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

三磷酸肌醇（IP_3）诱导的平滑肌细胞内Ca^<2+>释放是控制细胞内Ca^<2+浓度的重要机制。我们对IP_3诱导的Ca^<2+>释放机制的研究表明，Ca^<2+释放通道受Ca^<2+和腺嘌呤核苷酸的调节。本研究旨在探讨Ca^<2 +>自身调节IP_3诱导的Ca^<2 +>释放的分子机制。本实验的目的是研究IP_3通道在平面脂质双层膜中的单通道活性。虽然我们成功地从骨骼肌和小脑微粒体中获得了Ca^<2 +>诱导的Ca^<2+>释放通道活性，但我们发现在本研究期间很难获得关于IP_3通道的目标结果。因此，我们采取了另一种方法，使用笼状化合物来研究Ca^2+对皮肤平滑肌纤维IP_3通道的直接影响。结果表明，Ca^<2+>对IP_3诱导的Ca^<2+>释放有直接的增强和抑制作用，提示Ca^<2+>直接调节IP_3通道的门控。还发现通道活性对IP_3的依赖性存在弱的协同性（Hill系数为-2）。这些结果对了解IP_3诱导的Ca^<2+>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.