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

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Abstract
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 (πeff). Perfusate sodium concentration of 3.7 mmol l−1±0.9 (SD) resulted in an effluent Na+ of 3.3 mmol l−1±1.4. Potassium concentrations of 15 or 25 mmol l−1 remained unaltered. At this cationic steady-state, transmural net water flux (J v ) was linearly correlated (r=−0.96,n=24) to the logarithm of πeff.J v was zero at 634 mosm l−1.L p (μl h−1 cm−2 mosm−1 l) was 0.195 at πeff 85 mosm l−1 and 0.046 at 1,050 mosm l−1. These data characterize the distal colon as an epithelium with high capacity for salt conservation.