Cyclic GMP-Independent Mechanisms of Nitric Oxide-Induced Vasodilation

Abstract

1. The aim of the present study was to test in vitro if NO acts through a cyclic GMP-independent mechanism to activate Ca 2+-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.