Intracellular, interstitial and plasma spaces in the rat myocardium in vivo

The aim of this study was to determine tissue water spaces in rat heart in vivo using [14C]-inulin or D-[14C]-mannitol as extracellular markers, and 2,3DPG as whole blood tissue marker. Tracer was injected into the jugular vein of anaesthetised male Sprague-Dawley rats (60 mg/kg) and allowed to equilibrate for 30 min. A blood sample was then taken and the heart was freeze-clamped at liquid N2 temperatures within 5 s of opening the chest cavity. Extracellular space was calculated by dividing the total tissue counts (d/min/g) by plasma counts (d/min/g). The extracellular inulin and mannitol spaces were 0.212+/-0.010 and 0.236+/-0.012 ml/g wet weight tissue, respectively (n=5). Expressed as a percentage of total tissue water (0.79+/-0.012 ml/g wet weight tissue, n=9), the intracellular space was found to be 73.1 (inulin) and 70.1% (mannitol). 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 19.4 and 21. 9%; and the plasma space was 7.5 and 8.0% for the inulin and mannitol methods, respectively. We conclude that (i) there was no significant differences in the inulin or mannitol spaces in the in vivo heart (P<0.05), demonstrating that either marker provides a reliable and quantitative estimate of extracellular space, and (ii) 2,3-DPG can be used as a reliable indicator of tissue whole blood which can then be used in combination with the tracer method to calculate the interstitial and plasma spaces.