1. Two normal, healthy subjects volunteer for a study on insulin secretion . In Patient 1, blood glucose is increased to 150 mg/dL by direct intravenous infusion. In Patient 2, blood glucose is increased to 150 mg/dL by ingestio n of oral glucose. The peak plasma insulin concentration produced in Patient 1 is 70 mU/mL while in Patient 2, it is 95 mU/mL. Which of the following best explains the higher insulin concentration in Patient 2?