Luminal hyperosmolarity decreases Na transport and impairs barrier function of sheep rumen epithelium.

The effects of luminal hyperosmolarity on Na and Cl transport were studied in rumen epithelium of sheep. An increase of luminal osmotic pressure with mannitol (350 and 450 mosm/l) caused a significant increase of tissue conductance, G (T), which is linearly correlated with flux rates of (51)Cr-EDTA and indicates an increase of passive permeability. Studies with microelectrodes revealed, that an increase of the osmotic pressure caused a significant increase of the conductance of the shunt pathway from 1.23 +/- 0.10 (control) to 1.92 +/- 0.14 mS cm(-2) (450 mosm/l) without a change of fractional resistance. Hyperosmolarity significantly increased J (sm) and reduced J (net) Na. The effect of hyperosmolarity on J (ms) Na is explained by two independent and opposed effects: increase of passive permeability and inhibition of the Na(+)/H(+) exchanger. Hypertonic buffer solution induced a decrease of the intracellular pH (pH(i)) of isolated ruminal cells, which is consistent with an inhibition of Na(+)/H(+) exchange, probably isoform NHE-3, because NHE-3-mRNA was detectable in rumen epithelium. These data are in contrast to previous reports and reveal a disturbed Na transport and an impaired barrier function of the rumen epithelium, which predisposes translocation of rumen endotoxins and penetration of bacteria.