子宮平滑筋イオンチャネル研究の最近の展開

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Elevation of intracellular Ca^<2+> generates contractions of myometrial cells. The entry of Ca^<2+> is carried through L-type Ca^<2+> channels leading to action potentials and its openings are induced-by depolarization. The membrane potential is regulated by various types of ion channels. Many studies on ion channels in myometrial cells have been presented using the patch-clamp method and molecular techniques. This review will briefly outline the properties and function of these channels. L- and also T-type Ca^<2+> channel currents have been identified in human myometrial cells. Fast Na^+ currents were also detected. These inward currents produce the rising phase of action potentials and are controlled by the membrane potentials. K^+ channels play important roles in controlling the resting potential and repolarization of action potentials. Two types of voltage dependent K^+ channel currents are known to be present in the myometrium. One is the delayed rectifier K^+ (Kd) current which repolarizes the membrane during action potentials. The slow activating K^+ current (IKs) was detected in Kd currents but the rapid type (IKr) was not. The other is I_A-like transient K^+ (Kt) current. Ca^<2+> activated K^+ (K_<ca>) channel was identified in various myometrial cells. The single channel conductance was around 200 pS and this channel should be BK_<ca>. Recently, mRNA of Kir 6.1 and SUR2b, components of a ATP sensitive K^+ channel, have been identified in rat myometrium. Both channels regulate the resting potential to keep the myometrium quiescent and are suggested to be a target of β-stimulants. The excitation of myometrial cells is propagated through gap junctions to synchronize cell contractions. The junctional conductance was measured to be seven times higher during delivery comparing with the late stage of gestation using double patch-clamp methods. Non-selective cation channel currents have been isolated in rat myometrial cells and this channel has significant Ca^<2+> permeability. ATP enhanced ion conductance has been also observed in these preparations. Its properties are similar to those of the P2X channel in brain. Transient receptor potential channels have been identified in human myometrium and PHM1-41 cells using RT-PCR. These channels are supposed to relate to spontaneous depolarization and may be mechanisms of inhibition by MgS0_4 in tocolytic treatments. In the cells from circular myometrial layer hyperpolarizing activating inward currents have been identified, indicating relationship with pacemaker potentials. Ca^<2+> activating Cl^- channel was also reported to induce pacemaker activities of rat myometrial cells.
日本平滑筋学会の論文
2004-08-28

子宮平滑筋イオンチャネル研究の最近の展開

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