Somlyo A P, Walker J W, Goldman Y E, Trentham D R, Kobayashi S, Kitazawa T, Somlyo A V
Pennsylvania Muscle Institute, University of Pennsylvania School of Medicine, Philadelphia 19104-6083.
Philos Trans R Soc Lond B Biol Sci. 1988 Jul 26;320(1199):399-414. doi: 10.1098/rstb.1988.0084.
The identity of organelles storing intracellular calcium and the role of Ins(1,4,5)P3 in muscle have been explored with, respectively, electron probe X-ray microanalysis (EPMA) and laser photolysis of 'caged' compounds. The participation of G-protein(s) in the release of intracellular Ca2+ was determined in saponin-permeabilized smooth muscle. The sarcoplasmic reticulum (SR) is identified as the major source of activator Ca2+ in both smooth and striated muscle; similar (EPMA) studies suggest that the endoplasmic reticulum is the major Ca2+ storage site in non-muscle cells. In none of the cell types did mitochondria play a significant, physiological role in the regulation of cytoplasmic Ca2+. The latency of guinea pig portal vein smooth muscle contraction following photolytic release of phenylephrine, an alpha 1-agonist, is 1.5 +/- 0.26 s at 20 degrees C and 0.6 +/- 0.18 s at 30 degrees C; the latency of contraction after photolytic release of Ins(1,4,5)P3 from caged Ins(1,4,5)P3 is 0.5 +/- 0.12 s at 20 degrees C. The long latency of alpha 1-adrenergic Ca2+ release and its temperature dependence are consistent with a process mediated by G-protein-coupled activation of phosphatidylinositol 4,5 bisphosphate (PtdIns(4,5)P2) hydrolysis. GTP gamma S, a non-hydrolysable analogue of GTP, causes Ca2+ release and contraction in permeabilized smooth muscle. Ins(1,4,5)P3 has an additive effect during the late, but not the early, phase of GTP gamma S action, and GTP gamma S can cause Ca2+ release and contraction of permeabilized smooth muscles refractory to Ins(1,4,5)P3. These results suggest that activation of G protein(s) can release Ca2+ by, at least, two G-protein-regulated mechanisms: one mediated by Ins(1,4,5)P3 and the other Ins(1,4,5)P3-independent. The low Ins(1,4,5)P3 5-phosphatase activity and the slow time-course (seconds) of the contractile response to Ins(1,4,5)P3 released with laser flash photolysis from caged Ins(1,4,5)P3 in frog skeletal muscle suggest that Ins(1,4,5)P3 is unlikely to be the physiological messenger of excitation-contraction coupling of striated muscle. In contrast, in smooth muscle the high Ins(1,4,5)P3-5-phosphatase activity and the rate of force development after photolytic release of Ins(1,4,5)P3 are compatible with a physiological role of Ins(1,4,5)P3 as a messenger of pharmacomechanical coupling.
利用电子探针X射线微分析（EPMA）和“笼形”化合物的激光光解，分别探究了储存细胞内钙的细胞器的身份以及肌醇-1,4,5-三磷酸（Ins(1,4,5)P3）在肌肉中的作用。在皂素通透的平滑肌中测定了G蛋白参与细胞内Ca2+释放的情况。肌浆网（SR）被确定为平滑肌和横纹肌中激活Ca2+的主要来源；类似的（EPMA）研究表明，内质网是非肌肉细胞中主要的Ca2+储存位点。在所有细胞类型中，线粒体在细胞质Ca2+调节中均未发挥显著的生理作用。在20℃时，α1-激动剂去氧肾上腺素光解释放后豚鼠门静脉平滑肌收缩的延迟时间为1.5±0.26秒，在30℃时为0.6±0.18秒；从笼形Ins(1,4,5)P3光解释放Ins(1,4,5)P3后收缩的延迟时间在20℃时为0.5±0.12秒。α1-肾上腺素能Ca2+释放的长延迟及其温度依赖性与由G蛋白偶联激活磷脂酰肌醇-4,5-二磷酸（PtdIns(4,5)P2）水解介导的过程一致。GTPγS是一种不可水解的GTP类似物，可导致通透平滑肌中Ca2+释放和收缩。Ins(1,4,5)P3在GTPγS作用后期而非早期具有相加作用，并且GTPγS可导致对Ins(1,4,5)P3不敏感的通透平滑肌中Ca2+释放和收缩。这些结果表明，G蛋白的激活至少可通过两种G蛋白调节机制释放Ca2+：一种由Ins(1,4,5)P3介导，另一种与Ins(1,4,5)P3无关。青蛙骨骼肌中Ins(1,4,5)P3 5-磷酸酶活性较低，且对从笼形Ins(1,4,5)P3激光闪光光解释放的Ins(1,4,5)P3的收缩反应时间进程较慢（数秒），这表明Ins(1,4,5)P3不太可能是横纹肌兴奋-收缩偶联的生理信使。相比之下，在平滑肌中，Ins(1,4,5)P3的高5-磷酸酶活性以及Ins(1,4,5)P3光解释放后的力发展速率与Ins(1,4,5)P3作为药物-机械偶联信使的生理作用相符。
