NDP measurements. The agents indicated at the bottom were added at the times (minutes) shown on the x-axis. On the y-axis, PD is expressed in mV. The graph on the left was obtained from right nostril measurements; the right one was obtained from left nostril measurements. Baseline NPD value was -21 mV. After the addition of amiloride PD increased to 13 mV, falling to 2.8 mV after exposure to 0 Cl- and isoproterenol. NDP measurements were repeated three times and readings were always consistent with a non-CF phenotype.