Figure 4. Role of insulin on survival fitness during end-life. Plasma insulin declines (A) from hyperinsulinemic state while glucose (B) remains unchanged from 18-24 months of age. Decreases in survival rate (C) and glycogen storage (D) after insulin loss by streptozotocin injection (S group) is reversed by insulin replacement (SI group, 0.25 IU/kg twice a day) for old rats (20 months of age). Abbreviation: LS, lifespan; LS1 (short-lived, survival time < 579 days); LS2 (survival time 582-672 days); LS3 (survival time 672-738 days); LS4 (survival time 738-870 days); LS5 (long-lived, survival time 870-1032 days). ¶ Significant difference against long-lived rats (LS5), P < 0.05; * Significant difference against 6th month, P < 0.05; † Significant difference against 18th month, P < 0.01. ** Significant difference against control group, P < 0.05. Abbreviation: C, vehicle-injected control group; S, Streptozotocin-injected group; SI, Streptozotocin and insulin co-injected group.