SUMMARYSix mature sheep fitted with rumen fistulae underwent four consecutive feeding periods, of 15 weeks each, i.e. (1) hay only; (2) 36% hay, 64% concentrate; (3) 10% hay, 90% concentrate; and (4) hay only again. The net absorption of sodium, chloride and magnesium from the washed rumen filled with artificial rumen fluids was tested during each feeding period. The artificial rumen fluids varied either in osmolarity (315 and 422 mosmol 1-1) or in pH (6 78 and 4-79). Further, the surface area of rumen papillae was determined. The feeding of increasing proportions of concentrate resulted in an increase of the surface area of rumen papillae and in an elevated net absorption of sodium, chloride and magnesium. Hypertonicity (422 mosmol 1-1) of the artificial rumen fluid caused net influx of water into the rumen but did not influence net absorption of the electrolytes irrespective of the feeding regimen. When the pH was lowered this led to a decrease in net absorption of sodium, chloride and magnesium and in transmural potential difference. The extent of the pH-induced decrease in net ion transport and potential difference was less in sheep receiving high-concentrate diets. When only hay was fed after the period of feeding 90 % concentrate diets the surface area of the papillae, the absorptive capacity of the rumen epithelium and the resistance against low pH returned to control levels. The findings show a reversible adaptive response of rumen epithelial functions to different diets.

The effects of lactate, osmotic pressure and/or pH on net absorption of Na, CI, Mg and water and on transepithelial potential difference were determined in the temporarily isolated reticulo-rumen of sheep. Lactate (0 vs lOmmol/l) did not change the net absorption of Na, C1, Mg and water.Hypertonicity (422 mOsm/l) caused a net influx of water but did not influence the net transport of Na, CI and Mg or the potential difference. A low p H (4.79 vs 6.78) decreased the net absorption of Na by 75 YO and of C1 by 52 % and led to a net secretion of Mg into the rumen. The effects of a low p H were intensified by simultaneous application of lactate and increased osmolarity. It is concluded that ruminal acidosis impairs the absorptive mechanisms of the ruminal epithelium.