The relaxant effects of atrial natriuretic peptide (ANP) and nitroprusside were studied on prostaglandin F2 alpha (PGF2 alpha)-contracted preparations of pulmonary resistance vessels (internal diameter 200-500 microns) and main pulmonary arteries taken from rats with pulmonary hypertension induced by monocrotaline (105 mg kg-1, s.c., 4 weeks previously). Control rats received saline. 2. In preparations from monocrotaline-treated rats, the potencies (negative log EC50) of ANP on resistance vessels (8.45) and main pulmonary arteries (8.25) were similar to those obtained in vessels from control rats (8.78 and 8.53 respectively), whereas those of nitroprusside were significantly less than in controls in both resistance vessels (7.13 compared with 7.63 in controls; three fold decrease in potency) and main pulmonary arteries (6.92 compared with 8.17 in controls; 18 fold decrease in potency). 3. On pulmonary resistance vessels from monocrotaline-treated rats, the maximum relaxant responses to ANP and nitroprusside, and also to pinacidil, were increased compared with controls, and reversal of the PGF2 alpha-induced contraction by these drugs was greater than 100%. In contrast, on main pulmonary arteries from monocrotaline-treated rats, the maximum relaxations to ANP and nitroprusside were not increased relative to controls, and reversal of PGF2 alpha was not greater than 100%. 4. Since the high potency of ANP on pulmonary resistance and conduit arteries is retained in vessels from rats with pulmonary hypertension, whether induced by monocrotaline (this study) or by chronic hypoxia (previous findings), it is postulated that elevation of plasma levels of ANP by use of drugs that inhibit the breakdown of this endogenous peptide, may be one approach to the pharmacological treatment of pulmonary hypertension.
