Inhaled nitric oxide (NO) may downregulate the endogenous NO/cyclic guanosine monophosphate (cGMP) pathway, potentially explaining clinical rebound pulmonary hypertension. We determined if inhaled NO decreases pulmonary cGMP levels, if the possible downregulation is the same as with nifedipine, and if regulation also occurs with the cyclic adenosine monophosphate (cAMP) pathway. Rats were exposed to 3 wk of normoxia, hypoxia (10% O 2), or monocrotaline (MCT; single dose= 60 mg/kg) and treated with either nothing (control), inhaled NO (20 ppm), or nifedipine (10 mg· kg− 1· day− 1). The lungs were then isolated and perfused with physiologic saline. Perfusate cGMP, prostacyclin, and cAMP levels were measured. Perfusate cGMP was not altered by inhaled NO or nifedipine in normoxic or MCT rats. Although hypoxia significantly increased cGMP by 128%, both inhaled NO and nifedipine equally prevented the hypoxic increase. Inhibition of the NO/cGMP pathway with N G-nitro-L-arginine methyl ester (L-NAME) decreased cGMP by 72% and 88% in normoxic and hypoxic lungs. Prostacyclin and cAMP levels were not altered by inhaled NO or nifedipine. L-NAME significantly decreased cGMP levels, whereas inhaled NO had no effect on cGMP in normoxic or MCT lungs, suggesting that inhaled NO does not inhibit the NO/cGMP pathway. Inhaled NO decreased cGMP in hypoxic lungs, however, nifedipine had the same effect, which indicates the decrease is not specific to inhaled NO.

Implications

High pulmonary pressure after discontinuation of inhaled nitric oxide (NO) may be secondary to a decrease in the natural endogenous NO vasodilator. This rat study suggests that inhaled NO either does not alter endogenous NO or that it has similar effects as nifedipine.