[Characterization of HDL receptor and its ligand in cultured smooth muscle cells isolated from rabbit aorta].

The characteristics of HDL3 binding sites on cultured smooth muscle cells originally isolated from rabbit aorta were studied with 125I-labelled rabbit HDL3. The results indicated that specific and high-affinity HDL3 binding sites were present on the surface of smooth muscle cells (Bmax = 0.321 micrograms/mg cell protein; KD = 5.6 x 10(-8) mol/L). The presence of a high concentration of non-labelled apo A1 blocked the 125I-HDL3 binding rate by 50%. 125I-HDL3 pretreated (blocked) with rat antirabbit apo A1 IgG lost up to 70% of its original binding activity. Additionally, rabbit apo A1 was isolated, purified and its amino acid composition analyzed. The binding rate of 125I-HDL3 to smooth muscle cells was not affected by the presence of various concentrations of cholesterol. It was also relatively unaffected by trypsin or pronase treatment and independent of Ca2+ concentration. The optimal temperature and pH were 4 degrees C and 2 respectively. All of these characteristics differ from those of the LDL receptors. These results suggest that apo A1 is a major ligand of HDL3 receptors.