Kinetics of pentose permeation in the perfused rat heart.

1. The fractional permeation of heart cell water by L-arabinose and D-xylose after a standard period of perfusion is less the higher the extracellular pentose concentration.2. At any given pentose concentration the kinetics of permeation are consistent with passive diffusion through the cytoplasm or with transport across the cell membrane by a saturable carrier.3. The concentration dependence of permeation is inconsistent with passive diffusion.4. The kinetics of L-arabinose permeation are consistent with a carrier having V(max) = 2.2 m-mole/l. extracellular water per min and a half-saturation concentration [K] of 0.5 x 10(-4)M. The corresponding figures for D-xylose are V(max) = 1.4, [K] = 1.6 x 10(-4)M.