Measurement of resistance of barriers to solute transport in vivo in rat jejunum.

Assessment of intestinal absorptive function requires techniques for correcting transport constants for diffusion barrier resistance. These studies were done to develop such techniques for use in vivo. In one method the functional thickness of the unstirred water layer (d) in rat jejunum was quantitated electrically, and its minimal surface area (Sw) was measured directly. From these values the diffusion barrier resistance (d/Sw) decreased from 0.041 to 0.022 as the perfusion rate of the intestine was increased from 1.5 to 15 ml/min. In the second method apparent passive permeability coefficients (*P) were measured for a series of saturated fatty acids, and these increased with chain length. However, at the longest chain lengths tested, *P became proportional to their free diffusion coefficients, indicating that uptake was limited by the rate of diffusion up to the microvillus surface. From the rates of uptake of such diffusion-limited probes, the diffusion barrier resistance was again calculated and found to decrease from 0.041 to 0.022 as the rate of perfusion was increased from 1.5 to 15 ml/min. Over this same range of perfusion rates, the apparent Michaelis constant (*Km) for D-glucose transport decreased from 18.2 to 10.0 mM. Using either set of resistance terms and these apparent Km values, the true Km value for glucose transport in vivo was found to equal 0.8 mM when the barrier resistance was extrapolated to zero. Thus these data indicate that diffusion-limited probes can be utilized to measure unstirred layer resistance in the intestine of a live animal so that absolute transport parameters can be determined in vivo in experimental animals and, presumably, in humans.