Water, sodium, and thiourea transcapillary diffusion in the dog heart

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Abstract
First passage dilution curves of siderophilin-59Fe, HTO [tritiated water], 22Na, and thiourea-35s were simultaneously obtained in the blood-perfused isolated dog heart. Transcapillary passage of the diffusible tracers was estimated from the fractional extractions (E) computed from the zero-time extrapolated concentrations ratio (referred to the intravascular siderophilin-59Fe). E for HTO ranges between 0.90 and 0.96, for 22Na between 0.34 and 0.76, and for thiourea between 0.34 and 0.73. E for the last 2 tracers refers to plasma flow whereas HTO to whole blood flow. There appears to be no consistent difference between 22Na and thiourea extractions in the same experiment. Permeability-surface products (PS) were calculated. PS increased with perfusion rate and at the higher flows used, 1 ml/min. per g, did not reach a constant value. However, the fact PS 22Na/PS HTO (avg 0.26 [plus or minus] 0.10) is lower than the free diffusion in water constants ratio (0.52) is consistent with the hypothesis that there is a restriction to the passage of 22Na (and thiourea) compared to HTO. This is interpreted as due to HTO endo-thelial cell permeability. HTO distributes in only 50% of the organ water in 1 single passage through the capillary bed.