Absorption and exchange of water across rumen epithelium
Dobson, A.; Sellers, A.F.; Gatewood, V.H.
American Journal of Physiology 231(5 Pt. 1): 1588-1594
1976
ISSN/ISBN: 0002-9513 PMID: 998805 Document Number: 101580
The osmotic pressure of solutions in the ventral sac of the rumen of the conscious cow was varied with NaCl or mannitol. The mucosal blood flow measured by HTO clearance was least when the lumen contained an isotonic solution and rose threefold when the rumen was hypo- or hypertonic to plasma by 150 mosmol/kg; thus osmotic gradients across the rumen epithelium stimulated mucosal blood flow. Using osmotic gradients small enough to avoid blood flow stimulation, the net water flow could be enhanced by butyrate, a chemical stimulator of blood flow. Thus water movement was partly limited by blood flow. This implied an appreciable change in osmotic pressure of the capillary blood towards that of the rumen contents. The relative importance of blood flow, membrane permeability and solute uptake on water transport was assessed. The osmotic pressure in the rumen was stationary when the rumen solution was distinctly hypotonic to plasma. The absorbate in the absence of an osmotic gradient was thus hypertonic. The net uptake of solute increased rapidly when the solution in the lumen was hypertonic to plasma. This gave rise to a more rapid rate of change of osmotic pressure in the rumen.