Development of rat jejunum: lipid permeability, physical properties, and chemical composition.

This study reports the correlation between jejunal microvillus membrane's chemical composition, physical properties, and lipid permeability over the age range of 9-25 days in the rat. During this period of time the diet varies from entirely maternal milk (9 days, suckling) to milk plus chow (18 days, weaning) to finally predominantly chow (25 days, weaned). Lipid permeability was found to be greatest during the suckling period but decreased rapidly over the subsequent 2 wk of life. Alterations in lipid permeability were correlated with maturation of the microvillus membrane, both in terms of its lipid fluidity and chemical composition. The static component of membrane fluidity did not vary significantly over this time period, in agreement with a constant molar ratio of cholesterol-to-phospholipid (0.87 +/- 0.08, 0.94 +/- 0.07, and 0.84 +/- 0.04 at ages 9, 18, and 25 days, respectively). However, decreasing membrane lipid permeability correlated with decreasing lipid fluidity assessed by probes sensitive to the dynamic component of membrane lipid fluidity within the superficial regions of the bilayer. Furthermore, by measurement of the incremental change in free energy associated with a methylene group partitioning into the microvillus membrane, the hydrophobicity of the membrane could be assessed in vivo. These data demonstrate that suckling rats have the most hydrophobic microvillus membrane (-459 +/- 30 cal/mol), but with maturation a decline in membrane hydrophobicity occurred (-358 +/- 60 cal/mol). The results suggest that during the first 3 wk of life the jejunal microvillus membrane of the rat undergoes a maturation process that is well suited to the change in dietary nutrients ingested. Furthermore, changes in lipid permeability, in vivo properties of the microvillus membrane, and the associated change in lipid fluidity are similar to associations recently recognized in the adult animal.