Acidic pH and short-chain fatty acids activate Na+ transport but differentially modulate expression of Na+/H+ exchanger isoforms 1, 2, and 3 in omasal epithelium

Abstract

Low sodium content in feed and large amounts of salivary sodium secretion are essential requirements to efficient sodium reabsorption in the dairy cow. It is already known that Na+/H+ exchange (NHE) of the ruminal epithelium plays a key role in Na+ absorption, and its function is influenced by the presence of short-chain fatty acids (SCFA) and mucosal pH. By contrast, the functional role and regulation of NHE in omasal epithelium have not been completely understood. In the present study, we used model studies in small ruminants (sheep and goats) to investigate NHE-mediated Na+ transport and the effects of pH and SCFA on NHE activity in omasal epithelium and on the expression of NHE isoform in omasal epithelial cells. Conventional Ussing chamber technique, primary cell culture, quantitative PCR, and Western blot were used. In native omasal epithelium of sheep, the Na+ transport was electroneutral, and it was inhibited by the specific NHE3 inhibitor 3-[2-(3-guanidino-2-methyl-3-oxo-propenyl)-5-methyl-phenyl]-N-isopropylidene-2-methyl-acrylamide dihydrochloride, which decreased mucosal-to-serosal, serosal-to-mucosal, and net flux rates of Na+ by 80% each. The application of low mucosal pH (6.4 or 5.8) in the presence of SCFA activated the Na+ transport across omasal epithelium of sheep compared with that at pH 7.4. In cultured omasal epithelial cells of goats, mRNA and protein of NHE1, NHE2, and NHE3 were detected. The application of SCFA increased NHE1 mRNA and protein expression, which was most prominent when the culture medium pH decreased from 7.4 to 6.8. At variance, the mRNA and protein expression of NHE2 and NHE3 were decreased with low pH and SCFA, which was contrary to the published data from ruminal epithelial studies. In conclusion, this paper shows that (1) NHE1, NHE2, and NHE3 are expressed in omasal epithelium; (2) NHE3 mediates the major portion of transepithelial Na+ transport in omasal epithelium; and (3) SCFA and acidic pH acutely activate Na+ transport but suppress the expression of NHE2 and NHE3 in the longer term. By contrast, the expression of NHE1 is increased by SCFA and acidic pH, indicating a prominent role for NHE1 in the regulation of intracellular pH of omasal epithelium. Our results suggest a regulatable Na+ absorption in ruminal and omasal epithelium. It is of benefit for intracellular pH homeostasis and highly relevant to dairy cows fed on high-concentrate diets.