Permeation of long-chain fatty acids and alcohols in rat intestine.

Uptake of long-chain fatty acids and alcohols from micellar bile salt solutions has been studied with everted sacs of rat jejunum. These unidirectional uptake rates are linearly related to lipid concentration if bile salt concentration is constant, but are essentially independent of lipid concentration over a 2.5-fold concentration range if lipid and bile salt concentrations are maintained at a constant ratio. Uptake of lipid from various micellar bile salt solutions was directly related to the experimentally varied monomer activity of solute, thus allowing conversion of uptake rates to permeation coefficients. Natural logarithm of permeation coefficients (ln P) increased linearly for saturated fatty acids of 12--18 carbons, equivalent to an incremental free energy of -695 cal.mol-1 per --CH2--. Alcohols of 10- to 14-carbon chain lengths had a similar relationship on ln P to number of carbons. Previously determined permeation coefficients for fatty acids of 2--10 carbons are now seen to be a nonlinear portion of the curve for ln P versus chain length for all saturated fatty acids.