Mechanisms of Mg2+ transport in cultured ruminal epithelial cells

Abstract

Net Mg2+ absorption from the rumen is mainly mediated by a transcellular pathway, with the greater part (62%) being electrically silent. To investigate this component of Mg2+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 Mg2+ concentration ([Mg2+]i), the intracellular Na+ concentration ([Na+]i), and the intracellular pH (pHi) of REC under basal conditions, after stimulation with butyrate and [Formula: see text], and after changing the transmembrane chemical gradients for Mg2+, H+, and Na+. REC had a mean resting pHi 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 [Formula: see text] and[Formula: see text]/butyrate led to a stimulation of Mg2+ influx that amounted to 27.7 ± 5 and 29 ± 10.6 μM/min, respectively, compared with 15 ± 1 μM/min in control solution. The increase of [Mg2+]iwas dependent on extracellular Mg2+ concentration ([Mg2+]e). Regulation of pHi has been demonstrated to be Na+ dependent and is performed, for the most part, by a Na+/H+ exchanger. The recovery of pHi was fully blocked in nominally Na+-free media, even if [Mg2+]e was stepwise increased from 0 to 7.5 mM. However, an increase of [Mg2+]i was observed after reversing the transmembrane Na+ gradient. This rise in [Mg2+]i was pH independent, K+ insensitive, dependent on [Mg2+]e, imipramine and quinidine sensitive, and accompanied by a decrease of [Na+]i. The results are consistent with the existence of a Na+/Mg2+ exchanger in the cell membrane of REC. The coupling between butyrate, CO2/[Formula: see text] and Mg2+ transport may be mediated by another mechanism, perhaps by cotransport of Mg2+ and[Formula: see text].