Effects of intracellular and extracellular concentrations of Ca2+, K+, and Cl- on the Na+-dependent Mg2+ efflux in rat ventricular myocytes

Dimeric G-Quadruplex: An Effective Nucleic Acid Scaffold for Lighting Up Thioflavin T

As a recently identified higher-order quadruplex (G4) structure, the G4 dimer possesses unique structure and biological functions. In this work, we found accidentally that two tandem PW17 (one known G4-forming DNA) sequences can fold into a stable G4 dimer, and the G4 dimer can enhance dramatically the fluorescence intensity of thioflavin T (ThT). The G4 dimer/ThT fluorescence intensity is about ninefold that of the corresponding G4 monomer/ThT. Meanwhile, compared with the common G4/ThT system, G4 dimer/ThT exhibited more stable fluorescence emission in the media with various concentrations of Na⁺ and K⁺. On the basis of these findings, G4 dimer/ThT was used as a fluorescence indicator to construct one arched DNA probe for label-free detection of DNA. By incorporating a G4 dimer sequence in amplified products, we further designed one rolling circle amplification-based biosensing strategy to show the utility of this G4 dimer/ThT fluorescence indicator. This study demonstrates that dimeric G4 is an effective nucleic acid scaffold for lighting up ThT, showing promising applications in a label-free bioassay.

Divalent cations and molecular crowding buffers stabilize G-triplex at physiologically relevant temperatures

G-triplexes are non-canonical DNA structures formed by G-rich sequences with three G-tracts. Putative G-triplex-forming sequences are expected to be more prevalent than putative G-quadruplex-forming sequences. However, the research on G-triplexes is rare. In this work, the effects of molecular crowding and several physiologically important metal ions on the formation and stability of G-triplexes were examined using a combination of circular dichroism, thermodynamics, optical tweezers and calorimetry techniques. We determined that molecular crowding conditions and cations, such as Na⁺, K⁺, Mg²⁺ and Ca²⁺, promote the formation of G-triplexes and stabilize these structures. Of these four metal cations, Ca²⁺ has the strongest stabilizing effect, followed by K⁺, Mg²⁺, and Na⁺ in a decreasing order. The binding of K⁺ to G-triplexes is accompanied by exothermic heats, and the binding of Ca²⁺ with G-triplexes is characterized by endothermic heats. G-triplexes formed from two G-triad layers are not stable at physiological temperatures; however, G-triplexes formed from three G-triads exhibit melting temperatures higher than 37°C, especially under the molecular crowding conditions and in the presence of K⁺ or Ca²⁺. These observations imply that stable G-triplexes may be formed under physiological conditions.

Physiological pathway of magnesium influx in rat ventricular myocytes

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.

Magnesium homeostasis in cardiac myocytes of Mg-deficient rats

To study possible modulation of Mg²⁺ transport in low Mg²⁺ conditions, we fed either a Mg-deficient diet or a Mg-containing diet (control) to Wistar rats for 1–6 weeks. Total Mg concentrations in serum and cardiac ventricular tissues were measured by atomic absorption spectroscopy. Intracellular free Mg²⁺ concentration ([Mg²⁺]_i) of ventricular myocytes was measured with the fluorescent indicator furaptra. Mg²⁺ transport rates, rates of Mg²⁺ influx and Mg²⁺ efflux, were estimated from the rates of change in [Mg²⁺]_i during Mg loading/depletion and recovery procedures. In Mg-deficient rats, the serum total Mg concentration (0.29±0.026 mM) was significantly lower than in control rats (0.86±0.072 mM) after 4–6 weeks of Mg deficiency. However, neither total Mg concentration in ventricular tissues nor [Mg²⁺]_i of ventricular myocytes was significantly different between Mg-deficient rats and control rats. The rates of Mg²⁺ influx and efflux were not significantly different in both groups. In addition, quantitative RT-PCR revealed that Mg deficiency did not substantially change mRNA expression levels of known Mg²⁺ channels/transporters (TRPM6, TRPM7, MagT1, SLC41A1 and ACDP2) in heart and kidney tissues. These results suggest that [Mg²⁺]_i as well as the total Mg content of cardiac myocytes, was well maintained even under chronic hypomagnesemia without persistent modulation in function and expression of major Mg²⁺ channels/transporters in the heart.

KB-R7943 inhibits Na+-dependent Mg2+ efflux in rat ventricular myocytes

Na(+)-dependent Mg(2+) efflux activity was studied with the fluorescent Mg(2+) indicator furaptra in the presence of various potential antagonists known to inhibit other transporters and channels. Among the compounds tested, KB-R7943, an inhibitor of Na(+)/Ca(2+) exchange, most potently inhibited the Na(+)/Mg(2+) exchange with half inhibitory concentrations (IC(50)) of 21 μM: (25°C) and 16 μM: (35°C). These IC(50) values were a factor of three to four lower than those of imipramine, a widely used inhibitor of Na(+)/Mg(2+) exchange. Apart from the inhibitory effect on Na(+)/Mg(2+) exchange, relatively high concentrations of KB-R7943 (100 μM: at 25°C and ≥20 μM: at 35°C), in combination with prolonged UV-illumination, caused cell shortening, probably because of the phototoxicity of the compound and the formation of rigor crossbridges. We conclude that KB-R7943 may be a useful tool to study cellular Mg(2+) homeostasis if care is taken to minimize its phototoxicity.

Metabolic inhibition strongly inhibits Na+-dependent Mg2+ efflux in rat ventricular myocytes

We measured intracellular Mg2+ concentration ([Mg2+]i) in rat ventricular myocytes using the fluorescent indicator furaptra (25 degrees C). In normally energized cells loaded with Mg2+, the introduction of extracellular Na+ induced a rapid decrease in [Mg2+]i: the initial rate of decrease in [Mg2+]i (initial Delta[Mg2+]i/Deltat) is thought to represent the rate of Na+-dependent Mg2+ efflux (putative Na+/Mg2+ exchange). To determine whether Mg2+ efflux depends directly on energy derived from cellular metabolism, in addition to the transmembrane Na+ gradient, we estimated the initial Delta[Mg2+]i/Deltat after metabolic inhibition. In the absence of extracellular Na+ and Ca2+, treatment of the cells with 1 microM carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone, an uncoupler of mitochondria, caused a large increase in [Mg2+]i from approximately 0.9 mM to approximately 2.5 mM in a period of 5-8 min (probably because of breakdown of MgATP and release of Mg2+) and cell shortening to approximately 50% of the initial length (probably because of formation of rigor cross-bridges). Similar increases in [Mg2+]i and cell shortening were observed after application of 5 mM potassium cyanide (KCN) (an inhibitor of respiration) for > or = 90 min. The initial Delta[Mg2+]i/Deltat was diminished, on average, by 90% in carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone-treated cells and 92% in KCN-treated cells. When the cells were treated with 5 mM KCN for shorter times (59-85 min), a significant decrease in the initial Delta[Mg2+]i/Deltat (on average by 59%) was observed with only a slight shortening of the cell length. Intracellular Na+ concentration ([Na+]i) estimated with a Na+ indicator sodium-binding benzofuran isophthalate was, on average, 5.0-10.5 mM during the time required for the initial Delta[Mg2+]i/Deltat measurements, which is well below the [Na+]i level for half inhibition of the Mg2+ efflux (approximately 40 mM). Normalization of intracellular pH using 10 microM nigericin, a H+ ionophore, did not reverse the inhibition of the Mg2+ efflux. From these results, it seems likely that a decrease in ATP below the threshold of rigor cross-bridge formation (approximately 0.4 mM estimated indirectly in the this study), rather than elevation of [Na+]i or intracellular acidosis, inhibits the Mg2+ efflux, suggesting the absolute necessity of ATP for the Na+/Mg2+ exchange.