INTRACELLULAR, INTERSTITIAL AND PLASMA SPACES IN THE RAT MYOCARDIUM IN-VIVO

The aim of this study was to determine tissue water spaces in rat hear t in vivo using [C-14]-inulin or D-[C-14]-mannitol as extracellular ma rkers, and 2,3DPG as whole blood tissue marker. Tracer was injected in to the jugular vein of anaesthetised male Sprague-Dawley rats (60 mg/k g) and allowed to equilibrate for 30 min, A blood sample was then take n and the heart was freeze-clamped at liquid N-2 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 F 0.012 ml/g wet weight t issue, n=9), the intracellular space was found to be 73.1 (inulin) and 70.1% (mannitol). The interstitial space was calculated by subtractin g 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 met hods, respectively. We conclude that (i) there was no significant diff erences in the inulin or mannitol spaces in the in vivo heart (P<0.05) , demonstrating that either marker provides a reliable and quantitativ e 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 com bination with the tracer method to calculate the interstitial and plas ma spaces. (C) 1997 Academic Press Limited.