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Subcellular fractions of vascular smooth muscle exhibiting calcium transport properties.

Vascular smooth muscle relies on two general sources of activator calcium, one the extracellular fluid and the other intracellular sites in the coupling between stimulation at the external membrane and tension development. Electron microscopic studies have demonstrated at least two subcellular elements, mitochondria and sarcoplasmic reticulum, that may participate in the release and/or sequestration of this activator calcium. In order to obtain quantitative answers to questions concerning the rates of calcium transport by these subcellular elements attempts to use differential centrifugation to obtain subcellular fractions from whole muscle homogenate have been reported. Mitochondrial enriched preparations have received little quantitative study as yet, although they possess active calcium uptake mechanisms. Nonmitochondrial preparations having active calcium uptake mechanisms have been found in a variety of smooth muscles. The method of preparation and properties of these nonmitochondrial preparations show a great deal of variation, indicating they are probably a heterogeneous mixture of sarcoplasmic reticulum and plasma membrane. Some preliminary experiments on the effects of vasoactive agents and cyclic AMP on the calcium transport by these nonmitochondrial preparations have yielded conflicting results. Resolution of these problems awaits clear identification of the calcium transport mechanisms with their appropriate subcellular structure.