Deletion of HNF-4α in β cells results in hyperinsulinemia and impaired glucose tolerance in vivo. (A) In the fed state (Fed) and after an overnight (16-hour) fast (Fast), blood glucose concentrations are decreased in HNF-4α^loxP/loxP; Ins.Cre mice compared with littermate controls. (B) Plasma insulin levels are elevated in HNF-4α mutants in both the fed and overnight-fasted states. (C) Fasting plasma glucagon levels in the mutants were indistinguishable from controls. (D) The ratio of plasma insulin to plasma glucagon is elevated 70% in HNF-4α^loxP/loxP; Ins.Cre mice. (E) Glucose tolerance test. After an overnight fast, 3- to 5-month-old HNF-4α^loxP/loxP; Ins.Cre mice and littermate controls were challenged with 2 grams of glucose per kilogram of body weight. The blood glucose elevation is significantly higher in HNF-4α^loxP/loxP; Ins.Cre mice than in controls, indicating impaired glucose tolerance in the HNF-4α mutants. (F) Following glucose injection (3 g/kg body weight), HNF-4α mutants exhibit a diminished first-phase insulin secretory response in comparison to controls. (G) Insulin tolerance test. Mutant and control mice that had fasted for 4 hours were injected with 0.75 units of insulin per kilogram of body weight. The insulin sensitivity of HNF-4α mutants is indistinguishable from that of controls. *P < 0.05 by Student’s t test or ANOVA; n = 8–13 animals per group for each experiment.