Colloid osmotic pressure of interstitial fluid in rat subcutis and skeletal muscle: comparison of various wick sampling techniques.

Interstitial fluid for measurement of colloid osmotic pressure (COPi) was collected from rat subcutis and skeletal muscle with three wick methods; saline-soaked wicks implanted in vivo, titration with wicks preloaded with serial dilutions of rat plasma implanted postmortem (crossover method) and dry wicks implanted in vivo and post-mortem. When compared in 12 rats, the mean subcutaneous COPi of 14.2 and 13.3 mmHg as estimated with the crossover method and dry wick method, respectively, both exceeded the 10.7 mmHg obtained in saline-soaked wicks implanted for 60 min. Colloid osmotic pressure (COP) in fluid from dry wicks implanted subcutaneously was slightly lower than in plasma 5 min after implantation in vivo, then fell gradually, but remained stable the last 30 min of the 60 min in vivo implantation period and the first 30 min post-mortem, when again starting to rise. COP in fluid isolated from dry wicks implanted in muscle post-mortem approached or even exceeded that of plasma, suggesting leakage of intracellular proteins to wick fluid. Electrophoresis of dry wick fluid from subcutis showed a pattern similar to that of plasma, while protein fractions not present in plasma were observed in muscle wick fluid. For measurement of COPi we recommend the use of dry wicks implanted post mortem for subcutis and saline-soaked or dry wicks implanted in vivo for 90-120 min for skeletal muscle.