Cyclic GMP-independent mechanisms of nitric oxide-induced vasodilation

1. The aim of the present study was to test in vitro if NO acts through a cyclic GMP-independent mechanism to activate Ca2+-dependent potassium channels (K+(Ca)), leading to membrane hyperpolarization and vasodilation in rat tail artery. 2. Acetylcholine and sodium nitroprusside stimulated a significant increase in cyclic GMP (190+/-23 and 180+/-15 pmol/g, respectively) compared with agonist-free conditions (132+/-15 and 130+/-15 pmol/g, respectively); these agonist-mediated increases in cyclic GMP were completely abolished by treatment with the guanylate cyclase inhibitor methylene blue (122+/-10 and 60+/-8 pmol/g, respectively). 3. In contrast, relaxation to acetylcholine (10(-7) mol/l; 61+/-3%) and sodium nitroprusside (10(-8) mol/l; 97+/-1%) were significantly, but not completely, attenuated by methylene blue (30+/-5 and 79+/-3%, respectively); maximum relaxation to sodium nitroprusside (10(-7) mol/l) was unaffected by methylene blue. 4. Depolarization-induced contraction of vessels with KCl inhibited relaxation to both acetylcholine (10(-7) mol/l; 18+/-4%) and sodium nitroprusside (10(-8) mol/l; 57+/-7%). Furthermore, the specific K+(Ca) antagonist charybdotoxin significantly inhibited relaxation to sodium nitroprusside (10(-8) mol/l; 52+/-7%). 5. An additive inhibitory effect on relaxation to sodium nitroprusside (10(-8) mol/l) was observed with a combination of methylene blue and KCl (26+/-6%) or charybdotoxin (34+/-3%). 6. These data suggest that NO stimulates membrane hyperpolarization via K+(Ca) activation, in addition to guanylate cyclase, to cause relaxation in rat tail artery.