In Vitro Evidence That Carteolol Is a Nonconventional Partial Agonist of Guinea Pig Cardiac 1-Adrenoceptors: A Comparison with Xamoterol

The present study was designed to verify our previous hypothesis that carteolol, a 1/ 2-adrenoceptor-blocking agent, is a nonconventional partial agonist of cardiac 1-adrenoceptors. To this purpose, we characterized the effects of carteolol in guinea pig myocardial preparations and measured the affinities of carteolol for highand low-affinity sites of 1-adrenoceptors labeled by CGP12177 [( )4-(3-t-butylamino-2-hydroxypropoxy)-2-benzimidazol-2-one]. All experiments were performed in comparison with xamoterol, a cardioselective 1-adrenoceptor partial agonist. Both drugs caused cAMP-dependent positive inotropic and chronotropic effects, but carteolol was less effective and less potent than xamoterol, and its cardiac actions were not affected by conventional concentrations of the -blocker propranolol. Both carteolol and xamoterol antagonized the cardiac effects of isoprenaline, but although the antagonistic concentrations of xamoterol were almost equal to those producing cardiostimulation, the antagonistic concentrations of carteolol were 3 log units lower than those causing cardiostimulant effects. Both carteolol and xamoterol competed with ( )[H]CGP12177 for a high-affinity site of 1-adrenoceptors, but carteolol showed a higher affinity than xamoterol. Moreover, carteolol, unlike xamoterol, bound also to a low-affinity site of the receptors. The binding affinity constants of the drugs for the high-affinity site correlated well with the respective blocking potencies against isoprenaline, whereas the affinity constant of carteolol for the low-affinity site was well related to its agonist potency. In conclusion, our findings demonstrate that carteolol, unlike xamoterol, is a nonconventional partial agonist, which causes agonistic effects through interaction with the low-affinity propranolol-resistant site of 1-adrenoceptors and antagonistic actions through the high-affinity site of the same receptors. Carteolol is a 1/ 2-adrenoceptor-blocking agent (Yabuochi and Kinoshita, 1974; Chiba, 1979), currently used in the management of patients with cardiovascular and noncardiovascular (e.g., glaucoma) diseases. Like other -blockers, it may be classified as a partial agonist since it is endowed with intrinsic sympathomimetic activity (ISA) in several animal species, including humans (for review, see Odenthal, 1983; Frishman and Covey, 1990). Within the last 20 years, much evidence has been accumulated that partial agonists of the -adrenoceptors have two modes of action. Although conventional partial agonists produce stimulant effects at concentrations equal to those causing blockade of -adrenoceptors, other compounds, namely the nonconventional partial agonists, produce cardiostimulation at concentrations greater than those blocking the effects of catecholamines (Kaumann and Blinks, 1980; Kaumann, 1989; Takayanagi et al., 1989), their agonistic effects being resistant to blockade by the classic -antagonist propranolol (Sarsero et al., 1999). Currently, the behavior of these agents is ascribed to the existence of two different active sites or conformations of the 1-adrenoceptor: an H site through which the effects of catecholamines are blocked and an L site generally defined as the propranolol-resistant state of the 1-adrenoceptor (Bundkirchen et al., 2002) through which the stimulant effects of these partial agonists are mediated (Konkar et al., 2000; Granneman, 2001; Baker et al., 2003; Arch, 2004; Baker, 2005). Various compounds have been recently classified as nonconventional partial agonists of the This work was supported by grants from the Ministry of Instruction, University, and Research (Rome, Italy), and by Società Industria Farmaceutica Italiana S.p.A. (Catania, Italy). Article, publication date, and citation information can be found at http://jpet.aspetjournals.org. doi:10.1124/jpet.105.088963. ABBREVIATIONS: ISA, intrinsic sympathomimetic activity; H, high affinity; L, low affinity; ( )CGP12177, ( )4-(3-t-butylamino-2-hydroxypropoxy)2-benzimidazol-2-one; IBMX, 3-isobutyl-1-methylxanthine. 0022-3565/05/3153-1386–1395$20.00 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 315, No. 3 Copyright © 2005 by The American Society for Pharmacology and Experimental Therapeutics 88963/3064768 JPET 315:1386–1395, 2005 Printed in U.S.A. 1386 at A PE T Jornals on O cber 7, 2017 jpet.asjournals.org D ow nladed from 1-adrenoceptor (Sarsero et al., 1999; Bundkirchen et al., 2002; Lowe et al., 2002; Joseph et al., 2003). Among them, CGP12177 contains, like carteolol, an aryloxypropanolamine moiety (Fig. 1) (Sarsero et al., 1998, 1999; Konkar et al., 2000; Sarsero et al., 2003; Joseph et al., 2004). Therefore, we wondered whether carteolol also behaves as a nonconventional partial agonist of 1-adrenoceptors. We recently showed that, in rat-isolated myocardial preparations, the drug antagonized the effects of isoprenaline at concentrations 2 log units lower than those causing positive inotropic and chronotropic effects (Floreani et al., 2004). Moreover, cardiostimulation by carteolol was resistant to a concentration of propranolol (1 M) greater than that antagonizing the effects of catecholamines in the same preparations. Collectively, these findings indicate that carteolol behaves as a nonconventional partial agonist of rat 1-adrenoceptors. The aim of the present work was to verify our hypothesis that ISA of carteolol is mediated through the propranololresistant state of the 1-adrenoceptor. To this purpose, we characterized the effects of carteolol in myocardial preparations isolated from guinea pigs and measured the affinity of carteolol for the highand low-affinity sites of cardiac 1adrenoceptors labeled by ( )[H]CGP12177; xamoterol, a cardioselective 1-adrenoceptor partial agonist (Hattori et al., 1987; Hicks et al., 1987), was used as a reference drug. In our research, we used myocardial preparations obtained from reserpine-treated animals; the use of catecholamine-depleted tissues, to avoid the underlying sympathetic tone, is crucial because the ISA of -blockers critically depends on the activation state of the -adrenoceptors (Maack et al., 2003) and/or on the degree of the underlying sympathetic tone (Lipworth and Grove, 1997). Our results demonstrate, for the first time, that carteolol binds with different affinities to both the highand lowaffinity sites of 1-adrenoceptors present in guinea pig heart membranes. Moreover, here we show that carteolol behaves like a nonconventional partial agonist, eliciting agonistic responses through its interaction with the low-affinity propranolol-resistant site of 1-adrenoceptors while antagonizing the effects of catecholamines through its interaction with the high-affinity site. Therefore, carteolol markedly differs from xamoterol, which behaves as a conventional partial agonist. Materials and Methods