Voltage-dependent modulation of calcium channel kinetics and its contribution to physiological functions

点击翻译按钮获取中文摘要

In smooth muscle, it is well known that spike activities are largely due to activation of voltage-sensitive Ca^<2+> channels. Sustained Ca^<2+> influx seen during long term depolarization of the cell membrane is also considered to be through these voltage-sensitive Ca^<2+> channels. Since both phenomena are significantly suppressed by Ca^<2+> antagonists, (dihydropyridinesensitive) L-type Ca^<2+> channels seem to cover a wide range of smooth muscle behavior : from transient to persistent events.Previously, we have demonstrated the presence of multiple open states in smooth muscle L-type Ca^<2+> channels, in guinea-pig urinary bladder, taenia caeci and gastric antrum. During a large, long duration depolarization, the conformation of Ca^<2+> channels are transferred to the second open state in which these channels do not, or very slowly inactivate during depolarizaton, and deactivate very slowly upon repolarization.In 1999, using cell-attached patch clamp techniques, we mainly examined w … More hether smooth muscle Ca^<2+> channel undergoes voltage-dependent conversion of the channel conformation from normal to the second open state, even when its α_1-subunits are expressed in the plasma membrane. The answer was 'YES'. We demonstrated that α_1-subunit of cloned smooth muscle Ca^<2+> channel (α_<1C-b>) does not inactivate during large conditioning depolarization, and produced slow deactivating tail current upon repolarization of the cell membrane. These phenomena were essentially the same observed in intact smooth muscle Ca^<2+> channels.In 2000, we used whole-cell patch clamp techniques to investigate 1) the role of β-subunit in the voltage-dependent conversion of the C^<2+> channel conformation, and 2) interaction of depolarization and Ca^<2+> agonist. The experiments revealed that a smooth muscle β-subunit (β_3) decreased the ratio of the Ca^<2+> channels converted to the second open state during large depolarization, and that voltage-dependent conversion and Ca^<2+> agonistinduced mode 2 gating mechanism operated separately, causing four open states in α_1-subunit of cloned smooth muscle Ca^<2+> channel. Less