Page 123 - Magnesium-based supports for stem cell therapy of vascular disease - Mónica Echeverry Rendón

ENDOTHELIAL FUNCTION AFTER EXPOSITION OF MAGNESIUM DEGRADATION PRODUCTS
4. Discussion
Results from this study support the information that argue that c.p Mg has a strong effect on the cell behavior and demonstrated that the use of coatings obtained by PEO are an interesting alternative to improve the biological per- formance of the material [21][22]. The coating acts as a protective barrier that allows to decrease the accelerated release of Mg and the alkalization of the area. Comparison of the coatings obtained by HMT and MAN did not show significant differences between them according with the results obtained. Additionally, findings of this study allow us to demonstrate that ECs and SMCs, which play an important role in the cardiovascular function, are sensitive to changes of pH and concentrations of Mg2+. Similar observation were reported by Cipriano et al [23] who evaluated c.p Mg and Mg alloys containing zinc (Zn) and strontium (Sr) with HUVECs and founded that the limits for the pH was about 9 and for the Mg2+ concentration about 27.6mM. On the other hand, Wolf and Cittadini stated that Mg has an important effect in the growth and differentiation of cells. The intracellular concentration of Mg2+ varies depends of the cell type. On the other hand, extracellular Mg in the range of 0.5 to 0.05 mM stimulates the synthesis of proteins and the general cellular metabolism. Oppositely, when the levels of the Mg2+ are lower than 0.5mM, cell proliferation is reduced and apoptosis is induced [24]. Our results obtained in HUVECs are also similar with found by Cutaia et al, who studied the effect of alkaline stress on human pulmonary artery endothelial cells (HPAEC). After change the pH of the environment between 7.4 and 8.4 by different techniques, HPAEC showed changes in the cell morphology and potentiation of apoptosis was observed with the increment of the pH [25]. Nguyen et all, also evaluated the effect of extracts of c.p Mg under human embryonic stem cells (hESCs), observing a complete cell death after the 30h of culture. In the study they evaluated the independent effect of pH and concluded that even at pH of 8.1 the cells are still viable. However the normal cell behavior was affected after increase concentrations of Mg up to 10mM [26].
Another important parameter to take in count in these evaluations, it is the buffering system for controlling the pH. 7 Compounds like 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), tris-HCl, phosphates and NaHCO3/CO2
has been reported to be used as buffer agents. This last one is one of the most recommended according different
studies [27]. Contrary, the use of HEPES, for instance, has been showed acceleration in the degradation of the Mg
by the modification of the protective layer generated in the material and favoring the diffusion of ions of Cl- which are highly aggressive for the material [27]. For our case, NaHCO3 contained in the cell culture medium and regulated by CO2 from the incubator was used, however the capacity of the buffer because of the restricted volume used was not enough to control changes in the pH.
Although these evaluations allow us to have more information about the effect of the dissolution of Mg on the ECs and SMCs, the model is far from reality because the system in which the implant will be used is dynamic. There, the bloodstream contributes highly to the buffering of effects caused by pH changes and hydrogen evolution avoiding their accumulation in large quantities around the implant. Also the volume/area (V/A) ratio should be reconsidered due to the implant will be placed in a system where the total blood volume is around 5L considering an adult weighting 70Kg. The placement of a stent in the blood vessels as a treatment of arthrosclerosis, leads an imminent damage in the endothelial tissue due to the mechanical action of the insertion of the implant to open the lumen of the vessel and by the removal of the adipose tissue or plaque in order to guarantee normalization in the blood flow. The direct contact between the material and the vessel tissue is prime in the well-functioning of the stent and should be biocompatible.
Coatings proposed in this study can improve the biocompatibility of Mg which degrades rapidly in aqueous sys- tems and generates free Mg, and H2 which are toxic to cells. Static systems are not a good model for the dynamic body due to the increment of pH and dissolved Mg2+ as a consequence of the production of degradation products,
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