Regulation of the cardiac delayed rectifier K^+ channel by membrane PIP_2 and its physiological significance

我们已经提出了实验证据，支持<Ks>豚鼠心肌细胞膜磷脂磷脂酰肌醇4，5-二磷酸（PIP_2）对延迟整流钾电流（I_）的缓慢激活成分具有张力性抑制的观点（Ding，Toyoda &amp; Matsuura，J.Biol.Chem.279，50726 - 50734，2004）。本研究采用全细胞膜片钳技术进一步阐明了PIP_2对豚鼠心房肌细胞I_2调节的生理意义<Ks>。经<Ks>膜片电极在细胞外应用ATP（50 μM）或苯肾上腺素（50 μM）或暴露于70%低渗的细胞外溶液所引起的I_2增加，可被细胞内应用PIP_2（50 μM）或抗PIP_2单克隆抗体（1：40稀释）所明显减弱。这些结果表明，PIP_2的调节至少部分参与了<Ks>由刺激某些Gq-PLC偶联受体（例如，P2 Y和α_1受体）或低渗细胞肿胀。浴用ATP（50 μM）引起动作电位时程（APD）的双相缩短，即在ATP应用后10 min内观察到明显缩短（初始相），此后观察到较中度的缩短（晚期相）。上述结果支持ATP增强I_0是晚期APD缩短的主要<Ks>原因，而I_0 &lt;K，ACh&gt;的瞬时激活则是早期APD缩短的主要原因。因此，PIP_2对I_2的抑制作用<Ks>可能在调节豚鼠心房肌细胞膜兴奋性中起重要的生理作用。

We have presented experimental evidence supporting the view that the slowly activating component of delayed rectifier K^+ current (I_<Ks>) is tonically inhibited by membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP_2) in guinea-pig cardiac myocytes (Ding, Toyoda & Matsuura, J.Biol.Chem.279,50726-50734,2004). The present research project further elucidated the physiological significance of the PIP_2 regulation of I_<Ks> in guinea-pig atrial myocytes using the whole-cell patch-clamp method. Enhancement of I_<Ks> by extracellular application of ATP (50 μM) or phenylephrine (50 μM) or by exposure to 〜70% hyposmotic extracellular solution was significantly attenuated by intracellular application of PIP_2(50 μM) or anti-PIP_2 monoclonal antibody (1:40 dilution) via a patch-electrode. These results indicate that the PIP_2 regulation is involved at least partly in the potentiation of I_<Ks> evoked by stimulation of some Gq-PLC coupled receptors (e.g., P2Y-and α_1-receptors) or by hyposmotic cell swelling. Bath application of ATP (50 μM) evoked a biphasic shortening of the action potential duration(APD), namely, a marked shortening observed within 〜1 min of ATP application (an initial phase) and a more moderate shortening which remained thereafter (late phase). Our results support that APD shortening in the late phase can be primarily ascribed to the potentiation of I_<Ks> by ATP, while the transient activation of I_<K,ACh> mainly contributes to APD shortening in the initial phase. Thus, the inhibitory action of PIP_2 on I_<Ks> may play an important physiological role in the regulation of membrane excitability in guinea-pig atrial myocytes.