Beta adrenoreceptor in vascular smooth muscle with special reference to subcellular localization and number of binding sites.
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The binding of monoiodinated cyanopindolol (ICYP) to membranes from canine and rat aortas, mesenteric arteries and mesenteric nerves has been studied. Binding of ICYP was shown by differential centrifugation and sucrose gradient purification as well as digitonin treatment to be directly related to the content of arterial muscle plasma membrane assessed by marker enzymes. The Kd values obtained were all in the range of 10 to 30 pM but maximum binding site (Bmax) values varied depending upon the extent of contamination by nonplasmalemmal membranes. The more purified plasma membrane fractions have Bmax many times higher than those reported in the literature. Although purified membrane fractions all had ICYP binding characterized by a single binding site with similar Kd values, binding to some crude fractions was complex. In addition, plasma membranes for mesenteric nerves, removed carefully by dissection in our study, were shown to contain ICYP binding sites with a similar Kd and Bmax values to those in arterial muscle membrane. Using arterial muscle purified membranes after removal of nerves by dissection, Bmax values from mesenteric arteries were demonstrated to be much larger than those from aorta in both species. Although the necessity of purifying the membranes depends on the objectives of a given experiment, our result strongly suggest that measurement of changes in receptor densities requires an analytical approach which provides plasma membrane-enriched fractions derived from a single cell type and purified to a known and comparable degree, especially when changes in density related to experimental or disease processes are being investigated.
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