Mechanisms of Mg2+ transport in cultured ruminal epithelial cells

Net Mg2+ absorption from the rumen is mainly mediated by a transcellular pa thway, with the greater part (62%) being electrically silent. To investigat e this with isolated ruminal epithelial cells (REC). Using the fluorescent indicators mag-fura 2, sodium-binding benzofuran isophthalate, and 2',7' -b is(2-carboxyethyl)-5 (6)-carboxyfluorescein, we measured the intracellular free Mg2+ concentration ([Mg2+](i)), the intracellular Na+ concentration ([ Na+](i)), and the intracellular pH (pH(i)) of REC under basal conditions, a fter stimulation with butyrate and HCO,, and after changing the transmembra ne chemical gradients for Mg2+,H+, and Na+. REC had a mean resting pH(i) of 6.83 +/- 0.1, [Mg2+](i) was 0.56 +/- 0.14 mM, and [Na+](i) was 18.95 +/- 3 .9 mM. Exposure to both HCO3- and HCO3-/butyrate led to a stimulation of Mg 2+ influx that amounted to 27.7 +/- 5 and 29 +/- 10.6 mu M/min, respectivel y, compared with 15 +/- 1 mu M/min in control solution. The increase of [Mg 2+](i) was dependent on extracellular Mg2+ concentration ([Mg2+](e)). Regul ation 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 b locked in nominally Na+-free media, even if [Mg2+](e) was stepwise increase d from 0 to 7.5 mM. However, an increase of [Mg2+](i) was observed after re versing the transmembrane Na+ gradient. This rise in [Mg2+](i) was pH indep endent, K+ insensitive, dependent on [Mg2+](e), imipramine and quinidine se nsitive, and accompanied by a decrease of [Na+](i). The results are consist ent with the existence of a Na+/Mg2+ exchanger in the cell membrane of REC. The coupling between, butyrate, CO2/HCO3-, and Mg2+ transport may be media ted by another mechanism, perhaps by cotransport of Mg2+ and HCO3-..