A microsomal fraction isolated from longitudinal smooth muscle of guinea pig ileum actively sequesters calcium ion in the presence of magnesium and adenosine triphosphate in a fashion previously described for microsomes of the rabbit aorta. This activity in guinea pig ileum appears to be associated primarily with the plasma membrane as is found in the red cell. By contrast the uptake of calcium in aortic smooth muscle appears to be associated to an appreciable extent with intracellular membranes, possibly analogous to the sarcoplasmic reticulum of skeletal muscle.
A microsomal cell fraction derived from the intimal-medial layer of rabbit aorta takes up calcium in the presence of magnesium and adenosine triphosphate. The rate of uptake of calcium is slower than that observed in skeletal muscle microsomes. Uptake of calcium by mitochondria from the aorta is even more limited and, unlike microsomal uptake, is inhibited by azide.
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