Dual modulation of Ca channels by intracellular Ca^<2+> concentration

肌肉和神经元中L型钙通道的活动受细胞内钙浓度([Ca]i)的反馈控制，包括钙依赖的失活和增强。然而，人们对这两种相反功能之间的相互关系以及所需的[Ca]i水平知之甚少。通过使用FURA2同时光学监测[Ca]i，我们在单通道水平上研究了[Ca]i张力性变化是如何调制心脏L型钙通道的。当钾去极化状态下的细胞内钙浓度通过改变槽钙浓度而改变时，当[Ca]i超过静息水平的2倍时，钙通道的活性被可逆地增强，直到它在收缩阈值下不久达到“失活”水平。在这些观测期间，单一电流幅度是恒定的。钙依赖增强期间通道开放概率的增加与1.可用扫描次数增加，2.开放扫描期间Po增加，以及3.“模式2”行为的出现有关。我们还发现，钙依赖增强具有短时“记忆”，这种记忆对钙诱导的短暂失活有抵抗作用。本研究首次对钙通道活动的钙依赖性进行了直接和定量的分析。

Activities of L-type Ca channels in muscles and neurons are under feedback controls by intracellular Ca concentrations ([Ca]i), which include both Ca-dependent inactivation and potentiation. Little is known,however, about interrelationships between these two opposite functions and the levels of [Ca]i required. With simultaneous optical monitoring of [Ca]i using fura2, we investigated at the single-channel level how tonic changes in [Ca]i modulate cardiac L-type Ca channels. When cytosolic Ca concentrations of myocytes under K-depolarization is altered through changes in bath Ca concentration, activity of Ca channels is reversibly potentiated as [Ca]i exceeds 2 times the resting level, until it reaches the "inactivation" level shortly below the threshold for contraction. Unitary current amplitudes were constant during these observations. Increased probability for channel opening during Ca-dependent potentiation was associated with 1. increased number of available sweeps, 2. increased Po during open sweeps, and 3. increased appearance of "mode2" behavior. We also found that Ca-dependent potentiation was equipped with short term "memory" which is resistant to transient Ca-induced inactivation. The present study provide analysis of Ca-dependence of Ca channel activity for the first time in direct and quantitative way.