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Mechanisms of Mg 2+ transport in cultured ruminal epithelial cells

2000; American Physiological Society; Volume: 278; Issue: 3 Linguagem: Inglês

10.1152/ajpgi.2000.278.3.g400

1522-1547

Monika Schweigel, Jürgen Vormann, Holger Martens,

Ion Transport and Channel Regulation

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 [Formula: see text], 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 [Formula: see text] and[Formula: see text]/butyrate led to a stimulation of Mg 2+ 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 [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 /[Formula: see text] and Mg 2+ transport may be mediated by another mechanism, perhaps by cotransport of Mg 2+ and[Formula: see text].