Isolation of interstitial fluid from skeletal muscle and subcutis in mice using a wick method

Until recent years, mice were sparsely used in physiological experiments, and therefore, data on the basic cardiovascular parameters of mice are lacking. Our aim was to gain access to interstitial fluid and thereby study transcapillary fluid dynamics in this species. Using a modified wick method, we were able to isolate interstitial fluid from subcutis and skeletal muscle in mice. Three-stranded, dry, nylon wicks were inserted post mortem in an attempt to avoid local inflammation and thus eliminate protein extravasation and wick contamination. Colloid osmotic pressure (COP) was measured with a colloid osmometer for submicroliter samples and averaged (means +/- SE) 18.7 +/- 0.4 in plasma, 9.1 +/- 0.4 in subcutis, and 12.3 +/- 0.5 mmHg in muscle. HPLC of plasma and wick fluid showed similar patterns except for some minor peaks eluting in the <40-kDa region. Plasma protein extravasation as determined by 125I-labeled human serum albumin showed that contamination of wick fluid by plasma proteins was negligible (<2%). Capillary hyperfiltration induced by intravenous infusion of saline (10% of body wt) was reflected in tissue fluid isolated by wicks as shown by the average postinfusion COP values of 14.5 +/- 0.6, 6.8 +/- 0.3, and 7.7 +/- 0.4 mmHg in plasma, subcutis, and muscle, respectively. We conclude that the wick technique can be easily adapted for use in mice and may represent a reliable method to isolate interstitial fluid and study transcapillary fluid flux in this species.