Which of the following is considered a negative chronotropic agent?

A negative chronotropic agent is one that decreases the heart rate. Acetylcholine is classified as a negative chronotropic agent because it primarily acts on the muscarinic receptors in the heart, specifically the sinoatrial (SA) node, leading to a decrease in heart rate. This action promotes a parasympathetic response, which slows down the electrical conduction and reduces the frequency of heartbeats.

In contrast, the other options mentioned, such as dopamine and epinephrine, act as positive chronotropic agents. They increase heart rate through stimulating beta-adrenergic receptors, enhancing cardiac output. Glucagon, while it has various metabolic effects, can also increase heart rate but not by the same pathways as the classic positive chronotropes. Therefore, acetylcholine is uniquely positioned as a negative chronotropic agent, effectively lowering heart rate through parasympathetic stimulation.