Pharmacological study of intracellular Ca^<2+> mobilizing mechanism

肌醇1，4，5-三磷酸(IP_3)诱导的Ca~(2+)释放是控制细胞内Ca~(2+)浓度的重要机制。我们先前对IP_3诱导的钙释放机制的研究表明，钙释放通道受钙和腺嘌呤核苷酸的调节。本研究旨在探讨钙离子自身调节IP_3诱导的钙释放的分子机制。本研究的初衷是研究IP_3通道在平面脂质双层膜上的单通道活性。虽然我们成功地从骨骼肌和小脑微粒体中获得了钙诱导的钙释放通道活性，但在本研究期间，我们发现很难在IP_3通道上获得预期的结果。因此，我们采取了另一种方法，使用笼状化合物来研究钙离子对皮肤平滑肌纤维IP_3通道的即时影响。结果表明，钙离子对IP_3诱导的钙离子释放具有即刻的增强和抑制作用，提示钙离子直接调节IP_3通道的门控。研究还发现，通道活动对IP_3的依赖存在弱的协作性(Hill系数为-2)。这些结果对于了解IP_3诱导体内钙释放的生理调节有重要意义。

Inositol 1,4,5-trisphosphate (IP_3)-induced Ca^<2+> release is an important mechanism that controls the cytoplasmic concentration of Ca^<2+> in the smooth muscle cells. Our previous study on the mechanism of IP_3-induced Ca^<2+> release showed that the Ca^<2+> release channels are regulated by Ca^<2+> and adenine nucleotides. This study was conducted to look into the molecular mechanism of the modulation of IP_3-induced Ca^<2+> release by Ca^<2+> itself. The original intention was to study the single channel activities of the IP_3 channels incorporated into planar lipid bilayers. Although we were successful in obtaining Ca^<2+>-induced Ca^<2+> release channel activities derived from both skeletal muscle and cerebellar microsomal fractions, we found it difficult to obtain aimed results on IP_3 channels within the period of the current study. We, thus, took an alternative course and used caged compounds to study the immediate effects of Ca^<2+> on the IP_3 channels in skinned smooth muscle fibers. It was shown that Ca^<2+> has the immediate potentiating and inhibitory effects on the IP_3-induced Ca^<2+> release, suggesting that Ca^<2+> directly regulates the gating of the IP_3 channels. It was also found that there is a weak cooperativity (Hill coefficient of -2) in the IP_3 dependence of the channel activity. These results are important in the understanding of the physiological regulation of the IP_3-induced Ca^<2+> release in vivo.