Subcellular membrane fractions were isolated ‘from dog mesenteric arteries by differential and isopynic sucrose density gradient centrifugations. Isolated membrane fractions were characterized by marker enzyme activities, morphological features and sodium dodecyl sulfate-polyacrylamide gel electrophoretic patterns. Our results show that the microsomal fraction isolated by conventional differential centrifugation was highly heterogenous and contained substantial amount of plasma membranes which could be further enriched as a light density membrane fraction on a discontinuous sucrose density gradient. The microsomal fraction and its subfractions were vesicular in appearance under electron microscope and were capable of binding and actively transporting Ca2+. The binding of Ca2+ and ATP-supported Ca2+-transport in the presence or absence of oxalate paralleled the distribution of plasma membrane marker enzyme activities suggesting that plasma membranes in vascular smooth muscle may play a major role in handling Ca2+ and thus the control of contractile function.
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