Abstract
Na absorption across the cornified, multilayered, and squamous rumen epithelium is mediated by electrogenic amiloride-insensitive transport and by electroneutral Na transport. High concentrations of amiloride (>100 μM) inhibit Na transport, indicating Na(+)/H(+) exchange (NHE) activity. The underlying NHE isoform for transepithelial Na absorption was characterized by mucosal 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 short-chain fatty acid-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. Expression of NHE1 and NHE3 has been previously demonstrated by RT-PCR, and earlier experiments with isolated rumen epithelial cells have shown the activity of both NHE1 and NHE3. Obviously, both isoforms are involved in the regulation of intracellular pH, pH(i). However, transepithelial Na transport is only mediated by apical uptake via NHE3 in connection with extrusion of Na by the basolaterally located Na-K-ATPase. The missing involvement of NHE1 in transepithelial Na transport suggests that the proposed "job sharing" in epithelia between these two isoforms probably also applies to forestomach epithelia: NHE3 for transepithelial transport and NHE1 for, among others, pH(i) and volume regulation.
Highlights
SODIUM IS THE MAJOR CATION in isotonic saliva of ruminants [2], with 1.2–1.5 mol Na being daily secreted by an adult sheep [8], which is flowing into the rumen together with 0.1– 0.2 mol dietary Na
The short circuit current (Isc) and Jnet Na were almost identical in the presence of mucosal S3226 (1.07 Ϯ 0.05 vs. 1.12 Ϯ 0.28 Eq·cmϪ2·hϪ1), indicating inhibition of electroneutral Na transport via NHE3
The data of the present study are in agreement with findings in many previous studies of Na transport in rumen epithelium: Jnet Na in the mucosal-serosal direction, with Jnet Ͼ Isc (11, 15, 28, 36, 38 –39), and stimulation of Jnet Na by short-chain fatty acids (SCFA) [7, 15, 38]
Summary
SODIUM IS THE MAJOR CATION in isotonic saliva of ruminants [2], with 1.2–1.5 mol Na being daily secreted by an adult sheep [8], which is flowing into the rumen together with 0.1– 0.2 mol dietary Na. Jnet Na is larger than the short circuit current (Isc), indicating electroneutral Na transport [39]. This in vitro observation (Jnet Na Ͼ Isc) has been confirmed in all following studies with rumen from sheep [11], goats [5], cattle [5, 38], or deer [40]. The localization of NHE3 in the apical membrane and NHE1 in the basolateral membrane of polarized cells has led, in epithelia, to the generally accepted model of “job sharing” between these two NHE isoforms: NHE3 mediates transepithelial electroneutral Na transport [9], and NHE1 is involved in the regulation of pH and homeostasis of cell volume [33]
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callMore From: American Journal of Physiology-Regulatory, Integrative and Comparative Physiology
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
