Lipoprotein composition and transport in the pig and dog cardiac lymphatic system.

SUMMARY The cardiac lymphatic system of pigs was found to consist of valved efferent lymph vessels draining into a cardiac lymph node, which is similar to the lymphatic system found in dog and man. The interstitial lipoproteins of the heart were studied by sampling cardiac efferent lymph in anesthetized fasting pigs (n — 4) and dogs (n • • 5). In pigs, the lymph flow rate was 2.0 ± 0.7 ml/hr (mean ± SRM) and in dogs, 1.2 ± 0.3 ml/hr. The total cholesterol concentration was 54.9 ± 8.0 and 33.8 ± 6.3 mg per 100 ml in cardiac lymph of piga and dogs, respectively. The ratio of lymph :plasma cholesterol differed in the two species: 0.82 in pigs and 0.46 in dogs. More than 90% of cardiac lymph cholesterol was recovered in high density lipoprotein (HDL) and in low density lipoprotein (LDL) in both pigs and dogs. In pigs, LDL wag the major cholesterol-carrying lipoprotein in plasma and lymph, whereas in canine plasma and lymph, this was HDL. The appearance of '"I-LDL in canine cardiac lymph following iv administration was rapid and reached a plateau by 1 hour. The t'A appearance of '"I-LDL in lymph was 5 minutes compared to tV4 clearance of plasma (rapid phase) of 44 minutes. The lymph: plasma ratio of LDL specific activity wai 0.48 ± 0.15, suggesting continuous addition of cold LDL in the interstitial space. The high concentration and turnover of cholesterol-rich lipoprotein in cardiac lymph suggest that the predilection of the coronary bed for atherosclerotic degeneration may be a function of the interstitial lipoprotein concentration and composition as well as the plasma concentration of lipoprotein species. PIGS are particularly useful in the study of cardio-vascular disease (Detweiler, 1966) because of the many similarities between pig and human hearts and great vessels (Lumb, 1966) and the comparable physical and chemical properties of pig and human very low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL) (Mahley and Weisgraber, 1974a). Furthermore , LDL is the major cholesterol-carrying lipo-protein in swine (Mahley et al., 1975), as in humans (Fredrickson et al., 1978), and arteriosclerosis may be induced by dietary cholesterol feeding (Mahley et al., 1975). A significant proportion of LDL and HDL is metabolized and degraded by extravascular, extra-hepatic tissues. Sniderman et al. (1975) demonstrated that after iv administration of !2S I-LDL in hepatectomized pigs, the plasma clearance of 12fl I-LDL …