Effects of inhaled nitric oxide in rats with chemically induced pulmonary hypertension

To determine the model animal with pulmonary hypertension in which nitric oxide (NO) inhalation reduces pulmonary arterial pressure (PAP), we examined the inhalation of 20-100 ppm NO gas on normal rats and rats with monocrotaline induced pulmonary hypertension. In the control group, mean PAP showed no change after spontaneous breathing of NO at the concentration of 20 to 100 ppm for 5 min. On the contrary, in both the severe (mean PAP > 40 mmHg) and moderate (mean PAP < 40 mmHg) pulmonary hypertensive groups, NO inhalation produced a prompt reduction of the mean PAP which had been elevated by monocrotaline. 20 ppm NO inhalation reduced mean PAP from 64.4 +/- 3.7 mmHg to 56.2 +/- 4.4 mmHg (mean +/- SEM, P < 0.01) in the severe pulmonary hypertensive group, from 31.0 +/- 2.0 mmHg to 24.2 +/- 0.9 mmHg in the moderate pulmonary hypertensive group (mean +/- SEM, P < 0.05). The onset of the reduction of mean PAP occurred within 30 sec after the start of NO inhalation and maximum reduction occurred within 4 min. 20 ppm NO inhalation significantly reduced mean PAP, and mean PAP was reduced dose-dependently at the concentration of 20 to 60 ppm and reaction to NO was almost constant at the concentrations of over 60 ppm.