Cytosolic free magnesium concentration in human platelets

Actin filament attachments for sustained motility in vitro are maintained by filament bundling

We reconstructed cellular motility in vitro from individual proteins to investigate how actin filaments are organized at the leading edge. Using total internal reflection fluorescence microscopy of actin filaments, we tested how profilin, Arp2/3, and capping protein (CP) function together to propel thin glass nanofibers or beads coated with N-WASP WCA domains. Thin nanofibers produced wide comet tails that showed more structural variation in actin filament organization than did bead substrates. During sustained motility, physiological concentrations of Mg(2+) generated actin filament bundles that processively attached to the nanofiber. Reduction of total Mg(2+) abolished particle motility and actin attachment to the particle surface without affecting actin polymerization, Arp2/3 nucleation, or filament capping. Analysis of similar motility of microspheres showed that loss of filament bundling did not affect actin shell formation or symmetry breaking but eliminated sustained attachments between the comet tail and the particle surface. Addition of Mg(2+), Lys-Lys(2+), or fascin restored both comet tail attachment and sustained particle motility in low Mg(2+) buffers. TIRF microscopic analysis of filaments captured by WCA-coated beads in the absence of Arp2/3, profilin, and CP showed that filament bundling by polycation or fascin addition increased barbed end capture by WCA domains. We propose a model in which CP directs barbed ends toward the leading edge and polycation-induced filament bundling sustains processive barbed end attachment to the leading edge.

Risk factors and outcome associated with hypomagnesemia in massive transfusion

BACKGROUND

Electrolyte disturbances are common in patients with critical bleeding requiring massive transfusion. The risk factors and outcome associated with the occurrence of hypomagnesemia in massive transfusion remain uncertain.

STUDY DESIGN AND METHODS

A cohort of 353 consecutive patients who required massive transfusion, defined as 10 or more units of allogeneic red blood cells or whole blood transfusion within 24 hours, between 2002 and 2008 in a quaternary health care center in Western Australia was considered. Logistic regression was used to identify risk factors and outcome associated with hypomagnesemia during massive transfusion (<0.7 mmol/L or 1.7 mg/dL).

RESULTS

Of the 353 patients requiring massive transfusion during the study period, 298 patients (84%) had serum magnesium concentrations available for analysis. Hypomagnesemia was common (172 patients, 58%), and the mean magnesium concentration was 0.68 mmol/L (1.65 mg/dL; standard deviation, 0.15 mmol/L) in these patients. The risk factors for hypomagnesemia were hypocalcemia (odds ratio [OR], 1.67 per 0.1 mmol/L decrement; 95% confidence interval [CI], 1.36-2.01; p = 0.001) and hypofibrinogenemia (OR, 1.05 per 0.1 g/L decrement; 95% CI, 1.01-1.09; p = 0.009). The lowest serum magnesium concentrations were associated with the lowest ionized calcium concentrations (Spearman correlation coefficient, 0.377; p = 0.001). Both magnesium concentrations (OR, 0.91 per 0.1 mmol/L increment; 95% CI 0.31-2.69; p = 0.863) and the interaction term between magnesium and calcium concentrations were not associated with an increased risk of mortality after adjusting for other covariates.

CONCLUSIONS

Hypomagnesemia was common and associated with hypocalcemia in massive transfusion, but serum magnesium concentrations had no independent effect or interactive effect with hypocalcemia on mortality of patients requiring massive transfusion.

Calcium- and magnesium-dependent interactions between calcium- and integrin-binding protein and the integrin alphaIIb cytoplasmic domain

Calcium- and integrin-binding protein (CIB) is a small EF-hand calcium-binding protein that is involved in hemostasis through its interaction with the alphaIIb cytoplasmic domain of integrinalphaIIbbeta(3). We have previously demonstrated that CIB lacks structural stability in the absence of divalent metal ions but that it acquires a well-folded conformation upon addition of Ca(2+) or Mg(2+). Here, we have used fluorescence spectroscopy, NMR spectroscopy, and isothermal titration calorimetry to demonstrate that both Ca(2+)-bound CIB (Ca(2+)-CIB) and the Mg(2+)-bound protein (Mg(2+)-CIB) bind with high affinity and through a similar mechanism to alphaIIb cytoplasmic domain peptides, but that metal-free CIB (apo-CIB) binds in a different manner. The interactions are thermodynamically distinct for Ca(2+)-CIB and Mg(2+)-CIB, but involve hydrophobic interactions in each case. Since the Mg(2+) concentration inside the cell is sufficient to saturate CIB at all times, our results imply that CIB would be capable of binding to the alphaIIb cytoplasmic domain independent of an intracellular Ca(2+) stimulus in vivo. This raises the question of whether CIB can act as a Ca(2+) sensor in alphaIIbbeta(3) signaling or if other regulatory mechanisms such as fibrinogen-induced conformational changes in alphaIIbbeta(3), post-translational modifications, or the binding of other accessory proteins mediate the interactions between CIB and alphaIIbbeta(3). Differences in NMR spectra do suggest, however, that Ca(2+)-binding to the Mg(2+)- CIB-alphaIIb complex induces subtle structural changes that could further modulate the activity of alphaIIbbeta(3).

Magnesium therapy improves thromboelastographic findings before liver transplantation: A preliminary study

PURPOSE

Patients undergoing liver transplantation may be predisposed to hypomagnesemia and present a bleeding tendency. There are evidences suggesting that magnesium is a crucial constituent of the blood coagulation cascade and has a pro-coagulant activity. The aim of this study was to investigate the effect of magnesium therapy on thromboelastography (TEG) in patients undergoing liver transplantation.

METHODS

27 patients scheduled for liver transplantation were enrolled. 1.5 g of magnesium sulfate, diluted in normal saline, were infused to all patients over five minutes in the operating room, before skin incision. The TEG findings immediately before and ten minutes after the magnesium infusion were compared.

RESULTS

The TEG findings showed general hypocoagulability before magnesium therapy. The K time and coagulation times (r+k) were shortened significantly from 641.6 +/- 505.9 (mean +/- SD) to 464.6 +/- 387.7 sec and from 1664.7 +/- 772.5 to 1362.2 +/- 487.1 sec respectively (P < 0.05); the maximal amplitude, and TEG index showed significant increases from 38.5 +/- 13.5 to 45.3 +/- 12.2 mm and from -3.4 +/- 2.6 to -1.9 +/- 1.8 respectively after magnesium therapy (P < 0.01). R time, alpha angle and LY60 were not different after magnesium therapy.

CONCLUSION

Magnesium therapy significantly improved TEG findings suggestive of a general hypocoagulable state towards normal in patients about to receive liver transplantation.

Ionized magnesium in the homeostasis of cells: intracellular threshold for Mg(2+) in human platelets

The role of extracellular magnesium ions in the homeostasis of intracellular ionized magnesium ([Mg(2+)](i)) in human platelets was studied. For media containing 0.00 to 0.60 mmol/l of extracellular ionized magnesium ([Mg(2+)](o)), the mean [Mg(2+)](i) fluctuated between 533 and 760 micromol/l. As the [Mg(2+)](o) was increased to 1.5 mmol/l, the [Mg(2+)](i) increased proportionately and peaked at 1470.1 micromol/l. Additional increase in the [Mg(2+)](o) from 1.50 to 6.00 mmol/l resulted in decreased [Mg(2+)](i) until it equilibrated between 739 and 776 micromol/l. The influx of Mg(2+) at [Mg(2+)](o) of 0.60 and 1.50 mmol/l was studied using verapamil, a calcium channel inhibitor, and ouabain, an inhibitor of the Na/K pump, respectively. The verapamil (25 mmol/l) blocking experiments resulted in a 92.4% inhibition of the Mg(2+) influx into the platelet at a [Mg(2+)](o) of 1.50 mmol/l. Ouabain (0.5 and 2.5 mmol/l) showed an enhancement effect on the influx of Mg(2+) at [Mg(2+)](o) of 0.60 mmol/l and no effect at 1.50 mmol/l. The effect of verapamil indicates that ion channels that are homologous to calcium ion channels may be involved in the influx of Mg(2+) into the platelets. The inhibition of Mg(2+) influx for [Mg(2+)](o) greater than 1.50 mmol/l may illustrate a protective mechanism that attempts to maintain the viability of platelets at abnormally high [Mg(2+)](o). These results suggest that there is an intracellular Mg(2+) threshold of 1500 micromol/l, above which an active mechanism prevents further influx of Mg(2+).