The binding of calcium by isolated sarcoplasmic reticulum from cow uterus was studied. Sarcoplasmic reticulum was prepared by differential centrifugation. Three fractions were obtained: I, sedimented between 2,500–15,000 x g; II at 40,000 x g; and III, at 150,000 x g. Fraction II was further purified on a sucrose density gradient. All three fractions contained considerable amounts of intrinsic calcium, mostly in fraction I. Calcium binding in the presence of ATP1 and Mg also was greatest in fraction I, followed by fraction II, with less in fraction III. Without ATP no calcium was taken up. 5 and 10 mM sodium azide partially inhibited calcium binding in fraction I, but not in fraction II, suggesting the presence of some mitochondria or mitochondrial fragments in fraction I. Calcium binding in fraction II was completely inhibited by 3 mM salyrgan; this fraction thus appears to be sarcoplasmic reticulum. ATPase activity was found in all three fractions, highest in fraction II. It is computed that calcium binding in fractions I and II, on the basis of a 50% yield of protein, is sufficient to elicit contraction by supplying calcium to the contractile proteins of the smooth muscle cell and to regulate relaxation and contraction.

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