Receptor-independent low density lipoprotein transport in the rat in vivo. Quantitation, characterization, and metabolic consequences.

Receptor-independent low density lipoprotein (LDL) transport plays a critical role in the regulation of plasma cholesterol levels; hence, these studies were done to characterize this process in the tissues of the rat. High rates of receptor-independent clearance were found in the spleen, but other organs, like liver, gastrointestinal tract, and endocrine glands manifested lower clearance rates that varied from 3 to 9 microliter/h per g, while the rates in nervous tissue, muscle, and adipose tissue were less than 1 microliter/h per g. Receptor-dependent uptake was much higher in liver (85 microliter/h per g) and adrenal gland (219 microliter/h per g), but was also low in most other tissues. At normal plasma LDL concentrations, 67% of the receptor-dependent transport in the whole animal was accounted for by LDL uptake in the liver. In contrast, the receptor-independent uptake found in the whole animal took place in many organs, including skeletal muscle (20%), liver (16%), small bowel (15%), skin (10%), and spleen (7%). Furthermore, in liver, the rate of cholesterol synthesis could be varied 11-fold, yet the rate of receptor-independent LDL clearance remained constant at approximately 8 microliter/h per g. When the circulating levels of LDL were systematically increased, receptor-independent LDL clearance also remained constant, so that hepatic LDL-cholesterol uptake by this mechanism increased linearly, from 1 to 20 micrograms/h per g, as the plasma LDL-cholesterol level was increased from 10 to 250 mg/dl. Finally, when equal amounts of LDL-cholesterol were delivered into the liver by either the receptor-dependent or receptor-independent mechanism, there was significant suppression of cholesterol synthesis and an increase in cholesteryl esters. Thus, in any situation in which receptor-dependent LDL degradation is lost, cholesterol balance in the whole animal and across individual organs is maintained by receptor-independent mechanisms, although when the new steady state is achieved, circulating levels of LDL must necessarily be very much increased.