Heterogeneity of Dog Interstitial Fluid (peripheral Lymph) High Density Lipoproteins: Implications for a Role in Reverse Cholesterol Transport

The heterogeneity of dog interstitial fluid (peripheral lymph) high density lipoprotein (HDL) was investigated and compared to plasma HDL. Interstitial fluid and plasma HDL of normal and cholesterol-fed dogs was subfractionated by ultracentrifugation and affinity and molecular weight sieving chromatography. Both plasma (P) and interstitial fluid (L) HDL can be subfractionated into a larger fraction (P-I and L-I) and a smaller one (P-II and L-II). Cholesterol feeding induces a large increase in the P-I and L-I component of HDL, but the increase in L-I is far greater in proportion than that of P-I. Furthermore, L-I of cholesterol-fed dogs appears to be almost exclusively discoid in shape, while only approximately 15% of particles in P-I are discoidal. The discoid HDL of L-I is reflected in its chemical composition: 28% unesterified cholesterol, 6% cholesteryl ester, 45% phospholipid, and 21% protein. It contains large amounts of apoE in addition to apoA-I and apoA-IV. We found that the association of apoE with discoid particles is frequent, but not necessary. Calculations based on known protein mass and quantitation of discoid particles on electron micrographs suggest that the concentration of discoid particles in the peripheral lymph of cholesterol-fed dogs is about fourfold that of the plasma of the same animal. These findings provide strong circumstantial evidence for the peripheral formation of discoid HDL, perhaps as an early event in reverse cholesterol transport.