Mechanisms of Mg21 transport in cultured ruminal epithelial cells

Schweigel, Monika, Jürgen Vormann, and Holger Martens. Mechanisms of Mg21 transport in cultured ruminal epithelial cells. Am. J. Physiol. Gastrointest. Liver Physiol. 278: G400–G408, 2000.—Net Mg21 absorption from the rumen is mainly mediated by a transcellular pathway, with the greater part (62%) being electrically silent. To investigate this component of Mg21 transport, experiments were performed with isolated ruminal epithelial cells (REC). Using the fluorescent indicators mag-fura 2, sodium-binding benzofuran isophthalate, and 28,78-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, we measured the intracellular free Mg21 concentration ([Mg]i), the intracellular Na1 concentration ([Na]i), and the intracellular pH (pHi) of REC under basal conditions, after stimulation with butyrate and HCO3 , and after changing the transmembrane chemical gradients for Mg21, H1, and Na1. REC had a mean resting pHi of 6.83 6 0.1, [Mg]i was 0.56 6 0.14 mM, and [Na]i was 18.95 6 3.9 mM. Exposure to both HCO3 2 and HCO3 /butyrate led to a stimulation of Mg21 influx that amounted to 27.7 6 5 and 29 6 10.6 μM/min, respectively, compared with 15 6 1 μM/min in control solution. The increase of [Mg]i was dependent on extracellular Mg21 concentration ([Mg]e). Regulation of pHi has been demonstrated to be Na1 dependent and is performed, for the most part, by a Na1/H1 exchanger. The recovery of pHi was fully blocked in nominally Na1-free media, even if [Mg]e was stepwise increased from 0 to 7.5 mM. However, an increase of [Mg]i was observed after reversing the transmembrane Na1 gradient. This rise in [Mg]i was pH independent, K1 insensitive, dependent on [Mg]e, imipramine and quinidine sensitive, and accompanied by a decrease of [Na]i. The results are consistent with the existence of a Na1/Mg21 exchanger in the cell membrane of REC. The coupling between butyrate, CO2/HCO3 , and Mg21 transport may be mediated by another mechanism, perhaps by cotransport of Mg21 and HCO3 .