Increased negatively of interstitial fluid pressure in rat skin contributes to the edema formation induced by Zymosan.

Increased negatively of interstitial fluid pressure (Pif) contributes to rapid edema formation in several acute inflammatory reactions attesting to an "active" role for the loose connective tissues in the transcapillary fluid exchange and edema formation under these circumstances. The present study reports the effect of the complement activator Zymosan on Pif, transcapillary fluid, and albumin flux. Micropipettes (tip diameter 5 to 7 microns) connected to a servo-controlled counterpressure system were used to measure Pif in rat dermis. When compared to saline injection, subdermal injection of 1 mg Zymosan in 10 microliters 0.15 M NaCl increased total tissue water by 1.6 ml/g dry weight in 5 min, corresponding to about 150% increase in interstitial fluid volume. Pif increased from +0.4 to +3.7 mm Hg. Increased negativity of Pif can be masked by the edema formation which will increase Pif. Measurements were therefore also performed after circulatory arrest, when transcapillary fluid flux and edema formation are abolished. Using this experimental protocol Pif fell from +0.3 mm Hg to -2.5 mm Hg 5 min after subdermal injection of Zymosan and remained at this level throughout the observation period of 90 min. Injection of saline alone after circulatory arrest increased Pif transiently by about 1 mm Hg. Thus, subdermal injection of Zymosan causes increased negativity of Pif by about 4 mm Hg. Although the lowering of Pif itself will explain a minor part of the increased fluid filtration, the results attest to the role of loose connective tissues being active in the edema-generating process also in the inflammatory reaction induced by Zymosan.