Final answer:
The Adrenergic B1 receptor is responsible for mediating an increase in heart rate as a part of the sympathetic nervous system's response to stress or exercise.
Step-by-step explanation:
The autonomic receptor that mediates an increase in heart rate is the Adrenergic B1 receptor. B1 receptors are part of the sympathetic nervous system and are responsible for increasing the heart rate when activated by neurotransmitters like norepinephrine and epinephrine, which are released during stress or exercise. These receptors are located on the cardiac muscle, and when they are stimulated, they increase the force of contraction and the rate of the heartbeat.