Decrease in heart adrenoceptor gene expression and receptor number as compensatory tool for preserved heart function...

Area of Science:

• Cardiovascular Physiology
• Molecular Cardiology
• Pharmacology

Background:

• Muscarinic receptors (MR) are critical for cardioinhibition, primarily mediated by the M2 subtype.
• Parasympathetic stimulation via M2 MR normally slows heart rate (HR).
• M2 knockout (KO) mice lack these primary cardioinhibitory receptors.

Purpose of the Study:

• To investigate cardiac gene expression, receptor binding, and function in M2 KO mice.
• To assess changes in adrenergic receptors (BAR) and heart rate regulation.
• To determine if M2 receptor absence impacts cardiac function and HR biorhythm.

Main Methods:

• Gene expression analysis of M3 MR and beta-adrenergic receptors (BAR) in left (LV) and right ventricles (RV).
• Receptor binding assays for MR and BAR.
• Echocardiography to assess LV systolic and diastolic function.
• Pharmacological challenges with agonists/antagonents (isoprenaline, propranolol, atropine, carbachol) to measure HR response.

Main Results:

• M2 KO mice showed decreased M3 MR and BAR gene expression and binding in the LV.
• No significant changes in LV systolic/diastolic function or HR biorhythm were observed.
• Atropine failed to increase HR in M2 KO mice, while carbachol increased HR, indicating cardiostimulatory MR presence.

Conclusions:

• The absence of M2 MR in the heart does not significantly impair cardiac function.
• Reduced expression and binding of M3 MR and BAR may compensate for the lack of M2 receptors.
• Cardiostimulatory MR may play a role in maintaining cardiac function in M2 KO mice.