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The rumen epithelium adapts morphologically and functionally in response to variations of luminal nutrient and electrolyte availability. Changes in the expression and/or activity of active transport mechanisms may playa role in these processes. In addition to the Na+/K+-ATPase, a vacuolar-type H+-ATPase (vH+-ATPase) has recently been identified in ruminal epithelial cells, REG In this study, the expression and localization of the vH+-ATPase in the rumen epithelium as well as its contribution to pHi regulation have
Expression and localization of vH+-ATPase and Na+/K+-ATPase in rumen papillae were explored by use of RT-PCR and Western blotting combined with immuno-histochemistry.
The vH+-ATPase activity was estimated from the rate of intracellular pH (pHi) recovery after butyrate-induced acid-loading in isolated REG The pHi was measured using the fluorescence probe BCECF. Different types of active transport systems were identified
by pharmacological means.
All data confirmed the presence of a functional active vH+-ATPase in the rumen epithelium. Studies with the specific inhibitor foliomycin revealed a considerable role (30%) of the protein in cellular pHi regulation. Its activity was modulated by various diet-dependent
factors, e.g. extracellular CI- and short chain fatty acid concentrations. In rumen papillae, obtained from high yielding dairy cows, immunostaining clearly showed plasma membrane localization of Na+/K+- and vH+-ATPase. In contrast, dominant cytoplasmatic staining of vH+-ATPase has been observed in rumen tissue from animals at lower performance
and feeding levels. Also, a heterogeneous distribution of Na+/K+- and vH+-ATPase between
cells of the rumen epithelium has been verified at a morphological and functional level.
The results suggest an important role of the vH+-ATPase in ruminal transport
processes and in feed-induced adaptation of forestomach functions.