The distribution of labeled low-density lipoproteins across the rabbit thoracic aorta in vivo

Abstract

Transmural concentration profiles of 125I-labeled low-density lipoproteins (LDL) within the descending thoracic aorta were determined as a function of time following intravenous injection into normal conscious rabbits. The animals were sacrificed after 10 min to 67 h, and the descending thoracic aorta was immediately excised, opened longitudinally, rinsed and frozen. Samples of frozen aorta were sectioned parallel to the intimal surface and washed with trichloroacetic acid (TCA) prior to counting. Up to 4 h, transmural concentration profiles of TCA-precipitable radioactivity had steep gradients near the intimal surface, moderate gradients near the medial-adventitial border, and were relatively flat in the middle of the media. After 24 h, the steep intimal gradient had disappeared. Concentration levels were otherwise comparable to those at 4 h. All gradients disappeared and concentration levels were lower after 67 h. The rate of accumulation of TCA-precipitable radioactivity was initially rapid (measurable concentrations were found throughout the media after only 10 min), although it was less than that of [125I] albumin observed in a previous study. The results are consistent with entry of [125I]LDL into the media from both the luminal and adventitial sides and with gradual degradation of the labeled LDL within the aortic wall. Approximate calculations indicate that the LDL mass transfer resistance associated with the intimal endothelium is about an order of magnitude greater than that associated with the media.