Hydraulic permeability coefficient and sodium steady-state luminal concentration of the in vivo perfused rat distal colon.

The distal colon (rat) was perfused in vivo at low rates (1-2 ml/h). Dialyzed polyethyleneglycol 4,000 (PEG) was used to vary the luminal osmotic activity (pi eff). Perfusate sodium concentration of 3.7 mmol 1-1 +/- 0.9 (SD) resulted in an effluent Na+ of 3.3 mmol 1-1 +/- 1.4. Potassium concentrations of 15 or 25 mmol 1-1 remained unaltered. At this cationic steady-state, transmural net water flux (Jv) was linearly correlated (r = -0.96, n = 24) to the logarithm of pi eff. Jv was zero at 634 mosm 1-1. Lp (microliters h-1 cm-2 mosm-1 l) was 0.195 at pi eff 85 mosm 1-1 and 0.046 at 1,050 mosm 1-1. These data characterize the distal colon as an epithelium with high capacity for salt conservation.