Physiological Pathway of Magnesium Influx in Rat Ventricular Myocytes

Abstract

Cytoplasmic free Mg(2+) concentration ([Mg(2+)]i) was measured in rat ventricular myocytes with a fluorescent indicator furaptra (mag-fura-2) introduced by AM-loading. By incubation of the cells in a high-K(+) (Ca(2+)- and Mg(2+)-free) solution, [Mg(2+)]i decreased from ? 0.9 mM to 0.2 to 0.5 mM. The lowered [Mg(2+)]i was recovered by perfusion with Ca(2+)-free Tyrode's solution containing 1 mM Mg(2+). The time course of the [Mg(2+)]i recovery was fitted by a single exponential function, and the first derivative at time 0 was analyzed as being proportional to the initial Mg(2+) influx rate. The Mg(2+) influx rate was inversely related to [Mg(2+)]i, being higher at low [Mg(2+)]i. The Mg(2+) influx rate was augmented by the high extracellular Mg(2+) concentration (5 mM), whereas it was greatly reduced by cell membrane depolarization caused by high K(+). Known inhibitors of TRPM7 channels, 2-aminoethoxydiphenyl borate (2-APB), NS8593, and spermine reduced the Mg(2+) influx rate with half inhibitory concentrations (IC50) of, respectively, 17 ?M, 2.0 ?M, and 22 ?M. We also studied Ni(2+) influx by fluorescence quenching of intracellular furaptra by Ni(2+). The Ni(2+) influx was activated by lowering intra- and extracellular Mg(2+) concentrations, and it was inhibited by 2-APB and NS8593 with IC50 values comparable with those for the Mg(2+) influx. Intracellular alkalization (caused by pulse application of NH4Cl) enhanced, whereas intracellular acidification (induced after the removal of NH4Cl) slowed the Mg(2+) influx. Under the whole-cell patch-clamp configuration, the removal of intracellular and extracellular divalent cations induced large inward and outward currents, MIC (Mg-inhibited cation) currents or IMIC, carried by monovalent cations likely via TRPM7 channels. IMIC measured at -120 mV was diminished to ? 50% by 100 ?M 2-APB or 10 ?M NS8593. These results suggest that TRPM7/MIC channels serve as a major physiological pathway of Mg(2+) influx in rat ventricular myocytes.

Functional expression of TRPM7 as a Ca2+influx pathway in adipocytes

“…TRPM7‐like currents in both cell types were activated by a reduction of [Mg 2+ ] i , an elimination of extracellular divalent cations, and were inhibited by 2‐APB, NS8593, and FTY720 (Figs. and ), which were consistent with previous reports (Nadler et al ; Schmitz et al ; Chubanov et al ; Chokshi et al ,; Qin et al ; Tashiro et al , ). We previously reported that TRPM7 current was inhibited by H 2 O 2 in a [Mg 2+ ] i ‐ and [ATP] i ‐dependent manner in HEK293 cells overexpressing murine TRPM7 (Inoue et al ).…”

“…Whole‐cell patch‐clamp recordings were performed to investigate whether native mouse white adipocytes functionally express TRPM7 current. It has been reported that TRPM7 is (1) inhibited by intracellular Mg 2+ (Nadler et al ; Schmitz et al ; Chokshi et al ,); (2) activated by the elimination of divalent cations from the bath solutions, resulting in quasi‐linear current‐voltage ( I – V ) relationships (Nadler et al ; Kerschbaum et al ; Inoue et al ); (3) inhibited by H 2 O 2 irreversibly in a [Mg 2+ ] i ‐dependent manner (Inoue et al ); and (4) inhibited by 2‐aminoethyl diphenylborinate (2‐APB) (Li et al ; Macianskiene et al ; Chokshi et al ), N‐methyl maleimide (NMM) (Inoue et al ), NS8593 (Chubanov et al ; Tashiro et al , ), and FTY720 (Qin et al ; Takahashi et al ).…”

“…(2) activated by the elimination of divalent cations from the bath solutions, resulting in quasi-linear current-voltage (I-V) relationships (Nadler et al 2001;Kerschbaum et al 2003;Inoue et al 2014); (3) inhibited by H 2 O 2 irreversibly in a [Mg 2+ ] i -dependent manner ; and (4) inhibited by 2-aminoethyl diphenylborinate (2-APB) (Li et al 2006;Macianskiene et al 2012;Chokshi et al 2012a), N-methyl maleimide (NMM) ), NS8593 (Chubanov et al 2012;Tashiro et al 2014Tashiro et al , 2019, and FTY720 (Qin et al 2013;Takahashi et al 2018).…”

Activation of TRPM7 by naltriben enhances migration and invasion of glioblastoma cells

“…Activation of TRPM7 currents by naltriben have previously been confirmed in primary ventricular myocytes [16], as well as in HEK293 cells overexpressing TRPM7 [14]. In the current study, we first verified the potentiating effect of naltriben on the endogenous TRPM7-like currents in U87 cells by whole-cell patch-clamp recording.…”

Effect of Carvacrol, TRP Channels Modulator, on Cardiac Electrical Activity

“…The involvement of these channels in impaired cardiac automaticity and conduction defects has been implicated with a TRPM7 knock-in mouse model [31,32]. In addition, TRPM7-like currents have been measured in ventricular cells of various animal species [33,34] as well as in human atrial cardiomyocytes [16,29,30], and CAR, at a concentration of 100 μM, almost completely blocked that current [16]. The TRPM7 channels are ubiquitous Mg 2+ and Ca 2+ permeable [35] and are thought to be responsible for Mg 2+ transport into the cell at negative membrane potentials.…”