Characterization of HDL binding sites and its ligand in cultured smooth muscle cells of rabbit's aorta.

Cultured smooth muscle cells of rabbit aorta were studied by 125I labelled rabbit HDL3. Saturation curves, measured at 4 C, showed the presence of two different components: the low-affinity non-saturable binding portion and the high-affinity binding portion (Kd about 5.6 x 10(-8) mol/L and Bmax about 0.321 micrograms/mg cell protein). Scatchard analyses of the high-affinity binding portion suggest the presence of single class binding sites. Binding of rabbit HDL3 to cultured smooth muscle cells was relatively resistant to trypsin or pronase, and independent to Ca2+. The binding rate of 125I-HDL3 to the smooth muscle cells was highest at 4 C and the optimal pH was 2. Additionally, presence of high concentration apoAI reduced 50% of the binding rate of 125I-HDL3, and 125I-HDL3, being pretreated (blocked) with rat anti-rabbit apoAI IgG of different concentrations lost 70% of its original binding rate with smooth muscle cells. The results suggest that rabbit aorta smooth muscle cells possess a specific binding sites for apoE-free HDL which recognizes apoAI as a ligand.