Final answer:
Acetylcholine is a weak negative inotrope primarily in the atria, inhibiting contraction by hyperpolarizing the cell membrane. Parasympathetic stimulation releases acetylcholine, which reduces the strength of contraction and stroke volume. The primary site of action is in the upper chambers.
Step-by-step explanation:
Acetylcholine (ACh) acts as a weak negative inotrope primarily in the atria. This is because parasympathetic stimulation releases ACh at the neuromuscular junction from the vagus nerve, which then hyperpolarizes the cardiac muscle cell membrane. This hyperpolarization inhibits contraction, resulting in a decrease in the strength of contraction and stroke volume (SV) in the atria. The primary site of action is in the upper chambers, and a stronger parasympathetic stimulation can directly decrease the force of contraction in the ventricles as well.