Kuemmerle J F, Murthy K S, Makhlouf G M
Department of Medicine, Medical College of Virginia, Richmond 23298-0711.
Am J Physiol. 1994 May;266(5 Pt 1):C1421-31. doi: 10.1152/ajpcell.1994.266.5.C1421.
We have previously shown that Ca2+ mobilization in longitudinal muscle is not mediated by inositol 1,4,5-trisphosphate (IP3) and depends on an obligatory influx of Ca2+. The present study examined whether Ca2+ influx activates ryanodine-sensitive Ca2+ channels to cause Ca(2+)-induced Ca2+ release. Ryanodine bound with high affinity to longitudinal muscle cells [dissociation constant (Kd) 7.3 +/- 0.3 nM] and microsomes (Kd 7.5 +/- 0.4 nM) and induced concentration-dependent 45Ca2+ efflux [50% effective concentration (EC50) 1.3 +/- 0.5 nM], increase in cytosolic free Ca2+ (EC50 2.0 +/- 0.7 nM), and contraction (EC50 0.9 +/- 0.2 nM) but had no effect in circular muscle cells. Ryanodine binding and ryanodine-induced Ca2+ release were enhanced by caffeine and inhibited by dantrolene and ruthenium red but were not affected by IP3 or heparin. Changes in Ca2+ concentration (50-500 nM) caused Ca2+ release from permeabilized longitudinal but not circular muscle cells loaded with 45Ca2+. The contractile agonist cholecystokinin-8 elicited 45Ca2+ efflux in both circular and longitudinal muscle cells; efflux in longitudinal muscle cells was abolished by Ca2+ channel blockers and by pretreatment of the cells with ryanodine. Pretreatment with thapsigargin abolished agonist-induced 45Ca2+ efflux in both cell types. We conclude that ryanodine-sensitive IP3-insensitive Ca2+ release channels with properties similar to those in cardiac muscle are present in longitudinal but not circular muscle cells of intestine and that agonist-mediated Ca2+ influx activates these channels, leading to Ca(2+)-induced Ca2+ release.
我们之前已经表明，纵行肌中的钙离子动员不是由肌醇1,4,5 -三磷酸（IP3）介导的，而是依赖于钙离子的必需内流。本研究检测了钙离子内流是否激活了对兰尼碱敏感的钙离子通道，从而导致钙诱导的钙释放。兰尼碱以高亲和力结合纵行肌细胞[解离常数（Kd）7.3±0.3 nM]和微粒体（Kd 7.5±0.4 nM），并诱导浓度依赖性的45Ca2+外流[半数有效浓度（EC50）1.3±0.5 nM]、胞质游离钙离子增加（EC50 2.0±0.7 nM）和收缩（EC50 0.9±0.2 nM），但对环行肌细胞没有影响。咖啡因增强了兰尼碱结合和兰尼碱诱导的钙释放，丹曲林和钌红抑制了它们，但IP3或肝素对其没有影响。钙离子浓度变化（50 - 500 nM）导致加载了45Ca2+的通透化纵行肌细胞而非环行肌细胞释放钙离子。收缩性激动剂胆囊收缩素 - 8在环行肌细胞和纵行肌细胞中均引起45Ca2+外流；纵行肌细胞中的外流被钙离子通道阻滞剂和用兰尼碱预处理细胞所消除。用毒胡萝卜素预处理消除了两种细胞类型中激动剂诱导的45Ca2+外流。我们得出结论，肠道纵行肌细胞中存在与心肌中性质相似的对兰尼碱敏感、对IP3不敏感的钙释放通道，而环行肌细胞中不存在，并且激动剂介导的钙离子内流激活这些通道，导致钙诱导的钙释放。
