Compliance of the interstitial space in rats. I. Studies on hindlimb skeletal muscle.

Interstitial compliance, defined as the ratio between changes in interstitial fluid volume (delta IFV) and interstitial fluid pressure (delta IFP), was determined for rat skeletal muscle. IFV was measured as the extravascular distribution space for 51Cr-EDTA, while sharpened micropipettes connected to a servo-controlled counterpressure system were used to measure IFP. The experimental protocol was designed to bring about acute (2-4 h) and chronic (24-28h) tissue over- and dehydration. During dehydration, the average compliance was 0.056 ml/g dry weight . mmHg, corresponding to 1.40 ml/100 g wet tissue . mmHg, and was not significantly different in acute and chronic experiments. In hydration (acute and chronic), compliance increased several-fold when IFV increased. Even at greatly increased IFV, IFP did not rise more than 1 to 1.5 mmHg above control level. Since control IFV amounts to 10 ml/100 g wet tissue, IFV will decrease by 14% when IFP falls by 1 mmHg from this control level. Provided unchanged interstitial protein mass the dehydration will cause interstitial fluid colloid osmotic pressure to increase by somewhat more than 1 mmHg--from a control level of 9 mmHg. Furthermore, since IFP was not increased by more than 1 to 1.5 mmHg during hydration, an increase in IFP plays a minor role in edema-prevention compared to dilution and/or washout of interstitial proteins.