Tissue spaces in rat heart, liver, and skeletal muscle in vivo

Tissue spaces were determined in rat heart, liver, and skeletal muscle in vivo using isotopically labeled [¹⁴C]inulin. Tracer was injected into the jugular vein of pentobarbital-anesthetized male Sprague-Dawley rats. After a 30-min equilibration period, a blood sample was taken, and heart, liver, and gastrocnemius muscle were excised and immediately freeze clamped at liquid nitrogen temperatures. The extracellular inulin space was 0.209 ± 0.006 (n = 13), 0.203 ± 0.080 (n = 7), and 0.124 ± 0.006 (SE) ml/g wet wt tissue (n = 8) for heart, liver, and skeletal muscle, respectively. Total tissue water was 0.791 ± 0.005 (n = 9), 0.732 ± 0.002 (n = 9), and 0.755 ± 0.005 ml/g wet wt tissue (n = 10) for heart, liver, and skeletal muscle, respectively. Expressed as a percentage of total tissue water, the intracellular space was 73.6, 72.2, and 83.7% for heart, liver, and skeletal muscle, respectively. With use of 2,3-diphospho-D-glyceric acid as a vascular marker, the interstitial space was calculated by subtracting the counts in tissue due to whole blood from total tissue counts and dividing by plasma counts. The interstitial space was 18.8, 22.4, and 14.5% of total tissue water, with accompanying plasma spaces of 7.7, 5.3, and 1.8% for heart, liver, and gastrocnemius muscle, respectively. The tracer method used in this study provides a quantitative assessment of water distribution in tissues of nonnephrectomized rats that has applications for calculation of tissue ion and metabolite concentrations, gradients, and fluxes under normal and pathophysiological conditions.