|Budget Amount *help
¥2,100,000 (Direct Cost : ¥2,100,000)
Fiscal Year 1991 : ¥600,000 (Direct Cost : ¥600,000)
Fiscal Year 1990 : ¥1,500,000 (Direct Cost : ¥1,500,000)
To investigate the vasodilator mechanisms of the K^+ channel openers, cromakalim, pinacidil and nicorandil, we measured changes in cytoplasmic Ca^<2+> concentration ([Ca^<2+>]i) simultaneously with force by a microfluorimetric method using fura-2. Cromakalim is a more specific K^+ channel opener than pinacidil and nicorandil and that vasodilation produced by cromakalim in this study is predominantly a result of a reduction of[Ca^<2+>]i due to the closure of voltage-dependent Ca^<2+> channels by hyperpolarization. In contrast, additional mechanisms are involved in the vasodilator actions of pinacidil and nicorandil. One of these is related to a reduction in the sensitivity of contractile proteins to Ca^<2+>. The latter mechanism of nicorandil is akin to that of nitroglycerin. K^+ channels opened by these K^+ channel openers may be ATP-sensitive ones which are blocked by glibenclamide. A thromboxane A2 analogue U46619 increased [Ca^<2+>]i and force, upon cumulative application in canine
coronary arteries. Depolarization by 20 mM KCI potentiated the increases in [Ca^<2+>]i and force induced by U46619. Cromakalim and verapamil inhibited both increases. The inhibitory effect of cromakalim was counteracted by depolarization by 20 or 25 mM KCI. Upon single dose applications of U46619 at 3x10^<-7> M, [Ca^<2+>]i and force increased in phasic and tonic manners, which were almost abolished by cromakalim and Ki4032. In the absence of extracellular Ca^<2+>, U46619 induced a transient increase in [Ca^<2+>]i with a contraction. In the presence of cromakalim or Ki4032, the increase in[Ca^<2+>]i was abolished, which was blocked by the K^+ channel blocker tetrabutylammonium(TBA)and counteracted by the depolarization by 20 mM KCI. Cromakalim and Ki4032 did not affect caffeine-induced Ca^<2+> release. Thus, U46619 produces Ca^<2+> influx through L-type Ca^<2+> channels, which are deactivated by hyperpolarization induced by cromakalim and Ki4032. The IP_3-induced Ca^<2+> release from intracellular stores related to stimulation of the thromboxane A^2 receptor is selectively inhibited by hyperpolarization of plasma membrane by K^+ channel openers.
次にagonistのもう一方の収縮機構である筋小胞体(SR)からのCa遊離作用について検討した.細胞外液のCa^<2+>を0として,U46619による一過性の[Ca^<2+>]_1と張力の上昇に対する作用を検討すると,cromakalimはこの反応を抑制した.cromakalimの抑制作用は,Kチャンネルブロッカ-のTBAで拮抗され,[K^+]。を20mMとすると打ち消された.cromakalimはcaffeineによるCa遊離作用に影響しなかった.さらに,U46619とcaffeineとを続けて作用させると,それに応じてCa遊離が見られた.cromakalim存在下ではU46619によるCa遊離は抑制されるものの,caffeineによるものは抑制されずむしろ大きくなった.このようなU46619のCa遊離抑制作用は他の特異的なKCOによっても観察されている.すなわち,KCOは,thromboxane A_2受容体をはじめとしたagonistを介するIP_3によるCa遊離をも選択的に抑制すると考えられる. Less