Absorption of 'non-absorbable' markers and an improved method for determining cholesterol absorption rates

The determination of uptake rates for lipids across absorptive epithelia, using radiolabelled compounds has normally used non-absorbable volume markers to correct gross tissue counts for contamination by adherent mucosal fluid. There is evidence however to indicate that the macromolecules traditionally used as non-absorbable markers are in fact absorbed. This may result in an overestimate of the apparent adherent mucosal fluid with consequent errors in the calculation of lipid uptake. An in-vitro method was used to assess cholesterol absorption by human gallbladder from artificial bile, containing 3H-dextran 70,000 as a 'non-absorbable' volume marker. Both cholesterol and dextran were transported through the mucosa and appeared on the serosal side of the gallbladder. Confirmation of dextran absorption was obtained by electron microscopic examination of the gallbladder epithelium. By using the serosal appearance of dextran as a guide to the amount of absorbed (intracellular) dextran in the tissue samples, we have derived a formula to calculate tissue dextran absorption and, from the revised adherent mucosal volume, have consequently obtained more realistic estimates of tissue lipid uptake. In this study, comparison made between the previous estimate of lipid uptake compared to this new calculation, showed that the previous technique significantly underestimated true lipid uptake.