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The macromolecular transmission from the intestinal lumen into the circulation, and its cessation (intestinal closure), were investigated in young rats and pigs in relation to the enterocytes ability to internalize macromolecules. After gavage feeding of FITC-labelled dextran 70,000 (FITC-dextran) and bovine serum albumin (BSA), the uptake of FITC-dextran into the enterocytes was examined by fluorescence microscopy, and the intestinal transmission of both markers was estimated from their blood serum concentrations. In both preclosure rats (14-days old) and piglets (newborn, unsuckled), high serum concentrations of the markers were correlated with the presence of highly fluorescent enterocytes. Although the transmission of the markers to the blood had ceased in postclosure suckling pigs (24-h old), the enterocytes showed a high fluorescence, indicating that the cellular uptake of FITC-dextran was still high. In the 6-days old piglets, only the distal part of the intestine showed uptake of FITC-dextran. In postclosure rats (21- and 30-days old) and in pigs 4-8 weeks old, no fluorescence in the enterocytes and only trace amounts of markers in the serum could be detected. These results reflect differences in the closure process between the species. In the rat, closure is likely to be due to a decrease in the endocytotic activity of the enterocytes, whereas closure in the pig is related to a cessation of the passage of internalized material into the blood (transcytosis).

A new technique is described for the measurement of the self-diffusion coefficients of protein macromolecules in solution. The method makes use of the phenomenon of Taylor dispersion of a solute introduced into a solvent flowing in the laminar regime through a tube of circular section. Results are reported for the self-diffusion coefficient of cholesterol associated with lipoprotein molecules in dogs' serum at pH 7.4 in the temperature range 18-37 degrees C. The diffusivity of bovine serum albumin in serum has also been studied as a function of temperature at pH 7.4 and 4.7. In the more basic solution, measurements of the diffusivity as a function of protein concentration substantially agree with earlier work. For all the systems studied the diffusivity varies rapidly with temperature. The pH of the solution, in the case of bovine serum albumin, also has a significant effect on the diffusivity of the macromolecule. The latter observation is related to the amount of water bound to the protein molecule in solution.