Iino M
Department of Pharmacology, Faculty of Medicine, University of Tokyo, Japan.
J Gen Physiol. 1991 Oct;98(4):681-98. doi: 10.1085/jgp.98.4.681.
Effects of adenine nucleotides on the inositol 1,4,5-trisphosphate (IP3)-induced Ca release (IICR) mechanism were studied in smooth muscle cells of the guinea pig portal vein. A microfluorometry method of fura-2 was used to measure Ca release from saponin-skinned thin muscle strips (width approximately 200 microns, thickness 50-70 microns, length 2-3 mm). About 80% of ionomycin-releasable Ca store was sensitive to IP3, of which approximately 20% was also sensitive to caffeine. The rate of Ca release by 0.1 microM IP3 depended biphasically on ATP concentration in the absence of Mg2+; it was dose-dependently enhanced by ATP up to approximately 0.5 mM, and above this concentration the enhancement became smaller. However, the decline of enhancement of the IICR at the higher ATP concentrations was absent at IP3 concentrations greater than 1 microM. This suggests competitive antagonism between IP3 and ATP. Clear effects of ATP were observed not only at pCa 7 or 8, where the Ca-induced Ca release was not activated, but after a ryanodine treatment to excise the functional compartment that possessed the Ca-induced Ca release mechanism. ATP had no effect on the rate of Ca leakage in the absence of IP3 even at pCa 5.5 after the ryanodine treatment. Therefore, ATP has direct biphasic effects on the IP3-induced Ca release mechanism. The Ca release induced by 0.1 microM IP3 at pCa 7 was potentiated not only by ATP, but by 0.5 mM ADP, AMP, or beta, gamma-methyleneadenosine 5'-triphosphate. 0.5 mM GTP had only a little effect on the IP3-induced Ca release. These results extend the functional similarities between Ca- and IP3-induced Ca release mechanisms in that adenine nucleotides enhance Ca release. Millimolar concentration of ATP, which is present physiologically, will shift the dose-response relation of IP3 toward the higher IP3 concentration and enhance the maximal effect of IP3. Thus, ATP is expected to assist the Ca release by higher concentrations of IP3 while having less effect on the Ca release by low levels of IP3. These effects of ATP may be important in the switching of Ca release from the intracellular Ca store by IP3.
在豚鼠门静脉平滑肌细胞中研究了腺嘌呤核苷酸对肌醇 1,4,5 - 三磷酸（IP3）诱导的钙释放（IICR）机制的影响。采用fura - 2 微荧光测定法测量皂素处理的薄肌条（宽度约200微米，厚度50 - 70微米，长度2 - 3毫米）中的钙释放。约80%的离子霉素可释放钙库对IP3敏感，其中约20%也对咖啡因敏感。在不存在Mg2 + 的情况下，0.1微摩尔IP3诱导的钙释放速率对ATP浓度呈双相依赖；ATP浓度在约0.5毫摩尔以下时，钙释放速率呈剂量依赖性增加，高于此浓度时增加幅度变小。然而，当IP3浓度大于1微摩尔时，在较高ATP浓度下IICR增强作用的下降消失。这表明IP3与ATP之间存在竞争性拮抗作用。不仅在pCa 7或8（此时钙诱导的钙释放未被激活）观察到了ATP的明显作用，而且在使用ryanodine处理切除具有钙诱导的钙释放机制的功能区室后也观察到了该作用。即使在ryanodine处理后pCa 5.5且不存在IP3的情况下，ATP对钙泄漏速率也没有影响。因此，ATP对IP3诱导的钙释放机制具有直接的双相作用。在pCa 7时，0.1微摩尔IP3诱导的钙释放不仅被ATP增强，还被0.5毫摩尔ADP、AMP或β,γ - 亚甲基腺苷5'-三磷酸增强。0.5毫摩尔GTP对IP3诱导的钙释放只有很小的影响。这些结果扩展了钙诱导的钙释放机制与IP3诱导的钙释放机制之间的功能相似性，即腺嘌呤核苷酸可增强钙释放。生理状态下存在的毫摩尔浓度的ATP会使IP3的剂量 - 反应关系向更高的IP3浓度偏移，并增强IP3的最大效应。因此，预计ATP在高浓度IP3诱导钙释放时起辅助作用，而对低水平IP3诱导的钙释放影响较小。ATP的这些作用可能在IP3介导的细胞内钙库钙释放转换中起重要作用。
