Distribution of Ions and Water between Tissue Compartments in the Perfused Left Ventricle of the Rat Heart

Left ventricles from rat hearts were perfused through the coronary blood vessels for periods up to 90 minutes with solution containing radioactively labeled sulfate, sucrose, urea, glycerol, or chloride. Urea and glycerol equilibrate with all of tissue water. By contrast, 35 SO 4 and sucrose- 14 C equilibrate very rapidly with 40% of total water and slowly with an additional 20%; they are excluded from 40% of the tissue water. The two "cellular" compartments (C 2 , which equilibrates slowly with SO 4 and sucrose, and C 3 , from which SO 4 and sucrose are excluded) both lose water when hearts are perfused with a solution made hypertonic with NaCl. Chemical analyses for K, Na, and Cl, and measurements of the rate of equilibration of 36 C1 show that C 2 has low contents of Cl and Na. Experiments in which extracellular NaCl was replaced osmole for osmole by KCl according to the method of Boyle and Conway suggest that the boundaries of C 2 and C 3 may have different ionic permeabilities. These observations indicate that the division of mammalian heart muscle into tissue compartments is more complex than conventionally assumed, a conclusion reached by Bozler for frog heart muscle. They are inconsistent with the usual assumption that cardiac cellular water is homogeneous.