Mechanisms of Mg(2+) transport in cultured ruminal epithelial cells.
Net Mg(2+) absorption from the rumen is mainly mediated by a transcellular pathway, with the greater part (62%) being electrically silent. To investigate this component of Mg(2+) transport, experiments were performed with isolated ruminal epithelial cells (REC). Using the fluorescent indicators mag-fura 2, sodium-binding benzofuran isophthalate, and 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, we measured the intracellular free Mg(2+) concentration ([Mg(2+)](i)), the intracellular Na(+) concentration ([Na(+)](i)), and the intracellular pH (pH(i)) of REC under basal conditions, after stimulation with butyrate and HCO(-)(3), and after changing the transmembrane chemical gradients for Mg(2+), H(+), and Na(+). REC had a mean resting pH(i) of 6.83 +/- 0.1, [Mg(2+)](i) was 0.56 +/- 0. 14 mM, and [Na(+)](i) was 18.95 +/- 3.9 mM. Exposure to both HCO(-)(3) and HCO(-)(3)/butyrate led to a stimulation of Mg(2+) influx that amounted to 27.7 +/- 5 and 29 +/- 10.6 microM/min, respectively, compared with 15 +/- 1 microM/min in control solution. The increase of [Mg(2+)](i) was dependent on extracellular Mg(2+) concentration ([Mg(2+)](e)). Regulation of pH(i) has been demonstrated to be Na(+) dependent and is performed, for the most part, by a Na(+)/H(+) exchanger. The recovery of pH(i) was fully blocked in nominally Na(+)-free media, even if [Mg(2+)](e) was stepwise increased from 0 to 7.5 mM. However, an increase of [Mg(2+)](i) was observed after reversing the transmembrane Na(+) gradient. This rise in [Mg(2+)](i) was pH independent, K(+) insensitive, dependent on [Mg(2+)](e), imipramine and quinidine sensitive, and accompanied by a decrease of [Na(+)](i). The results are consistent with the existence of a Na(+)/Mg(2+) exchanger in the cell membrane of REC. The coupling between butyrate, CO(2)/HCO(-)(3), and Mg(2+) transport may be mediated by another mechanism, perhaps by cotransport of Mg(2+) and HCO(-)(3).
Schweigel
M
M
Department of Veterinary Physiology, Free University of Berlin, 14163 Berlin, Germany. shweigel@vetmed.fu-berlin.de
Vormann
J
J
Martens
H
H
D000818:Animals; D001639:Bicarbonates / Q000494:pharmacology; D001692:Biological Transport / Q000502:physiology; D002087:Butyrates / Q000494:pharmacology; D002478:Cells, Cultured; D003470:Culture Media; D004847:Epithelial Cells / Q000187:drug effects / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007425:Intracellular Membranes / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism*; D009994:Osmolar Concentration; D011188:Potassium / Q000008:administration & dosage / Q000494:pharmacology; D012417:Rumen / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D012756:Sheep; D012964:Sodium / Q000378:metabolism / Q000502:physiology; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism
D001639:Bicarbonates; D002087:Butyrates; D003470:Culture Media; D017923:Sodium-Hydrogen Exchangers; D012964:Sodium; D008274:Magnesium; D011188:Potassium