Some properties of inward currents and spontaneous excitation of the cell membrane in smooth muscle

1.平滑肌自发兴奋的机制是重要的，但尚未解决。然而，很少有报道描述离子电流如何引起平滑肌细胞膜的自发兴奋。在这项研究中，我们使用酶分离平滑肌细胞来研究向内Ca^<2+>电流的特性，这通常被认为是在动作电位期间引起尖峰活动的原因。在豚鼠逼尿肌平滑肌细胞中，全细胞膜片钳实验显示，当在测试步骤（0 mV,100 msec）之前施加大的预处理去极化（例如+80 mV，5秒）时，测试电流仅被少量抑制。此外，随后的再极化到保持电位（+60 mV）非常缓慢地（tau-10 msec）使Ca^<2+>尾电流失活。这种现象可以用一个模型来解释，在这个模型中，Ca^<2+>通道在大的去极化步骤中从正常状态转移到第二个开放状态。在第二开放状态下，Ca^<2+>通道不灭活，而且灭活非常缓慢。使用细胞贴附膜片钳技术（移液器中50-100 mM Ba^<2+>），我们记录了对应于第二开状态的单通道电流。在将Ca^<2+>通道大规模去极化tp转移到长开态后，我们应用斜坡脉冲测量了单一Ca^<2+>通道电流的I-V关系。坡电导为22 ~ 30ps，与正常开路状态相似。在其他平滑肌（如豚鼠胃、绦虫）中也得到了类似的结果。第二开放状态似乎是平滑肌的一种常见机制，并可能在平滑肌持续的Ca^<2+>内流和随后的细胞膜兴奋中起重要作用。

1.The mechanisms underlying spontaneous excitation in smooth muscle are important, but not resolved. However, there are very few reports describing how ionic currents cause spontaneous excitation in smooth muscle cell membranes. In this study, we used enzymatically isolated smooth muscle cells to investigate the characteristics of inward Ca^<2+> current which is usually thought to be responsible to spike activities during the action potential.2.In guinea-pig detrusor smooth muscle cells, whole-cell patch clamp experiments revealed that when large preconditioning depolarizations (e.g.+80 mV,5 sec) were applied prior to a test step (0 mV,100 msec), test currents were suppressed by only a small amount. Further, subsequent repolarizations to the holding potential (+60 mV) induced very slowly (tau-10 msec) deactivating Ca^<2+> tail currents.3.This phenomenon can be explained by a model in which Ca^<2+> channels are transfered from the normal to a second open state during large depolarizing steps. In the second open state Ca^<2+> channels are not inactivated and deactivate very slowly.4.Using cell-attached patch clamp techniques (50-100 mM Ba^<2+> in the pipette), we recorded single channel currents corresponding to the second open state. After a large depolarization tp transfer Ca^<2+> channels to the long open state, we applied a ramp pulse to measure I-V relationship of unitary Ca^<2+> channel current. The slope conductance was 22-30pS which is similar to that in the normal open state.5.Similar results were obtained from other smooth muscles (e.g.guinea-pig stomach, taenia caeci). The second open state seems to be a common mechanism in smooth muscles, and may play an important role in persistent Ca^<2+> influx and subsequent excitation of the cell membrane in smooth muscle.