Role of LDL Receptors in the In Vitro Uptake and Degradation of LDL in the Media of Rabbit Thoracic Aorta

The possible role of plasma low-density protein (LDL) receptors in the uptake and degradation of LDL in the whole arterial wall was investigated by comparison of the in vitro uptake of l-native LDL (nLDL) and l-methylated LDL (mLDL) by the media of deendothelialized rabbit thoracic aorta excised at in vivo length and pressurized to 70 mm Hg, taking the advantage that mLDL is not recognized by the LDL receptor. The distribution of the relative concentrations of nLDL (C± and mLDL (Cm) across the wall was obtained using a serial frozen sectioning technique. The aorta was incubated under three different conditions for varying periods of incubation in order to analyze separately the processes of binding, binding-internalization, and degradation. At 39°C, in which binding-internalization and degradation occurred, Cn was significantly higher than Cm at each position across the media. The mean medial Cn/Cm ratio was 1.36±0.15 (n=5) after 1 hour of incubation, and decreased to 1.23±0.22 (n=7) after 2 hours of incubation and to 1.13±0.11 (n=5) after 4 hours of incubation. At 4°C, in which internalization and degradation were blocked, the Cn/Cm ratio reflected the surface nLDL binding alone; the CnCm ratio was 1.47±0.20 (n=5) after 4 hours of incubation, higher than the value obtained at 39°C. To investigate whether degradation of nLDL occurred after receptor binding, the interstitial LDL was washed out by an LDL-free solution after 2-hour incubation at 39°C. After 30 minutes of washout, the Cn/Cm ratio decreased to 1.06±0.20 (n=5) in the inner media and was unchanged in the outer media. After 1 hour of washout, the ratio declined to 0.57±0.18 (n=7) in the inner part of the media and increased progressively to 1 at the media-adventitia boundary. The CnCm ratio, at 0.67±0.12 (n=5), was practically constant throughout the media after 2 hours of washout. The nLDL degradation rate across the media was obtained from the comparison of nLDL and mLDL before and after the washout. A steep decreasing gradient hi nLDL degradation rate was observed from the luminal to the external surface. The mean medial nLDL degradation rate value was 9.6±4.5 μl/cm wet tissue/hr. We concluded that functional LDL receptors participate in the uptake and degradation of LDL in the whole aorta.