Six 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
l-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 l-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.