Question

The β1 and β2 adrenergic receptor subtype agonist acting at atrioventricular node produces following responses

• A. Increases contractility and conduction velocity
• B. Increases automaticity and conduction velocity
• C. Increases contractility and automaticity
• D. Increases conduction velocity and heart rate

Answer 1

The Correct Option is B

To determine the correct response of β₁ and β₂ adrenergic receptor subtype agonists at the atrioventricular (AV) node, it is important to understand the effects mediated by these receptors.

1. β₁ Adrenergic Receptors:
   • Primarily located in the heart.
   • When stimulated, they increase heart rate (positive chronotropic effect), force of contraction (positive inotropic effect), and conduction velocity (positive dromotropic effect).
   • They also increase automaticity, which is the ability of cardiac cells to depolarize spontaneously and initiate an impulse.
2. β₂ Adrenergic Receptors:
   • Mostly located in the smooth muscles of blood vessels, but also present in cardiac tissue.
   • In the heart, they contribute to an increase in automaticity and can help in relaxation during diastole.

Given the role of these receptors, when an agonist acts on the β₁ and β₂ adrenergic receptors at the AV node, the following effects are expected:

• Automaticity Increase: Due to enhanced spontaneous activity.
• Conduction Velocity Increase: Due to a positive dromotropic effect.

This analysis points to the correct response as Increases automaticity and conduction velocity.

Let's evaluate the options provided:

• Increases contractility and conduction velocity: This is partly correct, as β₁ activation increases both, but automaticity is not mentioned.
• Increases automaticity and conduction velocity: This is the accurate choice as it corresponds directly to the actions mediated by β₁ and β₂ receptors at the AV node.
• Increases contractility and automaticity: While it describes some effects of β₁ activation, it misses conduction velocity.
• Increases conduction velocity and heart rate: Although these are effects of β₁, the automaticity increase is not mentioned.

Therefore, the correct answer is Increases automaticity and conduction velocity.