Characterization of Mg2+ transport in brush border membrane vesicles of rabbit ileum studied with mag-fura-2

Abstract

Mg2+ transport in rabbit ileal brush border membrane vesicles (BBMV) was characterized by means of a modified mag-fura-2 technique. In the presence of an i>o Na+ gradient, BBMV showed a saturable Mg2+ uptake with a Km of 1.64 mmol l-1. There was no evidence of an overshoot. K+, Li+, and choline+ were as effective as Na+ in stimulating Mg2+ transport. In contrast, only a small amount of Mg2+ transport was observed in the presence either of an o>i Na+ gradient, or in an Na+ equilibrium or in the absence of Na+. Moreover, the findings that Na+ efflux was not stimulated but inhibited by outside Mg2+ and that the nonfluorescent amiloride-analogues DMA and EIPA did not affect Mg2+ transport do not favour the idea of an Mg2+/Na+ antiport system. At Cl- equilibrium, independent of the Na+ gradient, the rate of Mg2+ transport was markedly suppressed compared with the transport rate noted in the presence of an i>o Cl- gradient. The stimulating effect of inside anions could be enhanced by SCN- and decreased by SO2-4. Furthermore, nonfluorescent anion transport antagonist H2-DIDS stimulated Mg2+ transport. These findings indicate that Mg2+ transport can be modulated by inside anions. Mg2+ transport appeared to be electroneutral because it was not dependent on membrane potential. Mg2+ transport was neither stimulated by Bay K8644, a Ca2+ channel agonist, nor inhibited by verapamil, diltiazem, nifedipine and imipramine, the Ca2+ channel antagonists. It, therefore, seems unlikely that Mg2+ uses the Ca2+ transport system.

Renal Control of Calcium, Phosphate, and Magnesium Homeostasis

“…When dietary magnesium intake is normal, transcellular transport mediates 30% of intestinal magnesium absorption. This fraction increases when dietary magnesium intake is lower (70)(71)(72). When dietary magnesium is higher, then the majority of intestinal magnesium absorption occurs via the paracellular pathway, which is regulated by proteins comprising the tight junction, including claudins, occludin, and zona-occludens-1 (73)(74)(75)(76).…”

Enhanced sodium-dependent extrusion of magnesium in mutant cells established from a mouse renal tubular cell line

“…7), membrane vesicles from rabbit ileum (16 mM; Ref. 10), and smooth muscle of guinea pig tenia cecum (27 mM; Ref. 16).…”

Magnesium in the Central Nervous System

“…Another similarity with the transporter operating in basolateral liver plasma membranes is the lack of inhibition by amiloride analogs. In good agreement with reports from Gunther and collaborators (Ebel and Gunther, 2003), this transporter is modulated by intravesicular anions, especially Cl --and SCN --, and markedly stimulated by antagonists of anion transport (e.g., H 2 --DIDS) (Juttner and Ebel, 1998). The main difference between plasma membrane vesicles from duodenum and jejunum (Baillien and Cogneau, 1995) is that a single Mg 2+ uptake mechanism operates in the duodenum with a K m of 0.8mM, whereas two transporters operate in the jejunum with K m values of 0.15 and 2.4mM, respectively.…”

“…This hypothesis is further supported by the observation that Mg 2+ accumulation is inhibited by amiloride but not by Ca 2+ channel antagonists. Consistent with the report by Juttner and Ebel (Juttner and Ebel, 1998), Mg 2+ accumulation is stimulated by an intravesicular electronegative potential or an alkaline pH o (Baillien and Cogneau, 1995). The effect of external pH, however, is lost when [Mg 2+ ] o >1mM (Baillien and Cogneau, 1995).…”

“…The operation of specific Mg 2+ accumulation mechanisms has also been observed in plasma membrane vesicles from brush border cells of rabbit ileum (Juttner and Ebel, 1998) and from the duodenum and jejunum of rat (Baillien and Cogneau, 1995). By using membrane vesicles from rabbit ileum and cell permeant and non--permeant Mag--Fura, Juttner and Ebel have observed the operation of a saturable Mg 2+ uptake mechanism when the intracellular Na + concentration is higher than the extracellular one (Baillien and Cogneau, 1995).…”