Protein concentration and colloid osmotic pressure of interstitial fluid collected by the wick technique: Analysis and evaluation of the method

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Abstract

The fluid contained in multifilamentous nylon wicks implanted subcutaneously in rats for 30 to 120 min has a protein concentration about one-half that of the serum. The determinants of wick fluid protein concentration, colloid osmotic pressure (COPw), and hydrostatic pressure (Pw) were further analyzed: Insertion of the wick greatly increased capillary protein permeability, as demonstrated by rapid transfer of labeled albumin from blood to wick fluid. The rapid blood-to-wick albumin transport almost ceased within 30 min and could be greatly reduced by indomethacin or cyproheptadine, indicating “normalization” of capillary permeability. Indomethacin treatment resulted in “subnormal” protein concentration and COP in saline-loaded wicks, but simultaneously wick fluid content and Pw were reduced. These changes were prevented by preloading the wicks with serum, or by using thinner wicks. In any case, the numerical sum of COPw and Pw averaged about 10 mm Hg. Thus the wick seems to act as a colloid osmometer, equilibrating against capillaries and more remote interstitium. Provided sufficient amounts of proteins are added to the wick fluid before or after insertion, Pw stabilizes at 0 to −2 mm Hg, equal to the tissue fluid pressure measured by the “wick-in-needle” technique, and the COPw of 8–9 mm Hg should then be equal to the COP of undisturbed interstitial fluid. The protein composition in wick fluid is not necessarily the same as in the interstitial fluid.

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Recipient of a Research Fellowship from the Norwegian Research Council for Science and the Humanities.

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