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Na transport across the cornified, multilayered, and squamous rumen epithelium is mediated by electrogenic amiloride-insensitive transport and by electroneutral Na transport, which is inhibited by high concentration of amiloride (> 100 μM) indicating Na+/H+ exchange (NHE) activity. The underlying NHE isoform for transepithelial Na transport was characterized by application of the specific inhibitor HOE642 for NHE1 and S3226 for NHE3 in Ussing chamber studies with isolated epithelia from bovine and sheep forestomach. S3226 (1 μM; NHE3 inhibitor) abolished electroneutral Na transport under control conditions and also the SCFA-induced increase of Na transport via NHE. However, HOE642 (30 μM; NHE1 inhibitor) did not change Na transport rates. NHE3 was immunohistochemically localized in membranes of the upper layers toward the lumen. We conclude that Na transport via NHE in bovine and sheep rumen epithelium is mediated by the isoform NHE3 and not by NHE1, which has recently been demonstrated by immunostaining in the stratum granulosum of bovine rumen. Since previous RT-PCR has demonstrated the expression of NHE1 and NHE3, and since experiments with isolated rumen epithelial cells have shown the activity of both NHE1 and NHE3, the proposed ?job sharing? in epithelia between these two isoforms probably also applies to forestomach epithelia: NHE3 for transepithelial transport and NHE1 for, amongst others, pH and volume regulation.