A: AIRs to arginine as a function of the prestimulus plasma glucose concentration in PS-CF (closed circles), PI-CF (open circles), and healthy control subjects (normal, open triangles). Data are given as mean ± SE. The glucose potentiation slope (GPS), calculated as the difference in the AIR at fasted and ∼230 mg/dL glucose levels divided by the difference in plasma glucose, is impaired in PI-CF vs. both PS-CF and normal (0.3 ± 0.2 vs. 1.0 ± 0.6 and 0.7 ± 0.4; **P = 0.002). β-Cell sensitivity to glucose is determined as the PG50 using the y intercept (b) of the GPS to solve the equation AIRmax/2 = GPS × PG50 + b and was not different across groups. B: Box plot of insulin sensitivity (M/I) by study group, given as median and IQR (box) and mean (open squares) and range (error bars), is similar across PS-CF, PI-CF, and normal control subjects.