A 16-year-old girl diagnosed with anorexia nervosa is brought to the emergency department after becoming very light-headed, breaking into a sweat, and developing palpitations while exercising. She exercises regularly. Her serum glucose is 40 mg/dL. The patient is given a soft drink to sip, and she feels better half an hour later. Which of the following hormones most likely triggered the sweating and palpitations the patient experienced?