It is 1:00 A.M. and a hard-working second-year medical student is intent on finishing her pathology reading assignment. Soon she begins to note that her concentration is fading because 7 hours have passed since she had dinner, and she is feeling famished. Having studied the chapter on ischemic heart disease, she decides to be prudent and forgoes her favorite chocolate cookies, and instead devours two apples, gulping them down with a glass of low-fat milk. Of the following substances, which one was most likely to have increased rapidly when she became hungry and decreased promptly after she finished her healthy snack?