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很重要，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。本研究采用酶促分离的平滑肌细胞，研究了与动作电位峰电位活动相关的内向Ca^<2+>电流的特性。2.在豚鼠逼尿肌细胞上，全细胞膜片钳实验显示，当大的预处理去极化（如+、-、80 mV，5 sec），测试电流仅被少量抑制。此外，随后的复极化到保持电位（+60 mV）诱导非常缓慢（τ-10 msec）的失活Ca^<2+>尾电流。3.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。在第二种开放状态下，Ca^2+通道没有失活，失活非常缓慢。4.使用细胞贴附膜片钳技术（在移液管中加入50-100 mM Ba^2+），我们记录了对应于第二种开放状态的单通道电流。在一个大的去极化将Ca^<2+>通道转换到长的开放状态后，我们施加一个斜坡脉冲来测量单位Ca^<2+>通道电流的I-V关系。斜率电导为22- 30 pS，与正常开放状态下的斜率电导相似。5.在其它平滑肌（如豚鼠胃、盲肠带）中也得到了类似的结果。第二开放状态似乎是平滑肌中的一种常见机制，并且可能在平滑肌中持续的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.