Effects of a newly synthesized K+ channel opener, Y-26763, on noradrenaline-induced Ca2+ mobilization in smooth muscle of the rabbit mesenteric artery

1. The mechanisms underlying the vasodilatation induced by (-)-(3S,4R)-4-(N-acetyl-N-hydroxyamino)-6-cyano-3,4-dihydro-2, 2-dimethyl-2H-1-benzopyran-3-ol (Y-26763) were investigated by measuring membrane potential, intracellular Ca2+ concentration ([Ca2+]i) and isometric force in smooth muscle cells of the rabbit mesenteric artery. 2. Y-26763 (0.03-1 microM) concentration-dependently hyperpolarized the membrane and glibenclamide (1-10 microM) inhibited this hyperpolarization. Noradrenaline (NA, 10 microM) depolarized the membrane and generated spike potentials. Y-26763 (1 microM) inhibited these NA-induced electrical responses. 3. In thin smooth muscle strips in 2.6 mM Ca2+ containing (Krebs) solution, 10 microM NA produced a large phasic, followed by a small tonic increase in [Ca2+]i and force with associated oscillations. In Ca(2+)-free solution (containing 2 mM EGTA), NA produced only phasic increases in [Ca2+]i and force. In ryanodine-treated strips, NA could not produce the phasic increases in [Ca2+]i and force even in the presence of 2.6 mM Ca2+, suggesting that ryanodine functionally removes the NA-sensitive intracellular storage sites. 4. Nicardipine (1 microM) partly inhibited the NA-induced tonic increases in [Ca2+]i and force but had no effect on either the resting [Ca2+]i or the NA-activated phasic increases in [Ca2+]i and force. By contrast, Y-26763 (10 microM) lowered the resting [Ca2+]i and also inhibited both the phasic and the tonic increases in [Ca2+]i and force induced by NA. All these actions of Y-26763 were inhibited by glibenclamide (10 microM). 5. In ryanodine-treated strips, nicardipine partly, but Y-26763 completely inhibited the NA-induced increases in [Ca2+]i, suggesting that Y-26763 inhibits both the nicardipine-sensitive and -insensitive Ca2+ influxes activated by NA. Y-26763 attenuated the phasic increase in [Ca2+]i and force in a Ca(2+)-free solution containing 5.9 mM K+, but not in one containing 50 mM K+, suggesting that Y-26763 inhibits NA-induced Ca2+ release, probably as a result of its membrane hyperpolarizing action. 6. In Beta-escin-skinned strips, Y-26763 (10 MicroM) had no effect on either the NA-induced Ca2+ release or the Ca2+-tension relationship in the presence and absence of NA (10 MicroM) with guanosine 5'-triphosphate(GTP, 10 MicroM), suggesting that Y-26763 has no direct action on either NA-induced Ca2+ release or the contractile proteins.7. It is concluded that Y-26763 inhibits NA-activated Ca2+ release and Ca2+ influx and thus inhibits the NA-contraction. Y-26763 also lowers the resting [Ca2+]i through an inhibition of the nicardipine insensitive Ca2+ influx. These actions of Y-26763 may be linked with the membrane hyperpolarization it produces by activation of the ATP-sensitive K+ channels.