Interstitial compliance and transcapillary Starling pressures in cat skin and skeletal muscle

Interstitial compliance, defined as change in local interstitial fluid volume (delta IFV) divided by the corresponding change in local interstitial fluid pressure (delta IFP), has been measured in skin and muscle of pentobarbital-anesthetized cats. IFP was measured with micropipettes and IFV as the difference between 51Cr-ethylenediaminetetraacetic acid space and 125I-human serum albumin space. IFV was altered by overhydration (saline infusion, 10% of body wt, or by femoral vein ligation plus saline infusion, 20% of body wt) or by dehydration (peritoneal dialysis with 20% glucose). Control IFV in skin and muscle averaged 1.45 (SD 0.44, n = 22) and 0.37 (SD 0.04, n = 22) ml/g dry wt, respectively, with corresponding IFP of -1.9 (SD 0.4, n = 22) and -0.8 (SD 0.6, n = 22) mmHg. During dehydration, IFV fell by 5.5 and 10.4% per mmHg fall in IFP in skin and muscle, respectively. The maximal rise in IFP during overhydration was about 2 mmHg in both tissues. Analysis of covariance showed that compliance was lower in cats than in rats during dehydration (P less than 0.05). At subnormal IFV, IFP will therefore be more powerful in opposing capillary absorption of interstitial fluid in cats than in rats. IFP is, however, of similar importance in edema prevention in the two species.