, Volume 154, Issue 3, pp 357-378

Ultrastructural distribution of calcium in smooth muscle cells of guinea-pig taenia coli

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In order to demonstrate the ultrastructural distribution of calcium in smooth muscle cells of the guinea pig taenia coli a precipitation method with potassium oxalate has been used. In addition, morphometric assessment (“stereology”) of cell components and direct quantitative estimation of calcium content in isolated subcellular fractions by atomic absorbtion analyses were carried out. After oxalate treatment smooth muscle cells show four major and distinct sites of calcium oxalate deposition: sarcoplasmic reticulum, mitochondria, cell boundaries (sarcolemma, surface microvesicles) and nucleus. Calcium oxalate deposits were frequently found at the level of close associations formed by different cell organelles. Correlating the morphometric analysis of electron micrographs with microchemical measurements of calcium in isolated subcellular fractions an estimation of calcium content in each cell component of one smooth muscle cell was performed. The nucleus has a significantly higher calcium content than that of any other cellular compartment, but the mitochondria are the main calcium-storage sites of smooth muscle cytoplasm. The highest calcium concentration was found in mitochondria. Although the amount of calcium stored in the sarcoplasmic reticulum represents only about a quarter of that accumulated in mitochondria, around 20% might be enough to produce contraction. On the other hand, the surface microvesicles may accommodate the entire amount of calcium translocated during activity. Functional implications of these findings in relation to the contraction-relaxation cycle are considered and the possible role of the surface microvesicles is discussed.

We thank Mrs. Natalia Cojocarasu for her excellent technical assistance.