PROSTACYCLIN AND NITRIC-OXIDE CONTRIBUTE TO THE VASODILATOR ACTION OF ACETYLCHOLINE AND BRADYKININ IN THE INTACT RABBIT CORONARY BED

The relative contribution of nitric oxide (NO) and cyclo-oxygenase products in the dilator response to equieffective doses of acetylcholine (ACh) and bradykinin (Bk) was studied in the isolated, saline-perfused rabbit heart under constant flow conditions. ACh (1 muM) and Bk (10 nM) induced a similar vasodilation, with a maximum reduction in coronary perfusion pressure (CPP) of 27 +/- 2%. The vasodilation induced by both agonists was associated with an enhanced release of 6-keto-PGF1alpha from the coronary bed, with the Bk-induced increase in 6-keto-PGF1alpha being threefold greater than that induced by ACh. The angiotensin converting enzyme (ACE) inhibitor ramiprilat (0.3 muM) selectively enhanced both the 6-keto-PGF1alpha outflow and the dilator response to Bk. The B2-receptor antagonist Hoe 140 (0.1 muM) blocked both Bk effects. The cyclo-oxygenase inhibitor diclofenac (1 muM) halved the dilator response to Bk, but did not affect the vasodilation to ACh. Both agonists induced the release of NO, as assessed by the increase in cyclic GMP content of platelets passing through the vascular bed. However, ACh induced a 2.5-fold greater increase in platelet cyclic GMP content, compared to Bk. Treatment of hearts with N(G)-nitro-L-arginine (L-NNA, 30 muM) halved the ACh- and Bk-induced maximum reduction in CPP. Combined infusion Of L-NNA and diclofenac completely blocked the dilator response to Bk, and inhibited the vasodilation to ACh more efficiently than L-NNA alone. We conclude that both NO and PGI2 contribute to the coronary dilator response to Bk and ACh in the rabbit Langendorff heart. Bradykinin induces a balanced release of these autacoids, which both contribute to and account for the vasodilation observed with this peptide. The relatively more important release of NO with ACh overwhelms the contribution of PGI2. However, the latter plays a significant role in the presence of an impaired L-arginine-NO pathway.