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 KCI 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 KCI (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. (C) 1998 Elsevier Science Inc.