Volume-pressure relationship (compliance) of interstitium in dog skin and muscle

The relationship between changes in interstitial fluid volume (IFV) and pressure (IFP) during over- and dehydration was estimated in hindlimb skin and skeletal muscle of 17 dogs. IFV was measured as the extravascular distribution volume of 51Cr-EDTA after nephrectomy and IFP with micropipettes, wick-in-needle technique (WIN), and perforated capsules. Control IFP (micropipettes) averaged -1.7 +/- 0.5 (n = 17) and 0.0 +/- 1.0 mmHg (SD; n = 17) in skin and muscle, respectively, not different from corresponding WIN and capsule pressures in subcutis. Control IFV was 0.93 +/- 0.34 (n = 17) and 0.42 +/- 0.11 ml/g dry wt (SD; n = 17) in skin and muscle, respectively. Peritoneal dialysis with 20% glucose reduced IFV by 27% in skin and by 44% in muscle, whereas micropipette IFP fell gradually by approximately 4 mmHg in skin and muscle. Compliance during dehydration was 7.6 and 12.5% change in IFV per millimeter Hg fall in IFP (micropipettes) in skin and muscle, respectively, whereas compliance in subcutis based on perforated capsule pressure was 2.0% change in IFV per millimeter Hg. The maximal increase in IFP during overhydration was approximately 2 mmHg in both skin and muscle. Skin and muscle compliance in dogs did not differ significantly from that of rats and cats. We conclude that the difference in compliance among previous studies mainly results from different methods for IFP measurement but is probably also influenced by the state of hydration before altering IFV.