Abstract
The rate of transfer of labeled molecules across a sheet of quiescent cat right ventricle separating two chambers containing chemically identical solutions was followed at 23 degrees C. For the diffusion of sucrose, SO(4), and Na the experimental points fit the entire time course of the plot of the diffusion equation for a plane sheet. The tortuosity factor of the extracellular diffusion channel, lambda was 1.44 +/- 0.05 (mean +/- SEM) for sucrose and similar for SO(4) and Na. The fractional area available for extracellular diffusion, calculated from lambda and the slope of the linear asymptote approached during steady state diffusion, was 0.17-0.23 for both impermeant species (sucrose, SO(4), Na) and permeant species (water, urea, glycerol). Permeant molecules showed a characteristic prolongation of the approach to the steady state, with an unexplained "hump" in the curve for water. The observed time courses for diffusion of permeant molecules are interpreted in terms of a model proposed by Fatt et al. for diffusion through linear porous media containing dead-end pore volume. Large molecules like inulin and dialyzed dextran (diameter 150 to 180 A) diffuse through the sheet. These molecules may have a reflection coefficient sigma > 0. The fraction of muscle water occupied by the sucrose diffusion channel is significantly smaller than the 3 hr. mannitol, sucrose, and inulin spaces.