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Constitutive activity of the human beta(1)-adrenergic receptor in beta(1)-receptor transgenic mice
S Engelhardt1, Y Grimmer, G H Fan
1Institut für Pharmakologie, Universität Würzburg, Würzburg, Germany.
Abstract:
We tested the hypothesis that the human beta(1)-adrenergic receptor displays constitutive activity and that beta-adrenergic antagonists differ in their ability to modulate this constitutive activity. Transfection of the cDNAs of the human beta(1)- and beta(2)-adrenergic receptors into COS-7 cells caused increases in basal cAMP that were proportional to the receptor levels, thus demonstrating constitutive activity for both subtypes. At comparable receptor levels, the increase in basal cAMP was about 5-fold higher for the beta(2)- than for the beta(1)-subtype. As a model for enhanced beta-adrenergic signaling at the whole-organ level, we used transgenic mice with heart-specific overexpression of the human beta(1)-adrenergic receptor. In this model, the beta(1)-adrenergic receptor displayed constitutive activity as evidenced by a higher spontaneous beating rate of isolated right atria from beta(1)-transgenic versus wild-type mice. This difference was abolished by the addition of CGP20712A, demonstrating inverse agonist properties of this compound. We then tested whether various beta-adrenergic antagonists currently in clinical use for the treatment of heart failure differ in their ability to modulate constitutive activity of the cardiac beta(1)-adrenergic receptor. The beta(1)-selective antagonists metoprolol and bisoprolol showed significant inverse agonist activity at the beta(1)-adrenergic receptor. Carvedilol behaved as a neutral antagonist and xamoterol displayed marked partial agonist activity. We conclude that the human beta(1)-adrenergic receptor displays constitutive activity that is considerably lower than that of the beta(2)-subtype. beta-Adrenergic antagonists currently in clinical use differ in their ability to exert inverse agonist activity at the human beta(1)-adrenergic receptor, which may contribute to their therapeutic effects.
Insights
The human beta(1)-adrenergic receptor has inherent activity, and different beta-adrenergic antagonists can modulate this. This constitutive activity, lower than beta(2), impacts heart failure drug efficacy.
Area of Science:
- Pharmacology
- Molecular Biology
- Cardiovascular Research
Background:
- The human beta(1)-adrenergic receptor (β1-AR) and beta(2)-adrenergic receptor (β2-AR) are key in cardiovascular regulation.
- Constitutive activity of receptors, independent of ligand binding, is increasingly recognized.
- Understanding β1-AR constitutive activity is crucial for cardiovascular drug development.
Purpose of the Study:
- To investigate the constitutive activity of the human β1-AR and β2-AR.
- To determine if clinically used β-adrenergic antagonists differentially modulate β1-AR constitutive activity.
- To explore the implications of β1-AR constitutive activity for heart failure treatment.
Main Methods:
- Transfection of human β1-AR and β2-AR cDNAs into COS-7 cells to assess basal cyclic AMP (cAMP) levels.
- Utilizing transgenic mice with heart-specific overexpression of human β1-AR to model enhanced cardiac signaling.
- Testing the effects of various β-adrenergic antagonists (metoprolol, bisoprolol, carvedilol, xamoterol) on isolated atrial preparations and cAMP assays.
Main Results:
- Both human β1-AR and β2-AR subtypes exhibit constitutive activity, evidenced by increased basal cAMP levels.
- β2-AR showed approximately 5-fold higher basal cAMP than β1-AR at comparable receptor levels.
- In a heart-specific β1-AR overexpression model, constitutive activity was confirmed by increased spontaneous beating rate, modulated by antagonists like CGP20712A (inverse agonist).
- Clinically used β1-selective antagonists (metoprolol, bisoprolol) demonstrated inverse agonist activity, carvedilol acted as a neutral antagonist, and xamoterol showed partial agonist activity at the β1-AR.
Conclusions:
- The human β1-AR possesses constitutive activity, albeit lower than the β2-AR.
- Existing β-adrenergic antagonists exhibit varied abilities to modulate β1-AR constitutive activity.
- Differential modulation of β1-AR constitutive activity by antagonists may contribute to their therapeutic effects in heart failure.