Transport of Na and Cl across the epithelium of ruminant forestomachs: rumen and omasum. A review.

Rumen: 1. It is generally accepted that sodium and chloride is transported across the rumen epithelium of ruminants in the mucosal-serosal direction by an active transport mechanism. In all in vitro studies the short circuit current, Isc, was significantly lower than the net transport of sodium, JNanet. It was concluded that most of the sodium is transported by an electrically silent mechanism. A Na, Cl cotransport and/or a double exchange system Na/H and Cl/HCO3 was proposed. 2. Recent in vitro studies in our laboratory revealed that the Na/H exchange inhibitor amiloride (1 mmol/l) reduced JNanet by 70%. The Na/K/2Cl-cotransport inhibitor bumetanide or furosemide (1 mmol/l) had no effect. 3. Replacement of permeable anions led to an inhibition of JNanet by 87%. However, under these experimental conditions Isc and JNanet were not significantly different. The remaining small electrogen transport of sodium was not influenced by mucosal amiloride. 4. It is concluded that two transport systems are operating in the ruminal epithelium: a Na/H exchange system which accounts for 80-90% of JNanet and an electrogen Na transport which is not sensitive against amiloride. Omasum: 1. The data concerning the transport of sodium and chloride in the omasum are very limited. In vivo measurements of flow rates demonstrated that sodium is absorbed by the omasum (40-60% of inflow in bull calves, 10-20% in sheep) and chloride is secreted. 2. In vitro studies with isolated preparation of sheep omasum epithelium demonstrated a net transport of Na and Cl in the mucosal-serosal direction. JNanet is completely abolished by ouabain (0.1 mmol/1) and JClnet by replacement of Na.