Functions of the two types of calcium release channels in the sarcoplasmic reticulum of skeletal muscle.

在青蛙骨骼肌的肌浆网（SR）中，分别对应于哺乳动物1型和3型的两种类型的兰尼碱受体（钙释放通道）以大约相等的量表达。本研究旨在阐明这两种类型的通道的生理作用。我们得到了以下结果。(1)通过使用仅表达3型通道的1型敲除小鼠的骨骼肌检查3型通道的性质，发现其显示出Ca诱导的Ca释放（CICR）活性，但需要较高的Ca浓度来激活。(2)我们制作了3型基因敲除小鼠，其骨骼肌显示正常的EC偶联。(3)在1型基因敲除小鼠的骨骼肌中，只有1型通道而2型和3型通道都不能挽救受损的EC偶联。(4)We已经能够制造出1型和3型通道都被敲除的突变小鼠。这些小鼠骨骼肌的SR可以积累Ca，但CICR不存在，表明在小鼠骨骼肌中表达的Ca释放通道仅为1和3型，而不是2型。还发现“去极化”诱导的Ca释放是1型通道的特性。(5)发现钙调素在低激活浓度的Ca下增强CICR，除了迄今为止报道的在较高激活浓度的Ca下显示的CICR抑制之外。它主要表现出对1型通道的增强作用和对3型通道的抑制作用。(6)Ryanodine不与闭合通道结合，但与通过生理刺激打开的通道结合，并固定在打开状态以引起挛缩，此外，众所周知，Ryanodine与通过Ca打开的通道结合（在CICR模式下打开）。因此，有可能用放射性兰尼碱测定生理收缩时1型和3型通道开放的相对大小。

In the sarcoplasmic reticulum (SR) of frog skeletal muscle, two types of ryanodine receptors (calcium release channels) , corresponding to mammalian type 1 and type 3, respectively, are expressed in about equal amount. This study is aiming at elucidation of the physiological role of each of these two types of channels. We have obtained the following results. (1) The properties of type 3 channels were examined by using skeletal muscles of type 1-knockout mice that expresses only type 3 channels, and found to show Ca-induced Ca release (CICR) activity but require a higher Ca concentration for activation. (2) We made type 3-knockout mice, the skeletal mucle of which showed normal EC coupling. (3) only type 1 channel but neither type 2 nor type 3 channels can rescue impaired EC coupling in skeletal muscle of type 1-knockout mice. (4)We have been able to make mutant mice in which both type 1 and type 3 channels are knocked out. The SR of skeletal muscle of these mice can accumulate Ca but CICR is absent, indicating that in mouse skeletal muscle the expressed Ca release channels are only types 1 and 3, but not type 2. It was also found that "depolarization" -induced Ca release is the property of type 1 channels. (5) Calmodulin was found to enhance CICR at low activating concentration of Ca in addition to inhibition of CICR so far reported that is shown at higher activating concentrations of Ca. It shows mainly an enhancing effect on type 1 channels and an inhibitory effect on type 3 channels. (6) Ryanodine, which does not bind to closed channels, binds to channels open by physiological stimulation and fixes in an open state to cause contracture, in addition to the wellknown fact that it binds channels open by Ca (open in CICR mode). Therefore, it will be possible by using radioactive ryanodine, to determine the relative magnitude of opening of type 1 and 3 channel in physiological contraction.