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Beta4-adrenoceptors are more effective than beta1-adrenoceptors in mediating arrhythmic Ca2+ transients in mouse
N S Freestone1, J F Heubach, E Wettwer
1Laboratory of Molecular Signalling, The Babraham Institute, Cambridge, UK.
Abstract:
Putative beta4-adrenoceptors mediate cardiostimulation and arrhythmias in mammalian heart. Both beta1- and putative beta4-adrenoceptors mediate arrhythmias but through different mechanisms. To elucidate further the mechanisms of cardiostimulation and arrhythmias we measured Ca2+ transients and L-type Ca2+ currents in mouse ventricular myocytes. We used (-)-CGP 12177, an antagonist of beta1- and beta2-adrenoceptors with agonist properties at the putative beta4-adrenoceptor, and (-)-isoprenaline as an agonist for beta1- and beta2-adrenoceptors. (-)-CGP 12177 increased Ca2+ transients in electrically stimulated cells loaded with Indo-1. The maximum increase of Ca2+ transients caused by (-)-CGP 12177 amounted to approximately one-third of that caused by maximally effective (-)-isoprenaline concentrations. Both (-)-CGP 12177 and (-)-isoprenaline caused concentration-dependent arrhythmic Ca2+ transients. The arrhythmias appeared at paced Ca2+ transients and between paced Ca2+ transients. The arrhythmic potency of (-)-CGP 12177 (-logEC50=9.4) was approximately 40 times greater than that of (-)-isoprenaline (-logEC50=7.8). L-type Ca2+ current was measured in the whole cell configuration of the patch clamp technique. In the presence of both 3-isobutyl 1-methylxanthine (6 micromol/l) and (-)-propranolol (500 nmol/l), (-)-CGP 12177 (100 nmol/l) increased significantly L-type Ca2+ current by 19% of the effect of (-)-isoprenaline. The (-)-CGP 12177-evoked increase of Ca2+ transients contrasts with the smaller effects on L-type Ca2+ current, suggesting that activation of the putative beta4-adrenoceptor causes a more efficient Ca2+-induced Ca2+ release than activation of the beta1-adrenoceptor. Beta4-Adrenoceptors mediate arrhythmias with smaller Ca2+ transients and smaller increases of L-type Ca2+ current than beta1-adrenoceptors, in line with different but still unknown mechanisms as previously suggested for the intact heart.
Insights
The study reveals that beta4-adrenoceptors mediate cardiac arrhythmias and calcium transients more potently than beta1-adrenoceptors. This suggests distinct mechanisms for beta4-adrenoceptor-induced arrhythmias, involving more efficient calcium release.
Area of Science:
- Cardiology
- Pharmacology
- Molecular Biology
Background:
- Beta-adrenoceptors play crucial roles in cardiac function, with beta1 and beta2 subtypes being well-characterized.
- The existence and function of putative beta4-adrenoceptors in the mammalian heart are less understood but implicated in cardiostimulation and arrhythmias.
Purpose of the Study:
- To investigate the specific roles of beta4-adrenoceptors in mediating cardiac calcium (Ca2+) transients and L-type Ca2+ currents.
- To compare the arrhythmogenic potency and underlying mechanisms of beta4-adrenoceptor activation versus beta1-adrenoceptor activation.
Main Methods:
- Utilized mouse ventricular myocytes to measure Ca2+ transients using Indo-1 fluorescence and L-type Ca2+ currents via patch-clamp electrophysiology.
- Employed specific pharmacological agents: (-)-CGP 12177 (a beta4-adrenoceptor agonist) and (-)-isoprenaline (a beta1/beta2-adrenoceptor agonist).
- Conducted experiments in the presence of beta1/beta2 antagonists ((-)-propranolol) and a phosphodiesterase inhibitor (3-isobutyl 1-methylxanthine) to isolate beta4-adrenoceptor effects.
Main Results:
- (-)-CGP 12177 significantly increased Ca2+ transients and induced concentration-dependent arrhythmias, with approximately 40 times greater arrhythmogenic potency than (-)-isoprenaline.
- Activation of beta4-adrenoceptors by (-)-CGP 12177 resulted in a smaller increase in L-type Ca2+ current compared to its effect on Ca2+ transients.
- The data suggest that beta4-adrenoceptor activation leads to more efficient Ca2+-induced Ca2+ release than beta1-adrenoceptor activation.
Conclusions:
- Putative beta4-adrenoceptors are potent mediators of cardiac arrhythmias, acting through mechanisms that enhance Ca2+ release more efficiently than beta1-adrenoceptors.
- Beta4-adrenoceptor-mediated arrhythmias involve smaller increases in Ca2+ transients and L-type Ca2+ current compared to beta1-adrenoceptors, indicating distinct signaling pathways.
- Further research is warranted to fully elucidate the unique mechanisms underlying beta4-adrenoceptor function in the intact heart.