| Summary: | Absorption rates for many biologically important compounds are determined by the relative hydrophobicity of the jejunal microvillus membrane. An estimate of this parameter may be obtained by measuring the incremental change in free energy that occurs when a methylene group partitions into the bilayer form an external aqueous solution. Although sensitive, this measurement has been difficult to quantitate in vivo; therefore, these studies have historically been performed in vitro. We describe a rapid, simple technique to measure this parameter in vivo. Furthermore, this method directly quantitates the resistance of aqueous unstirred layers that lie external to the microvillus membrane.
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