Rumen epithelial cells adapt magnesium transport to high and low extracellular magnesium conditions

Abstract

A protein of~70-kDa was identified as a candidate Na + /Mg 2+ exchanger in rumen epithelial cells (REC). Melastatin-related Transient Receptor Potential 7 (TRPM7) and Magnesium Transporter 1 (MagT1) transcripts and, from them, encoded proteins were also detected. The regulation of these Mg transport pathways by extracellular [Mg] changes was the main focus of this study. Therefore, a 24-h pre-incubation of ovine REC in control (1.2 mM), low (0.12 mM)-Mg, and high (5 mM)-Mg medium was performed. Na + /Mg 2+ exchangers, TRPM7 and MagT1 abundance and activity were investigated by Western blot analysis, flow cytometry, immunocytochemistry and fluorescence spectroscopic measurements of [Mg 2+ ] i changes. Inhibitors were employed to differentiate Na + /Mg 2+ exchanger-mediated (imipramine) and channel-mediated (cobalt(III)hexaammine, nitrendipine) Mg transport. Basal [Mg 2+ ] i (0.40 ± 0.02 mM) was not influenced by pre-incubation in low-or high-Mg medium. However, compared with control REC (4.1 ± 0.7 μM/min), such cells showed reduced (2.8 ± 0.6 μM/min) or elevated (6.4 ± 0.9 μM/min) Mg extrusion rates that correlated with a decreased (25%) and increased (38%) expression of the putative Na + /Mg 2+ exchanger protein, respectively. Low-and high-Mg pre-incubated REC were both characterized by an increased (30-40%) influx capacity. In low-Mg REC, the latter resulted mainly from a strong activation of the TRPM7-related transport component. The data thus clearly demonstrate the intrinsic regulation of REC transmembrane Mg transport.

The Mg2+ transporter MagT1 partially rescues cell growth and Mg2+ uptake in cells lacking the channel-kinase TRPM7

“…From our review of the literature, MagT1 gene expression does not appear to be uniformly regulated in all tissues. Other studies have shown that in low concentrations of Mg 2+ , mammary epithelial MagT1 expression remains unchanged [20], while rumen epithelial cells actually downregulate MagT1 expression [17,18]. These results imply that there may be differential regulation of Mg 2+ and its transport machinery in various cell types.…”

Human gene SLC41A1 encodes for the Na+/Mg2+ exchanger

“…Although not selective, these are the current pharmacological agents of choice for manipulating the Na ϩ /Mg 2ϩ exchanger functional activity (1,14,38,52). In a majority of studies (14,17,37,48,52), a Ϸ50% inhibition of Mg 2ϩ efflux has been reported after the application of imipramine (100 to 500 M).…”

“…It is long thought that a key element in this process is regulated Mg 2ϩ efflux either via an anion-dependent mechanism (36) or a putative Na ϩ /Mg 2ϩ exchanger (28). Evidence for the existence of both transport systems has been derived from functional studies with various cell types (1,2,14,17,24,38), but their molecular identity is unclear until today. However, within the last few years several genes (summarized in Ref.…”

Solute carrier family SLC41: what do we really know about it?

“…The A1-related Mg 2+ extrusion was not directly linked to anion (Cl − , HCO 3 − ) transport. However, it showed typical features of NME such as 1) activation by an increased [Mg 2+ ] i [26], 2) strict dependence on extracellular Na + [31, 32], 3) inhibition by imipramine and quinidine [33] and, 4) regulation via cAMP-dependent PKA [33]. Thus, unquestionably, A1 represents NME, the predominant Mg 2+ efflux system in mammalian cells [32, 34].…”