Salt and water permeability of the epithelium of the coprodeum and large intestine in the normal and dehydrated fowl ( Gallus domesticus ). In vivo perfusion studies

1. The transmural net flow of salt and water in the coprodeum and large intestine of normal and dehydrated hens was investigated by means of an intraluminal in vivo perfusion technique. 2. The lumen was perfused with a raffinose‐electrolyte solution having a low sodium concentration (Na + = 1 m‐equiv/l.). The osmolality of the solution was adjusted in the range 66–585 m‐osmolal by adding raffinose. Polyethylene glycol 4000 (PEG) served as a water marker. The experiments permitted estimation of the passive transport parameters: the reflexion coefficient (σ) of the penetrating solutes (predominantly NaCl), the Na + mobility (ω s ), and the osmotic water permeability coefficient ( P osm ). 3. When the luminal fluid had the same osmolality as plasma the net water flow ( J v ) was zero, indicating a σ of unity. The net flow of Na + was zero, (at J v = 0) and a transmural electric potential difference close to zero was present both in normal and in dehydrated birds. This indicates an ω s of zero. When lumen osmolality was higher than that of plasma, no ‘solvent drag’ effect on Na + was demonstrated in the serosa to mucosa (s—m) direction. 4. The P osm appeared to be independent of the luminal osmolality in the range of ±200 m‐osmolal from plasma osmolality. In normal birds the P osms—m was 3·2 μl./kg.hr.m‐osmolal, the P osmm—s 5·8 μl./kg.hr.m‐osmolal. In dehydrated birds these values were 3·6 and 10·0 respectively. Thus there seems to be rectification of water flow, and it varies with the state of hydration. 5. A net K + flow of 15–50 μ‐equiv/kg.hr in the m—s direction and a net Cl μ flow of 10–50 μ‐equiv/kg.hr in the s—m direction were observed. No relationship was observed between the flow of these solutes and the net water flow.