Physiological effects of inverse agonists in transgenic mice with myocardial overexpression of the β2-adrenoceptor

Abstract

G-protein-coupled receptors are thought to have an inactive conformation (R), requiring an agonist-induced conformational change for receptor/G-protein coupling. But new evidence suggests a two-state model in which receptors are in equilibrium between the inactive conformation (R), and a spontaneously active conformation (R*) that can couple to G protein in the absence of ligand (Fig. 1). Classic agonists have a high affinity for R* and increase the concentration of R*, whereas inverse agonists have a high affinity for R and decrease the concentration of R*. Neutral competitive antagonists have equal affinity for R and R* and do not displace the equilibrium, but can competitively antagonize the effects both of agonists and of inverse agonists. The lack of suitable in vivo model systems has restricted the evidence for the existence of inverse agonists to computer simulations and in vitro systems. We have used a transgenic mouse model in which there is such marked myocardial overexpression of beta 2-adrenoceptors that a significant population of spontaneously activated receptor (R*) is present, inducing a maximal response without agonist. We show that the beta 2-adrenoceptor ligand ICI-118,551 functions as an inverse agonist, providing evidence supporting the existence of inverse agonists and validating the two-state model of G-protein-coupled receptor activation.

If channel inhibitor ivabradine lowers heart rate in mice with enhanced sympathoadrenergic activities

“…Animals of 3-4 months age had a genetic background of C57BLK/SJL crossing and were individually genotyped. The TG mice have a life-time tachycardia phenotype due to constitutively activated b 2 -adrenergic signalling, with a marked increase in receptor density (Bond et al, 1995;Du et al, 1996). The NTG and TG mice were housed one to three per cage, with a 12 h/12 h day/ night cycle, and with free access to water and chow diet.…”

Different intrinsic activities of bucindolol, carvedilol and metoprolol in human failing myocardium

“…Transgenic mice overexpressing the human b 2 -adrenoceptor (Bond et al, 1995) or a constitutively active mutant (CAM) adrenoceptor (Samana et al, 1993) have led to the expansion of the classical ternary complex model of receptor action (De Lean et al, 1980). The b-adrenoceptor exists in an equilibrium between an active (R*) and an inactive (R) conformation.…”

“…Some antagonists are able to stabilize the inactive conformation (R), thus decreasing the basal activity of the receptor. These antagonists are termed inverse agonists, as they exert a`negative' intrinsic activity, driving the equilibrium to the opposite direction than the agonist (Bond et al, 1995).…”

“…The intrinsic activity of several partial and full agonists was shown to correlate with the degree of receptor phosphorylation and in turn desensitization and downregulation (Benovic et al, 1988). In this light of receptor action of antagonists, many therapeutically used agents may need reclassi®cation (Bond et al, 1995). In particular, in failing human tissue, direct evaluation of intrinsic activity of relevant compounds has not been performed.…”

Constitutive activity of the M1–M4 subtypes of muscarinic receptors in transfected CHO cells and of muscarinic receptors in the heart cells revealed by negative antagonists

“…The muscarinic antagonists quinuclidinyl benzilate and AF-DX 116 had no effect in some cases and acted like atropine and NMS in others. We conclude that the M~-M 4 subtypes of muscarinic receptors are constitutively active in the CHO cell lines expressing them and in cardiomyocytes and that atropine and NMS act as negative antagonists on these receptor subtypes by stabilizing them in the inactive conformation.antagonists permitted the demonstration of the constitutive activity of ~ opioid receptors [7], c~2-adrenoceptors [9], fl2-adrenoceptors [10][11][12], 5-hydroxytryptaminezc receptors [13] and bradykinin receptors [14].The present report describes the effects of four muscarinic antagonists on two processes which are controlled by muscarinic receptors, namely the synthesis of cyclic AMP (normally inhibited by the M 2 and M 4 subtypes of muscarinic receptors) and the hydrolysis of phosphoinositides (normally activated by the Ml, M3 and M5 subtypes of muscarinic receptors -review [15]). No reports were available on the action of muscarinic antagonists applied alone on these processes by the time our experiments were started.…”