Distribution volumes and macromolecular mobility in rat tail tendon interstitium.

This study explores a centrifugation technique for estimating interstitial fluid composition and macromolecular transport. Rat tail tendon supported by a nylon net was subjected to three consecutive 15-min centrifugations at 3,000, 6,000, and 14,000 revolutions per minute (rpm) or 3, 10, and 20 min at 6,000 rpm. Colloid osmotic pressure (COP) and concentrations of albumin, total protein, and hyaluronan in subsequent centrifugate fell as exponential functions of accumulated centrifuged volume, reaching 10-30% of initial level at an accumulated volume corresponding to 8% of tendon volume. Intercepts for zero centrifugation were 11 mmHg (COP), 22 mg/ml (albumin), and 39 mg/ml (total protein), probably reflecting concentrations in protein-accessible interstitial volume. Corresponding serum values were 19 mmHg, 34 mg/ml, and 63 mg/ml. Tendon distribution spaces were 0.62 (H2O), 0.57 (51Cr-labeled-EDTA), and 0.22 ml/g wet wt (albumin). The progressive fall in centrifugate concentrations probably reflects increasing resistance to macromolecular transport, with a sieving coefficient for albumin falling from 1 to 0.35, or increasing contribution of fluid from protein-excluded space. The effect was reversed by rehydration, which caused increased concentrations in centrifugate. Low hyaluronan concentrations in centrifugate (0.25 mg/ml) compared with that of whole tendon (0.4 mg/g wet wt) reflect either a large "bound" fraction in tissue or marked sieving of hyaluronan in normohydration.