Quantification in vivo of increased LDL content and rate of LDL degradation in normal rabbit aorta occurring at sites susceptible to early atherosclerotic lesions.

While the exact mechanisms that initiate atherosclerotic lesions are unknown, considerable evidence supports a role for low density lipoprotein (LDL). We investigated whether in the normal rabbit, LDL metabolism in areas of aorta that are destined to become lesioned during cholesterol feeding differed from the metabolism in adjacent lesion-resistant aorta. These studies took advantage of the predictable pattern of early atherosclerotic lesions in the cholesterol-fed rabbit. Early lesions occur diffusely in the aortic arch and ascending aorta and distal to branch orifices in the abdominal aorta and the descending thoracic aorta. Arterial rates of irreversible degradation of LDL and concentrations of intact LDL were measured in susceptible and resistant sites with homologous doubly labeled LDL. LDL was labeled directly with 131I and with 125I-tyramine cellobiose. The latter label provides a highly sensitive means to determine the sites and rates of lipoprotein degradation in vivo. The arterial concentration of intact LDL in the lesion-prone aortic arch was 3.12 +/- 0.45 micrograms LDL cholesterol/g (n = 14), 3.6 +/- 0.69 times that in the relatively lesion-resistant descending thoracic aorta (p less than 0.001). The rate of LDL degradation in the aortic arch was 3.14 +/- 0.41 micrograms LDL cholesterol/g/day, 2.14 +/- 0.24 times that in the descending thoracic aorta (p less than 0.001). In the abdominal aorta, the LDL (per gram wet weight) concentration and degradation rate (per square centimeter surface area) at branch sites exceeded that at nonbranch sites by 88 +/- 11% (p less than 0.001) and by 61 +/- 8% (p less than 0.001), respectively. These data provide evidence that in the normal rabbit, which does not develop atherosclerotic lesions, focal elevations of arterial LDL degradation rate and concentrations of intact LDL occur at sites that first develop atherosclerotic lesions in the hypercholesterolemic animal. These differences in LDL metabolism may be linked causally to the propensity to develop atherosclerotic lesions at these sites.