Submitted on March 22, 2005
Accepted on December 20, 2005
Lymphatic chylomicron size is inversely related to biliary phospholipid secretion in mice
Anniek Werner1*, Rick Havinga1, Frank G. Perton1, Folkert Kuipers1, and Henkjan J. Verkade1
1 Pediatric Gastroenterology, Department of Pediatrics, Groningen University Institute for Drug Exploration, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands
* To whom correspondence should be addressed. E-mail: a.werner{at}med.umcg.nl.
Background: Biliary phospholipids (PL) stimulate dietary fat absorption by facilitating intraluminal lipid solubilization and by providing surface components for chylomicron assembly. Impaired hepatic PL availability induces secretion of large VLDL, but it is unclear whether chylomicron size depends on biliary PL availability. Biliary PL secretion is absent in Mdr2(-/-) mice, whereas it is strongly increased in essential fatty acid (EFA) deficient mice. We investigated lymphatic chylomicron size and composition in mice with absent (Mdr2(-/-)) or enhanced (EFA deficiency) biliary PL secretion and in their respective controls, under basal conditions and during intraduodenal lipid administration.
Methods: EFA deficiency was induced by feeding mice a high-fat EFA-deficient diet for eight weeks. Lymph was collected by mesenteric lymph duct cannulation, with or without enteral lipid administration. Lymph was collected in 30-minute fractions for up to 4 hours, and lymphatic lipoprotein size was determined by dynamic light scattering techniques. Lymph lipoprotein subfractions were isolated by ultracentrifugation and lipid composition was measured.
Results: Lymphatic lipoproteins were significantly larger in Mdr2(-/-) mice than in Mdr2(+/+) controls, either without (+50%) or with (+25%) enteral lipid administration, and molar core-surface ratios were increased [TG-to-PL ratio: 4.4±1.4 (Mdr2(-/-)) vs. 2.7±0.8 (Mdr2(+/+)), p<0.001]. In contrast, EFA-deficient mice secreted significantly smaller lipoproteins into lymph than EFA-sufficient controls (173±32 nm vs. 236±47 nm; p<0.001), with correspondingly decreased core-surface ratios [TG-to-PL ratio: 3.0±1.0 (EFA-deficient) vs. 6.0±1.9 (EFA-sufficient), p<0.001]. Chylomicron size increased during fat absorption in both EFA-deficient and EFA-sufficient mice, but the difference between the groups persisted.
Conclusions: Present results strongly suggest that the availability of biliary PL is a major determinant of the size of intestinally produced lipoproteins, both under basal conditions and during lipid absorption. Altered chylomicron size may have physiological consequences for postprandial chylomicron processing.
