Morimoto T, Ogihara S, Takisawa H
Department of Biology, Faculty of Science, Osaka University, Japan.
J Cell Biol. 1990 Jul;111(1):79-86. doi: 10.1083/jcb.111.1.79.
The ultrastructural changes in electropermeabilized bovine platelets that accompany the Ca2(+)-induced secretion of serotonin were investigated in ultra-thin sections of chemically fixed cells. Such preparations permitted us to study both the localization of and the structures associated with serotonin-containing dense granules. Localization of dense granules within cells was examined by measuring the shortest distances between the granular membranes and the plasma membrane. About 40% of total granules were located close to the plasma membrane at an average distance of 10.8 +/- 1.6 nm. 71% of the total number of granules were localized at a similar average distance of 12.5 +/- 2.7 nm in intact platelets. The percentage of granules apposed to the plasma membrane corresponded closely to the percentage of total serotonin that was maximally secreted after stimulation of the permeabilized (38 +/- 4.9%) and the intact platelets (72 +/- 3.6%). Furthermore, the percentage of granules anchored to the membrane, but not of those in other regions of permeabilized cells, decreased markedly when cells were stimulated for 30 s by extracellularly added Ca2+. The decrease in the numbers of granules in the vicinity of the plasma membrane corresponded to approximately 22% of the total number of dense granules that were used for measurements of the distances between the two membranes and corresponded roughly to the overall decrease (15%) in the average number of the granules per cell. Most dense granules were found to be associated with meshwork structures of microfilaments. Upon secretory stimulation, nonfilamentous, amorphous structures found between the plasma membrane and the apposed granules formed a bridge-like structure that connected both membranes without any obvious accompanying changes in the microfilament structures. These results suggest that the dense granules that are susceptible to secretory stimulation are anchored to the plasma membrane before stimulation, and that the formation of the bridge-like structure may participate in the Ca2(+)-regulated exocytosis.
在化学固定细胞的超薄切片中，研究了电通透处理的牛血小板中伴随Ca2 +诱导的5-羟色胺分泌的超微结构变化。这样的制备使我们能够研究含5-羟色胺的致密颗粒的定位及其相关结构。通过测量颗粒膜与质膜之间的最短距离来检查细胞内致密颗粒的定位。约40%的总颗粒位于靠近质膜处，平均距离为10.8±1.6 nm。在完整血小板中，71%的颗粒总数以12.5±2.7 nm的相似平均距离定位。与质膜相邻的颗粒百分比与通透处理的血小板（38±4.9%）和完整血小板（72±3.6%）刺激后最大分泌的总5-羟色胺百分比密切对应。此外，当细胞用细胞外添加的Ca2 +刺激30 s时，锚定在膜上的颗粒百分比（但通透细胞其他区域的颗粒百分比未变）显著下降。质膜附近颗粒数量的减少约占用于测量两膜之间距离的致密颗粒总数的22%，大致相当于每个细胞中颗粒平均数量的总体减少（15%）。发现大多数致密颗粒与微丝的网状结构相关。在分泌刺激时，在质膜和相邻颗粒之间发现的无丝状、无定形结构形成了连接两膜的桥状结构，而微丝结构没有任何明显的伴随变化。这些结果表明，易受分泌刺激的致密颗粒在刺激前锚定在质膜上，并且桥状结构的形成可能参与Ca2 +调节的胞吐作用。
