adreno- 35,38,39) and (+)-cyanopindoloW9 cepto+. Nonconventional partial on the human right atrium. Recent agonists not only enhance the heart work has also identified positive rate but also increase contractile inotropic and lusitropic effects of (-)- force through the fourth cardiac CGP12177, resistant to blockade by p-adrenoceptor, as observed with (-)-propranolol, in human ventricular

preparationsP45. Additionally, (--)- CGP12177 elicits marked cardios- timulant effects in ferret heart prepa- rations (Lowe and Kaumann, unpublished), thus providing an excellent model for the cardiac

fourth P-adrenoceptor. When the existence of the fourth

cardiac p-adrenoceptor was pro- posed (at that time designated as a third cardiac P-adrenoceptor)g, some of its pharmacological properties resembled those of p,-adrenoceptors.

TIPS - March 1997 (Vol IX) 7 3

CURRENT AWARENESS

Table 2. Comparison of the pharmacology of the fourth cardiac P-adrenoceptor and the colonic P,-adrenoceptor of rat and man

Species Fourth cardiac P-adrenoceptor Colonic P,-adrenoceptor Refs

Rat

Non-conventional partial agonists

(-)-CGPI 2177 7.3-7.6 (pD,) 7.3 (piI,). 6.8-7.3 (pK,) 36 (+)-Cyanopindolol 7.1-7.7 (PO,) 7.0 (p&j, 7.6-7.7 (pK,) 36

Catecholamines (-)-Noradrenaline ~5.7 (pK,)? 7.2 (PD,) 36 (-)-R0363 ? 5.9 (PD,) 36

p,-adrenoceplor-selective agonists

BRL37344 inactive (5.2) 8.9 (PO,) 36 CL31 6243 inactive (4.2) 9.0-9.1 (pD,) 36 202079 inactive (4.2) 7.0 (P&I 36 SR5861 IA inactive (5.2) 7 6 (P&I 36

p,-adrenoceptor-selective antagonist

SR59230A 5.1-5.4 (pK,) 6.3-7.5 (pK,) 36

p,-adrenoceptor-selective antagonist

CGP20712A 6.3-6.4 (pK,) inactive (5.5) 36

Antagonists with high affinity for p,- and p,-adrenoceptors

(-)-Bupranolol 6.4-6.8 (pK,) 6.3 (P&I 36 (-I-Propranolol ~5.7 (PK,) 6.0 (p&i 36

Human

(4CGP12177 7.3 (PD,) 6.2 (~&)a 35,49

(-)-R0363 -7.5 (PO,) ? 23 (+)-Cyanopindolol 6.9 (PD,) ? 39

SR59230A inactive (5.2) 8.3 (PK,) 49,39

(-)-Bupranolol 7.3 (P&I ? 35

(-)-Propranolol inactive (6.7) ? 35

Concentrations are given in log form, tog of constants (pl(l are defined between parentheses K,. KP and KS are the equilibrium dissociation constant for an agonist, partial agonist and antagonist, respectively; when an antagonist was inactive, the highest concentration used is given between parentheses.

“(+CGP12177.

Nonconventional partial agonists cause agonist effects in fat cells28 and the guW6 (Table 2) mediated

through P,-adrenoceptors, as con- firmed with recombinant /3,-adreno-

ceptors transfected into host cells”,28”6. Agonist potencies of non-

conventional partial agonists are

similar for heart, gut28 (Table 2) and adipose tissue28 across species. For example, the cardiostimulant potency of (-)-CGP12177, through the fourth cardiac P-adrenoceptor, and the colon-relaxant potency of (-)- CGP12177, through the P,-adreno- ceptor, are quite similar (Fig. 2, Table 2). Both the fourth cardiac p- adrenoceptor and the p,-adrenoceptor are resistant to blockade by propra-

nolo128 (Tables 1 and 2). (-)-Bupra- nolo1 not only antagonizes the cardiac effects of nonconventional partial agonists, but also the effects of &- adrenoceptor-selective agonists on fat cells, host cells expressing recom- binant P,-adrenoceptors28,4 and iso-

lated human ventricular prepara- tion+ (Tables 1 and 2).

However, the differences between the fourth cardiac p-adrenoceptor and the P,-adrenoceptor greatly out- weigh the similarities. The following criteria are used to accept the in- volvement of p,-adrenoceptors in a physiological system2*,36: (1) the receptor should be selectively stimu- lated by P,-adrenoceptor-selective agonists; (2) the receptor should be

stimulated by nonconventional par-

tial agonists; (3) the receptor should be resistant to block by antagonists possessing only high affinity for PI- and p,-adrenoceptors; and (4) the receptor should be blocked by P,-adrenoceptor-selective antagon- ists. For agonist-evoked stimulation

of rat heart function, criteria (1) and (4) were not fulfilled”6,“7, while for agonist-induced relaxation of the rat colon36 all four criteria were fulfilled (Table Z), demonstrating that p3- adrenoceptors are active in the colon but not in the heart. p,-Adreno- ceptor-selective agonists failed to cause both cardiostimulant effects through the fourth p-adrenoceptor and blockade of the cardiostimulant

7 4 TiPS - March 1997 (Vol. 18)

CURRENT AWARENESS

effects of (-)-CGP12177 at concen-

trations exceeding 2.4-4.9 log units higher than those causing half-maxi- mal relaxation of the colon through

putative P,-adrenoceptors36. The &- adrenoceptor-selective antagonist,

SR59230A (Ref. 47), only caused marginal block of the cardiostimu- lant effects of (-)-CGP12177 at con- centrations 0.9-2.4 log units greater than those that antagonize the colon- relaxant effects of several agonists3h

(Table 2). In addition, the &- adrenoceptor-selective antagonist, (-)-CGP20712A, also has some affin- ity for the fourth P-adrenoceptor of

the rat atrium but not for the &- adrenoceptors of the rat colon36

(Table 2). Although only scant information

exists for human tissues, already some marked differences have been found between the fourth cardiac

P-adrenoceptor and colonic P,-adrenoceptors (Table 2). The nonconventional partial agonist, CGP32177, is more potent as a

cardiostimulant than as a colon re- laxant (Table 2). More importantly,

the p,-adrenoceptor-selective antag- onist, SR59230A (Refs 47, 481, does not antagonize the atria1 effects of (-)-CGP12177 (Ref. 39) at a concen-

tration 1000 times higher than its equilibrium dissociation constant, ob tained from antagonism for human

colonic P,-adrenoceptorsJ9 (Table 2). The P,-adrenoceptor-selective cat- echolamine (-)-R0363 (for chemis-

try, see Fig. l), not only increases human atria1 contractility through p,-adrenoceptors, but apparently also through the fourth P-adrenoceptor23 (Table 2). In addition, (-)-R0363 is also a partial agonist of rat colonic P,-adrenoceptors but its potency on this system is lower than on the human atrium21 (Table 2). In healthy volunteers (-)-isoprenaline causes positive inotropic and lusitropic

effects that are partially resistant to block by P,-selective atenolol and non-p, / &-selective nadolol, sug- gesting mediation through the fourth P-adrenoceptor50.

The fourth cardiac P-adrenocep- tor differs from cardiac P,-adreno- ceptors in two other improtant ways:

coupling and susceptibility to de- sensitization. The fourth cardiac p-adrenoceptor appears to be posi-

tively coupled to the G,/adenylate cyclase pathway in both the rat atrium51 and the human atrium3s,“9,“s

and ventricle39,45, while the human

ventricular P,-adrenoceptor appears to be coupled to the G, protein2h. Consistent with the participation of

a CAMP-dependent pathway, the positive inotropic effects of (-)- CGP12177 are potentiated by a nonselective phosphodiesterase in-

hibitor in both the rat51 and human atrium38139; the effects of (+-)- cyanopindolol are also potentiated

in the human atrium”Rs39. Moreover, (-)-CGP12177 increases CAMP levels39 and the activity of PKA (Ref. 51) through the fourth

P-adrenoceptor in the rat atrium. Matching positive chronotropic and

inotropic effects of (-)-CGP12177 (mediated through the fourth p-adrenoceptor) and (-)-isoprena-

line (mediated mainly through the P,-adrenoceptor) were associated with matching increases of PKA activitysl, suggesting, but not proving,

similar coupling of both receptors to the G,/adenylate cyclase cascade.

Three observations are consist- ent with activation of a CAMP-

dependent pathway through the fourth P-adrenoceptor: (1) in human atria1 and ventricular preparations

(-)-CGP12177 not only increases contractile force, but also hastens

relaxation, presumably through PKA- catalysed phosphorylation of pro- teins (phospholamban, troponin I) leading to cardiomyocyte relax- ation”9,45; (2) submicromolar concen- trations of (-)-CGP12177 increase the amplitude and usually shorten the duration of Ca2+ transients of atrials” and ventricular (Sarsero, Molenaar and Kaumann, unpublished) rat cardiomyocytes. Shortening of the duration of CaZ+ transients is again attributed to PKA-catalysed phos- phorylation of phospholamban and resultant stimulation of the sar- coplasmic reticulum Ca2+ ATPase. (3) (-)-CGP12177 causes arrhythmic Ca2+ transients in both the atria1 and ventricular cardiomyocytes of the

rat (ASPET Meeting 1997, San Diego,

USA). The arrhythmias are presum- ably due to PKA-dependent phos- phorylation of the L-type Ca?+ chan- nel and resultant Ca?+ overload.

What is the density of the fourth

cardiac P-adrenoceptor? The popu- lation of this receptor, labelled

with [3H](-)-CGP12177, appean, to be twice as high as that of p,- adrenoceptors (also assessed with [3H](-)-CGP12177) in rat atria1 mem-

branes (Sarsero, Molenaar and Kau- mann, unpublished). The binding

affinities of the nonconventional

partial agonists (-)-CGP12177, (-) - pindolol, (+_)-carazolol and (+)-

cyanopindolol matched their car- diostimulant potencies. A series of P,-adrenoceptor agonists did not

compete for binding at concentra- tions of up to 10000 times greater than those that activate p,-adreno-

ceptors. The antagonist SR59230A

failed to compete for binding at a concentration (1 FM) that blocks pi- adrenoceptors of the rat colonYi,.

However, (-)-bupranolol competed for the receptor with an affinity sim- ilar to its K, as an antagonist of

effects mediated through the fourth p-adrenoceptor. Furthermore, cate-

cholamines also competed for the receptor with the affinity rank order (-)-isoprenaline>(-)-noradrenaline>(-)- adrenaline >>(t)-isoprenaline. The

affinity profile and stereoselective interaction of catecholamines supports the concept that the fourth adreno-

ceptor is, indeed, a P-adrenoceptor. Unlike the human @,-adrenocep-

to+, the human atria1 fourth P-adrenoceptor appears to undergo some desensitization3”. The positive inotropic effect of (-)-CGP1217Y7, mediated through the fourth p-adrenoceptor in the human atrium, tends to fade ~10~1~3”. The (-)-CGP12177-evoked desensitiz- ation is partially prevented by (-)- bupranolol35, suggesting that it 11s mediated directly through the fourth cardiac P-adrenoceptor. Interest- ingly, chronic treatment of patients with P-adrenoceptor antagonists (usually selective for p,-adrenocep- tors) does not appear to modify the atria1 inotropic responsiveness I-o

7iPS - March 1997 (Vol 18) 7 5

CURRENT AWARENESS

(-)-CGP12177 (Ref. 35) while it does cause hyperresponsiveness of other G,-coupled receptorsl3,19,20-24.

Outlook There is now sufficient evidence

to consider the fourth cardiac P-adrenoceptor as pharmacologically distinct from the cardiac P,-adreno- ceptor. The coupling of the fourth car- diac p-adrenoceptor to the GJ adenylate cyclase cascade could fa- cilitate the cloning of its cDNA, due to possible sequence homology with other G,-coupled receptor proteins. Furthermore, it needs to be elucidated whether this receptor is not only expressed in different cardiac regions but also in extracardial tissues. To decide whether the fourth cardiac p- adrenoceptor is beneficial or harmful to heart function, several questions require investigation: (1) What is the density of the fourth p-adrenoceptor population in several regions of the human heart compared to the densi- ties of pl-, p2- and P,-adrenoceptors? (2) Is the receptor activated by nor- adrenaline and adrenaline? (3) Does the receptor mediate arrhythmias in the human heart? (4) What is the mechanism of desensitization of the fourth P-adrenoceptor? (5) Is the receptor phosphorylated, uncoupled and internalized?

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The author’s research IS

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Chemical names

BRL34377: (RR+SS)(‘&[2-[2-(3.

chlorophenyl)-2-hydroxy- ethyllphenoxyllacetic acid)

(-)-CGP12177: ((-)-4-(3+butyl- amino-2-hydroxypropoxy)benz- imidazol-2-one)

CGP20712A: 2-hydroxy-5[2-[[2- hydroxy-3-[4-[(l-methyl-b trifluoromethyl) lH-imidazole-2- yl]-phenoxy]propyl]amino] ethoxyl-benzamide

CL316243: (disodium (RR)-5[2-[2-(3-

chlorophenyl)-2-hydroxyethyl- amino]propyl]-1,3-benzodioxole- 2,2_dicarboxylate)

ICI118551: erythro-oL-1(7-methylin- dan-4-yloxy)-3-isopropylamino- butan-2-01

(-)-R0363: ((-)-l-(3,4-dimethoxy- phenethylamino)-3-(3,4-dihy- droxy)-2-propanol)oxalate)

SR58611A: (ethy1((7S)-7-[(2R)-2-(3- chlorophenyl)-2-hydroxyethyl- amino]-5,6,7,8_tetrahydron- aphtyl-2-yloxy]acetate)

SR59230A: (3-(2-ethylphenoxy)-l- [(ls)-1,2,3,4-tetrahydronapht-l- ylaminol-2s-2-propanol oxalate)

7 6 TiPS - March 1997 (Vol. 18)