Histochemische, elektronenmikroskopische und biochemische Untersuchungen �ber die ATPasen im Vormagenepithel der Ziege

Butyrate Permeation across the Isolated Ovine Reticulum Epithelium

Simple Summary

Short-chain fatty acids are the main source of energy for ruminants. The effective uptake of these substrates from the forestomach is a prerequisite for the health and performance of these animals. Thus far, the mechanisms of uptake have been investigated almost exclusively in the epithelium of the largest forestomach section, the rumen. Previous research suggests that the reticulum is also involved in the uptake of short-chain fatty acids, but the mechanisms involved have not been studied and may differ from those known from the rumen epithelium due to the different milieu in this compartment. To investigate this, ovine reticulum epithelium was mounted in Ussing chambers, and the transport of radiolabeled butyrate (as a representative of short-chain fatty acids) across the tissue was measured with and without the addition of inhibitors of particular transport proteins. Our results show that butyrate can be taken up effectively across the reticulum epithelium via pathways that are energized by the Na⁺/K⁺-ATPase and may involve monocarboxylate transporters, sodium-proton exchangers, and anion channels. However, our results are not completely congruent to those obtained in the rumen epithelium. These modifications could assure the effective uptake of short-chain fatty acids from the reticulum lumen under the particular conditions (p. e. high pH) of this forestomach compartment.

Abstract

We hypothesized that, due to the high pH of this compartment, the reticulum epithelium displays particular features in the transport of short-chain fatty acids (SCFA). Ovine reticulum epithelium was incubated in Ussing chambers using a bicarbonate-free buffer solution containing butyrate (20 mmol L⁻¹). p-hydroxymercuribenzoic acid (pHMB), 5-(N-Ethyl-N-isopropyl)amiloride (EIPA), or ouabain were added to the buffer solution as inhibitors of monocarboxylate transporters, sodium-proton-exchangers, or the Na⁺/K⁺-ATPase, respectively. The short-circuit current (I_sc) and transepithelial conductance (G_t) were monitored continuously while the flux rates of ¹⁴C-labelled butyrate were measured in the mucosal-to-serosal (J_ms^but) or serosal-to-mucosal direction (J_sm^but). Under control conditions, the mean values of I_sc and G_t amounted to 2.54 ± 0.46 µEq cm⁻² h⁻¹ and 6.02 ± 3.3 mS cm⁻², respectively. J_ms^but was 2.1 ± 1.01 µmol cm⁻² h⁻¹ on average and about twice as high as J_sm^but. Incubation with ouabain reduced J_ms^but, while J_sm^but was not affected. The serosal addition of EIPA did not affect J_ms^but but reduced J_sm^but by about 10%. The addition of pHMB to the mucosal or serosal solution reduced J_ms^but but had no effect on J_sm^but. Mucosally applied pHMB provoked a transient increase in the I_sc. The serosal pHMB sharply reduced I_sc. Our results demonstrate that butyrate can be effectively transported across the reticulum epithelium. The mechanisms involved in this absorption differ from those known from the rumen epithelium.

Evidence for NHE3-mediated Na transport in sheep and bovine forestomach

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, pHi. 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, pHi and volume regulation.

Measurement of the total concentration of functional Na+, K(+)-pumps in rumen epithelium

Using the technique of vanadate-facilitated [3H]ouabain binding we have developed a simple and reliable assay for measuring the concentration of [3H]ouabain binding sites in small fresh or frozen biopsies of rumen epithelium papillae. In bovine and ovine rumen epithelium obtained from the cranio-ventral rumen sac the concentration of [3H]ouabain binding sites was 1.6-4.9 nmol g dry wt-1 (n = 32) and 3.7-5.2 nmol g dry wt-1 (n = 6), respectively. When incubated in oxygenated Krebs-Ringer bicarbonate buffer fresh biopsies of rumen epithelium maintained a high K+ and low Na+ content for at least 6 h. Na+ loading of the biopsies induced about 20-fold increase of the Na+, K(+)-pump activity based on measurement of ouabain suppressible net [86Rb+] influx. The ouabain suppressible net influx of [86Rb+] measured in Na+ loaded biopsies showed a close correlation to the [3H]ouabain binding capacity (r = 0.80, P < 0.01) and corresponded to 47 +/- 2% (n = 9) of the theoretical maximum flux rate. The ouabain suppressible net influx of K+ and [86Rb+] were linearly related (r = 0.73; P < 0.001). The net Na+ efflux was 1.21 times the net K+ influx. It is concluded that rumen epithelium has a large capacity for active Na+/K+ transport and that there is agreement between the concentration of [3H]ouabain binding sites in the epithelium and the ouabain suppressible rate of net [86Rb+] influx in Na+ loaded biopsies in spite of some uncertainty about the maximum turnover number of the Na+, K(+)-pump in rumen epithelium.

Mechanisms of sodium and chloride transport across isolated sheep reticulum

1. 22Na+ and 36Cl- fluxes across isolated reticular epithelium of sheep were measured by using the Ussing-chamber technique. 2. Net NaCl absorption driven by Na(+)-K(+)-ATPase was observed under short-circuit conditions. 3. Evaluation of fluxes measured under voltage-clamp conditions indicated that Na+ absorption is mainly electroneutral. 4. Mucosal application of bumetanide, hydrochlorothiazide, or low dose amiloride (10(-4) M) produced no changes in Na+ transport whereas addition of higher doses of amiloride (> or = 10(-3) M) led to a reduction in net Na+ transport. Short chain fatty acids (SCFA) enhanced the amiloride-sensitive Na+ transport. 5. Alterations of JmsNa induced by inhibitors or by SCFA were always accompanied by qualitatively similar changes of JsmNa. Amiloride-sensitive JsmNa was also decreased at low mucosal Na+ concentration. 6. DIDS, SITS, and nitrate reduced both JmsCl and JsmCl. SCFA did not influence chloride transport. 7. It is concluded that Na+ transport is mediated by Na(+)-H+ exchange and by transport processes operating as Na+ self-exchange. Mucosal-to-serosal chloride transport seems partly to depend on anion exchange systems.