Effects of short chain fatty acids and carbon dioxide on magnesium transport across sheep rumen epithelium.
Short chain fatty acids (SCFAs) and CO2 have been shown to stimulate net Mg2+ efflux from the isolated reticulorumen in vivo. To investigate the underlying mechanisms of Mg2+ transport we performed Ussing chamber and microelectrode experiments and measured 28Mg2+ fluxes across sheep rumen epithelium in vitro. In the presence of SCFAs mucosal-to-serosal Mg2+ flux (Jm-sMg) amounted to 82.3 +/- 7.8 nmol cm-2 h-1 and serosal-to-mucosal Mg2+ flux (Js-mMg) to 3.2 +/- 0.7 nmol cm-2 h-1. Replacing SCFAs with gluconate caused a 50% reduction of Jm-sMg, whereas Js-mMg was not affected. Among the SCFAs, n-butyrate was more effective in stimulating Jm-sMg than acetate, propionate or iso-butyrate. Eliminating HCO3(-)-CO2 from SCFA-containing solutions did not affect Mg2+ fluxes, whereas the same replacement in SCFA-free solutions led to a further reduction in Jm-sMg. Jm-sMg decreased after the addition of ethoxyzolamide to SCFA-free, bicarbonate buffered solutions. Decreasing mucosal pH from 6.4 to 5.4 increased Jm-sMg in SCFA-free, bicarbonate buffered solutions. SCFAs had no effect on the apical membrane potential of rumen epithelial cells. The experiments show that both SCFAs and CO2 stimulate Mg2+ transport through an increase in Jm-sMg, most probably via stimulation of a Mg(2+)-2H+ exchange mechanism. SCFAs may have additional metabolic effects on Mg2+ transport.
Leonhard-Marek
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Department of Physiology, School of Veterinary Medicine, Hannover, Germany. head@physiology.tiho-hannover.de
Gäbel
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Martens
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D000818:Animals; D001639:Bicarbonates / Q000494:pharmacology; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology; D002245:Carbon Dioxide / Q000494:pharmacology / Q000502:physiology*; D002257:Carbonic Anhydrase Inhibitors / Q000494:pharmacology; D005016:Ethoxzolamide / Q000494:pharmacology; D005232:Fatty Acids, Volatile / Q000737:chemistry / Q000494:pharmacology / Q000502:physiology*; D005753:Gastric Mucosa / Q000378:metabolism*; D005942:Gluconates / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008274:Magnesium / Q000378:metabolism*; D012417:Rumen / Q000378:metabolism*; D012756:Sheep; D013329:Structure-Activity Relationship
D001639:Bicarbonates; D002257:Carbonic Anhydrase Inhibitors; D005232:Fatty Acids, Volatile; D005942:Gluconates; D002245:Carbon Dioxide; D008274:Magnesium; C030691:gluconic acid; D005016:Ethoxzolamide