Wick sampling of interstitial fluid in rat skin: further analysis and modifications of the method

We compared modifications of the wick technique for analysis of interstitial fluid in rat subcutis. Nylon wicks were implanted for 60 min in back skin of rats after anesthesia with pentobarbital or after sacrifice by potassium chloride injection. Wicks were implanted dry or loaded with saline or varied dilutions of rat serum. Implantation of dry wicks and wicks loaded with diluted serum in living, anesthetized animals produced similar results; the protein concentration of wick fluid averaged about 60% that of the plasma protein concentration. The saline loaded wicks produced wick fluid with a lower protein concentration, average about 45% that of plasma protein concentration. The lower concentrations apparently resulted from simple dilution. Wick fluid sampled from dead animals had similar total protein concentrations, but in the dead animals there was a lower concentration of the large plasma proteins and a relatively higher concentration of the smaller proteins.

Wick implantation in living animals causes a transitory inflammatory reaction and a decrease in the size selectivity of macromolecular sieving, but local osmotic forces bring about a concentration equilibrium with undisturbed interstitium. Implantation of dry wicks in subcutis either in vivo or post mortem provides a simple, direct method for sampling the total protein concentration and colloid osmotic pressure of interstitial fluid. Implantation of dry wicks postmortem permits measurement of individual component protein concentrations and evaluation of molecular selectivity between plasma and interstitium.