The radial protein concentration profile in the interstitial space of the rat ileal mesentery

The radial distribution of the protein concentration in the interstitium between arteriolar and venular vessels of the ileal mesentery of the rat was examined. Protein mass was determined by means of uv ultramicrospectrophotometry (UMS) and the relative volume distribution by means of fluorescence microscopy (FM) using the Na fluorescein and FITC-dextran (10,000 mol wt). UMS revealed gradients for protein mass from the vessels out into the interstitial space. FM showed a uniform distribution of fluorescence in the interstitium between the vessels. A gradient for protein mass without a gradient for volume distribution signifies the presence of a concentration gradient for protein in the interstitial space. The protein concentration across the arteriolar wall drops from 5.4 +/- 0.24 (SD) to 2.6 +/- 0.65% and across the venular wall from 5.4 +/- 0.24 to 3.3 +/- 0.43%. From the perivascular site the protein concentration declines exponentially reaching a minimum average interstitial concentration of 1.6 +/- 0.56%. Minimal protein concentration occurred at a point 37 +/- 6.4% of the 295 +/- 37 micron distance from the arteriolar to the venular vessels. In view of this distribution, it is unlikely that lymph or direct samples of interstitial fluid are representative of the perivascular protein concentration.