QUANTITATIVE AND QUALITATIVE CHARACTERIZATION OF APOLIPOPROTEIN-B CONTAINING LIPOPROTEINS PRODUCED BY THE VISCERAL RAT YOLK-SAC IN 2 DIFFERENT IN-VITRO SYSTEMS - ORGAN-CULTURE AND ISOLATED EPITHELIAL-CELLS IN SUSPENSION-CULTURE

Tissue pieces as well as isolated epithelial cells taken from visceral rat yolk sacs at the 18th day of gestation were able to synthesize an d to secrete apo B containing lipoproteins floating in the density ran ges of VLDL, IDL and LDL. In all three density classes only the high m olecular weight apo B was detectable. VLDL secreted from yolk sac tiss ue or isolated epithelial cells lacked apo E and apo C. Studies with c ycloheximide revealed that about 77% of the particles was delivered fr om a pool of preformed lipoproteins. Evidence was given that also de n ovo-synthesis of apo B containing lipoproteins took place in the tissu e segments and the isolated epithelial cells. Most of apo B mass (90%) and of apo B radioactivity (60%) secreted by the tissue pieces of the visceral yolk sac floated in the density range 1.020-1.064 g/ml, the remainder being found in the d less than or equal to 1.020 g/ml fracti on. In contrast, only 40% of apo B mass and 45% of apo B radioactivity delivered from isolated epithelial cells belonged to the d = 1.020-1. 064 g/ml fraction. LDL released from yolk sacs and isolated epithelial cells contained more triacylglycerols (41% vs. 25%) and had a larger mean diameter (24 nm vs. 21.8 nm) than those obtained from fetal rat s erum, whereas the comparatively small VLDL produced in vitro (mean dia meter = 34 nm) contained less triacylglycerols (46% vs. 60.5%) and mor e protein (20% vs. 10.2%) in comparison with fetal serum VLDL (mean di ameter = 42.3 nm). Incubation experiments with [I-125]VLDL led to the conclusion that the lipase secreted by yolk sac tissue into the medium could not be responsible for the conversion of VLDL into LDL, thus su pporting our view of a direct LDL secretion by visceral rat yolk sacs.