Rapid tachyphylaxis to hemodynamic effects of PACAP-27 after inhibition of nitric oxide synthesis

Departments of Pharmacology and Psychology and Cardiovascular Center, University of Iowa, Iowa City, Iowa 52242 The vasodilator effects of pituitary adenylate cyclase-activating polypeptide (PACAP)-27 are subject to tachyphylaxis in rats treated with the nitric oxide (NO) synthase inhibitor N G -nitro- L -arginine methyl ester ( L -NAME). We examined whether this tachyphylaxis could be prevented by administration of the putative endothelium-derived nitrosyl factor S -nitroso- L -cysteine ( L -SNC) and whether L -SNC may exert its effects via increases in cGMP levels in vascular smooth muscle. Five doses of PACAP-27 (2 nmol/kg iv) produced pronounced vasodilator responses in saline-treated rats. These responses were not subject to tachyphylaxis. The first injection of PACAP-27 (2 nmol/kg iv) in L -NAME-treated (50 µmol/kg iv) rats produced vasodilator responses similar to those in saline-treated rats, whereas subsequent injections produced progressively smaller responses. The injection of L -SNC (1,200 nmol/kg iv) before each injection of PACAP-27 prevented tachyphylaxis to the G s protein-coupled receptor agonist in L -NAME-treated rats, whereas equihypotensive doses of the NO donor sodium nitroprusside (100 µg/kg iv) did not. The injection of the membrane-permeant cGMP analog 8-(4-chlorophenylthio)guanosine 3',5'-cyclic monophosphate (8-CPT-cGMP; 30 µmol/kg iv) to L -NAME-treated rats restored resting hemodynamic values to pre- L -NAME levels but did not prevent the development of tachyphylaxis to PACAP-27. These results suggest that nitrosyl factors prevent the development of tachyphylaxis to the hemodynamic actions of PACAP-27. These nitrosyl factors may act independently of their ability to generate cGMP in vascular smooth muscle. G s protein-coupled receptors; vasodilation