Do forestomach epithelia exhibit a Mg2+/2H(+)-exchanger?
In ruminants the forestomachs (reticulum, rumen and omasum) represent the main site of Mg absorption. Readily fermentable carbohydrates enhance Mg availability for ruminants in vivo. The beneficial effect of carbohydrate addition on Mg absorption occurs preintestinally and is associated with changes in the rumen fluid (short chain fatty acids, SCFA, luminal pH, carbon dioxide, NH3, lactate, osmolarity). SCFA and HCO3-/CO2 increase Mg efflux from the isolated reticulorumen in vivo and stimulate 28Mg flux from mucosal to serosal across rumen epithelium in vitro. The stimulatory effect of SCFA on Mg absorption differs between acids, being a function of SCFA absorption and possibly SCFA metabolism. The stimulatory effect of HCO3-/CO2 is mediated by carbonic anhydrase activity. Experimental data suggest that the availability of protons inside the cell might be involved in the stimulation of Mg absorption. The contribution of Mg/H exchange to Mg2+ uptake across the apical membrane of rumen epithelium is discussed in comparison with other possible ways of stimulation (Mg/Na exchange, Mg/anion cotransport, electrogenic and metabolic effects of SCFA). It remains to be shown that Mg2+ uptake into rumen epithelium is directly linked to an efflux of protons. However, an apical Mg/H exchange (in contrast to other suggested Mg transporters) would explain a variety of in vivo and in vitro observations on ruminal Mg absorption.
Leonhard-Marek
S
S
Department of Physiology, School of Veterinary Medicine, Hannover, Germany. leomarek@physiology.tiho-hannover.de
D000042:Absorption; D000818:Animals; D017920:Antiporters / Q000378:metabolism*; D001692:Biological Transport; D050260:Carbohydrate Metabolism; D004848:Epithelium / Q000378:metabolism; D005227:Fatty Acids / Q000378:metabolism; D006859:Hydrogen / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008274:Magnesium / Q000378:metabolism*; D011522:Protons; D012964:Sodium / Q000378:metabolism; D013279:Stomach, Ruminant / Q000378:metabolism*
D017920:Antiporters; D005227:Fatty Acids; D011522:Protons; D006859:Hydrogen; D012964:Sodium; D008274:Magnesium