Final answer:
The muscarinic receptors are responsible for the slowing of the heart rate. They are activated by the parasympathetic nervous system and decrease heart rate, as opposed to beta-adrenergic receptors which generally increase heart rate and contractility. The correct answer to the question is (D) Muscarinic receptors.
Step-by-step explanation:
The muscarinic receptors are responsible for mediating the slowing of the heart. Specifically, the activation of muscarinic receptors by acetylcholine, which is released by the parasympathetic nervous system, leads to a decrease in heart rate.
This is opposed to the action of β1-adrenergic and β2-adrenergic receptors, which, when stimulated by epinephrine and norepinephrine, generally have a cardiac stimulatory effect, with β1-adrenergic receptors in particular increasing heart rate and contractility. Beta blockers, which block β1 receptors, can be used to decrease heart rate, acting as antagonists to the natural ligand norepinephrine.
It's important to highlight that β-adrenergic receptors have subtypes: β1, β2, and β3, with β1 being primarily associated with heart muscle contraction. The β1 receptors, when stimulated by norepinephrine, cause an increase in heart rate. However, when these receptors are blocked, for instance by beta blockers, the heart rate is reduced.
In contrast, activation of alpha receptors, such as α1 and α2, leads to smooth muscle contraction and is associated with vasoconstriction. The question specifically refers to which receptor mediates a decrease in heart rate, and the correct answer is (D) Muscarinic receptors.