Which of the following would NOT increase heart rate?

Stimulation by the vagal nerve is known to decrease heart rate. The vagus nerve is part of the parasympathetic nervous system, which works to slow down heart rate and promote a state of rest and relaxation. When the vagus nerve is stimulated, it releases acetylcholine, which acts on the heart to lower heart rate by increasing vagal tone.

In contrast, the other options listed involve mechanisms that increase heart rate. The sympathetic nervous system is responsible for the 'fight or flight' response and, when activated, it releases catecholamines like norepinephrine, which increase heart rate. Similarly, increased adrenaline levels (a result of sympathetic activity) boost heart rate and cardiac output. During physical exercise, several physiological changes, including increased sympathetic nervous system activity and adrenaline release, work together to elevate heart rate to meet the body’s increased demand for oxygen and nutrients.