The present study revealed the unexpected result of a significant decrease in matrix macromolecule synthesis of cartilage after exposure to a 3-tesla EMF. These data are based on the sGAG synthesis rate in cartilage as well as gene expression profiling of articular chondrocytes, demonstrating a decrease in aggrecan mRNA synthesis, whether exposed to the high-energy EMF as whole joint or as a cartilage explant. Because lower-energy EMFs have not been shown to induce a decrease in cartilage biosynthetic activity in this and previous studies [5
], the results obtained seem to be a consequence of the exposure to a 3-tesla high-energy EMF.
It has been hypothesized that an EMF might act like a mechanical load that causes a movement of fluid, which contains charged particles, relative to the solid matrix structures such as proteoglycans and collagens with their fixed charges [9
]. This fluid flow generates an electrical potential, the so-called 'streaming potential' [33
], which transduces mechanical stress into an electrical phenomenon capable of stimulating chondrocytes to synthesize matrix components. Physiologically, mechanical stresses on cartilage range from about 0 to 20 MPa [35
] and stimulate the synthesis of matrix constituents [36
]; exceeding this threshold causes physical damage to the cartilage [37
]. Taking these facts into account, a low-energy EMF may mimic mechanical stresses within physiological amplitudes, potentially leading to cellular stimulation [5
], whereas a high-energy EMF is likely to resemble stresses above physiological ranges, thereby initiating an inadequate flow of electrolytes and charges that ultimately impair cartilage activity. This assumption is fostered by our observations of a marked decrease in anabolic activity, as shown by sGAG synthesis and aggrecan gene expression. The studies of Lee and colleagues [40
] and Trinidade and colleagues [41
], who showed an impaired cartilage activity after applying mechanical stresses, are in line with our findings.
It is noteworthy that collagen type II expression was not appreciably changed, which may be attributed to the very low basal turnover of the collagen network [18
]. Beyond that, the release of sGAGs to the supernatant remained unchanged from that in the controls, indicating no major catabolic activity. Though not catabolic by itself, our inability to find increased expression of IL-1β upon exposure of cartilage to high-energy EMF is in line with the above results. A limiting factor to our study could be the fact that tissue manipulation and digestion can affect chondrocyte gene expression. Although the results from the RT–PCR analysis support the [35
S]sulfate incorporation data, the effects of the exposure to the EMF may have been masked by changes in gene expression resulting from enzymatic digestion and associated events. However, the decrease in both anabolic and catabolic activity led us to speculate that a high-energy EMF may to some extent compromise the biosynthetic activity and/or function of articular chondrocytes.
Although it is known that mechanical stress contributes to the induction of chondrocyte cell death [42
], in our experimental settings we found no difference in cell death rates between EMF-exposed and control samples, either in TUNEL or in Annexin V assays, which excludes cell death and a consequent decrease in chondrocyte numbers as a cause of the findings. Additionally, the 3-tesla EMF had no impact on the DNA content of the cartilage specimens, making a loss of chondrocytes very unlikely as a reason for the impaired biosynthetic activity.
Whether the results obtained also relate to the situation in vivo and in humans will have to be confirmed in similar analyses of human cartilage or in animal studies. However, it is also unknown whether this impairment of chondrocyte activity has any implication for the development of cartilage damage as seen in osteoarthritis. To address these questions, animal studies or studies in humans, for instance by the delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) technique, will be necessary.
The ability of articular chondrocytes to recover from mechanical strains has been proposed previously [39
]. When investigating the effects of the 3-tesla EMF over a period of 6 days, we did in fact find a recovery of cartilage biosynthetic activity. Furthermore, we tested whether there was a difference in the susceptibility to a high-energy EMF between young and old cartilage. The results on sGAG and aggrecan mRNA synthesis obtained on day 0 resembled the data from our initial measurements, in both young and old samples. Subsequently, the chondrocytes regained their biosynthetic activity over the course of time. At the end of the culture period an increase in sGAG/aggrecan mRNA production was found in the young group but not in the adult group after EMF exposure. This observation may be seen as a 'rebound phenomenon' caused by a higher metabolic rate of these cells in young cartilage compared to adult cartilage. In line with a lower metabolic activity of old chondrocytes [46
], such a rebound was not seen in tissues from aged cartilage. Our data therefore suggest that the effects of a 3-tesla EMF are transient and articular chondrocytes recover from the initial impairment.