Sodium chloride absorption and solute-linked water flow across 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. The lumen was perfused with hypo-, iso-, and hyperosmotic salt solutions. Polyethylene glycol (PEG 4000) and [(14)C]inulin served as water markers.2. The maximal net Na(+) flow (J(Na)) from the mucosal to the serosal side was nearly the same in the two states of hydration: normal birds 308 mu-equiv/kg.hr, dehydrated birds 281 mu-equiv/kg.hr, while the J(Na) was half maximal at luminal Na(+) concentrations of 99 and 43 mu-equiv/l. respectively. The ;affinity' for Na(+) in the dehydrated bird was thus twice that in the normal bird. K(+) was secreted into the gut lumen at a constant rate against the electrochemical gradient, J(K) = -97 mu-equiv/kg.hr (S.E. = 5). Cl(-) was absorbed from lumen to plasma down the electrochemical gradient with J(Cl) ranging from 0 to 94 mu-equiv/kg.hr. The low J(Cl) was observed at low luminal NaCl concentrations when the J(Na) was also small.3. The solute-linked water flow, J(vs), occurring in the absence of an osmolality difference across the epithelium, was 1.1 mul. H(2)O/mu-equiv Na(+) in normal birds and 1.5 in dehydrated birds. The J(vs) was calculated as an operational parameter in experiments with luminal osmolalities different from plasma osmolality by subtracting the water flow observed in an experiment without Na(+) in the perfusion fluid from the water flow in an experiment with Na(+) containing perfusion fluids, both fluids being of the same osmolality. J(vs) was maximal at luminal osmolalities close to plasma osmolality. These observations are compatible with the hypothesis that the J(vs) is due to an osmotic flow into a confined region between the cells.4. When the perfusion rate was lowered from 5-9 ml./kg.hr to 0.8-1.0 the incoming perfusion fluid osmolality at which net water flow across the epithelium was zero went up from 100 to 180 m-osmolal higher than plasma osmolality. This observation suggests that a significant fraction of ureteral urine in the dehydrated bird may be absorbed in the coprodeum and large intestine.