DETERMINATION OF ENDOGENOUS IONS IN INTERCELLULAR FLUID USING CAPILLARY ULTRAFILTRATION AND MICRODIALYSIS PROBES

Capillary ultrafiltration probes are novel sampling tools for continuo usly monitoring small molecules in the extracellular fluid of awake an imals. Capillary ultrafiltration uses a vacuum applied to hydrophilic membrane fibres and extracts intercellular fluid and quantitatively re covers many small hydrophilic molecules. The effects of continuously r emoving a small amount of fluid from the interstitial space are not kn own. The concentration of sodium, potassium, calicium and inorganic ph osphorus were determined in the collected ultrafiltrates from subcutan eous tissue. These values were compared to literature values and to co ncentrations determined for the same animals using microdialysis. The concentrations of sodium, potassium, calcium and inorganic phosphorous were found to be 140 +/- 4, 3.7 +/- 0.1, 1.1 +/- 0.1 and 1.7 +/- 0.1 mM, respectively, in the subcutaneous ultrafiltrates obtained from rat s. These corresponded very well with literature values and microdialys ates obtained, using pure water as the perfusate, in subcutaneous tiss ue. The concentration of sodium and potassium were determined to be 14 2 +/- 2 mM and 3.6 +/- 0.2 mM, respectively, for the dialysates. Hyper insulinemic-induced decrease in intercellular potassium levels under a euglycemic clamp were monitored using capillary ultrafiltration probe s in rats to further validate this technique for monitoring small mole cule dynamics in the intercellular space. The intercellular level of p otassium in rats decreased from 3.6 +/- 0.5 to 2.6 +/- 0.3 mM after an acute dose of pork insulin.