Review Question - QID 214346

A 38-year-old woman presents to her primary care physician with a 3-week history of headache and muscle weakness. She says that she started noticing a headache at work but thought it was just due to the stress of recent projects. She began to become concerned when she started finding it difficult to climb the 3 flights of stairs to her apartment. Her past medical history is significant for obesity, diabetes, and hypertension. On presentation, her temperature is 98.6°F (37°C), blood pressure is 181/120 mmHg, pulse is 98/min, and respirations are 19/min. A basic metabolic panel is obtained with the following results:

Serum:
Na+: 148 mEq/L
K+: 2.8 mEq/L
HCO3-: 33 mEq/L

The substance that is most likely responsible for this patient's symptoms acts in which of the following ways after binding to its receptor?