Increased negativity of interstitial fluid pressure during the onset stage of inflammatory edema in rat skin.

Interstitial fluid pressure (Pif) was measured in skin of pentobarbital-anesthetized rats during anaphylaxis toward dextran and after subdermal injection of histamine by using sharpened glass capillaries (tip diam 5-7 microns) connected to a servo-controlled counterpressure system. Control Pif averaged -1.5 mmHg (SD = 1.0). After intravenous dextran Pif in the rat paw fell transiently to -3 mmHg up to 20 min and thereafter increased to +1-2 mmHg when edema had developed. To study the full magnitude of the increased negativity of Pif, circulatory arrest was induced 1 min after dextran injection. This procedure prevents accumulation of edema that will cause underestimation of Pif. In this group Pif fell to about -10 mmHg in 20 min and remained at this level throughout the observation period of 90 min. Subdermal injection of 1-10 micrograms histamine in 10 microliters saline reduced Pif to about -6 mmHg within 5 min after injection. Injection of 10 microliters saline increased Pif by +2 mmHg. Indomethacin or cyproheptadine did not alter the response in the above situations. The increased negativity in Pif of about 6-8 mmHg will add directly to normal transcapillary net filtration pressure of 0.5 mmHg and increase the latter pressure 10-20 times.