AJP - Gastrointestinal and Liver Physiology, Vol 250, Issue 6 727-G735, Copyright © 1986 by American Physiological Society
Measurement of resistance of barriers to solute transport in vivo in rat jejunum
H. Westergaard, K. H. Holtermuller and J. M. Dietschy
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.
