Nitric oxide relaxes human myometrium by a cGMP-independent mechanism

The role of intracellular guanosine 3',5'-cyclicmonophosphate concentration(cGMP]_i) in nitricoxide (NO)-mediated relaxations in the uterus has become controversial.We found the NO donor S-nitroso-L-cysteine(CysNO) to potently (IC₅₀ = 30 nM)inhibit spontaneous contractions in the nonpregnant human myometrium. CysNO treatment increasedcGMP]_i significantly(P < 0.001), and this increase wasblocked by the guanylyl cyclase inhibitors methylene blue (10 µM) orLY-83583 (1 µM); however, pretreatment with these guanylyl cyclaseinhibitors failed to block CysNO-mediated relaxations. IntracellularcAMP concentrations were not altered by treatment of tissues with 10 µM CysNO. Incubation with the cGMP analogs 8-bromo-cGMP or-phenyl-1,N²-etheno-cGMPdid not significantly affect spontaneous contractility. Pretreatment oftissues with charybdotoxin a calcium-dependent potassium channel(BK) blocker] completely reversed CysNO-induced relaxations. Weconclude that NO is a potent inhibitor of spontaneous contractileactivity in the nonpregnant human uterus and that, although guanylylcyclase and BK activities are increased by NO, increases incGMP]_i are notrequired for NO-induced relaxations in this tissue.