| Abstract: | These studies were undertaken to determine the role of receptor-independent low density lipoprotein (LDL) transport in cholesterol balance across individual tissues and the whole animal. Homologous LDL, which measures total LDL transport, and methylated heterologous LDL, which measures receptor-independent LDL uptake, were cleared from the plasma at very different rates in the NZ control rabbit (3,900 and 1,010 microliter/hr per kg, respectively) whereas in the WHHL rabbit both preparations were cleared at essentially the same rate (approximately 1,070 microliter/hr per kg). Receptor-independent LDL clearance was detected in all tissues of the NZ control rabbit and these varied from 32 (spleen) to less than 0.5 (skeletal muscle) microliter/hr per g. In contrast, receptor-dependent LDL uptake was found in only about half of these same organs. In the WHHL rabbit, the rates of receptor-independent LDL transport were the same as in the NZ control rabbit, but no receptor-dependent uptake was detected. Using these clearance values it was calculated that in the control rabbit nearly 70% of LDL-cholesterol was removed from the plasma by the liver and 89% of this was receptor-mediated. With loss of receptor activity, however, the burden of LDL degradation was shifted away from the liver so that approximately 70% of LDL-cholesterol uptake took place in the extra-hepatic tissues of the WHHL rabbit. Thus, in the normal animal, the primary function of receptor-dependent LDL transport is to promote the rapid uptake and disposal of plasma LDL by the liver. In the absence of such receptor activity, cholesterol balance across most individual organs and the whole animal remains essentially normal and is mediated by the receptor-independent process. Because of the much lower absolute clearance rates manifested by this transport mechanism, however, substantial and predictable elevations in the circulating plasma LDL-cholesterol levels are required to maintain this balance. |
