Effects Of Extremely Low Frequency Magnetic Field On Ca～(2+) Transport Of Skeletal Muscle Sarcoplasmic Reticulum Vesicles

As a ubiquitous second messenger, Ca²⁺ plays a critical role in the cellular signal transduction. To discuss the effects of extremely low frequency (ELF) magnetic field (MF) on cytosolic free Ca²⁺ concentration ([Ca²⁺]i), it is very meaningful for understanding the mechanisms of ELF MF bioeffects, especially for the nonthermal effects of weak MF, in view of molecular level and signal transduction. A few studies showed that ELF MF had no effects or even decreased in cytosolic [Ca²⁺]i, whereas the others demonstated that ELF MF exposure increased [Ca²⁺]i to a marked level. Since these results apparent lack of consistence, whether and how ELF MF influences cytosolic [Ca²⁺]i are still controversial. In this study, with isolated sarcoplasmic reticulum (SR) vesicles, the molecular mechanisms of ELF MF affect cytosolic [Ca²⁺]i were investigated for the first time.Using the method of dynamic Ca²⁺ dye spectrum, the Ca²⁺ uptake and the Ca²⁺-Mg²⁺-ATPase activity were observed with or without exposure to ELF MF, the function of the ryanodine receptor (RyR1) was measured by the Ca²⁺ release assays and by the [³H]-ryanodine binding assays, and SR membrane fluidity was examined by the fluorescence polarization. Results showed that 50 Hz, 0.4 mT MF exposure for 30 min at 25 ℃ caused the initial Ca²⁺ uptake rate and the Ca²⁺ uptake capacity to decrease 31% and 20% respectively. The RyRl inhibitor ruthenium red (RR) could partially restore the Ca²⁺ uptake inhibited by the ELF MF, namely the net Ca²⁺ uptake initial rate, and the Ca²⁺ uptake capacity of MF group was still 16% and 14% less than that of control respectively. Further research demonstrated that MF exposure caused a 26% decline of the Ca²⁺-Mg²⁺-ATPase activity. Moreover, the results of DPH experiments indicated that SR membrane fluidity was not the major cause of the inhibition on Ca²⁺-Mg²⁺-ATPase activity due to MF.exposure. On the other hand, the results showed that MF caused a 17% increase of the initial Ca²⁺ release rate and a 5% increase of the [³H]-ryanodine binding, and 1 mM NADH magnified the activation effects by MF exposure, which caused about a 23% increase of the initial Ca²⁺ release rate and a 8% increase of the [³H]-ryanodine binding.These results demonstrated that the decline of the Ca²⁺-Mg²⁺-ATPase activity and the increase of RyR1 function, resulted in a down-regulation of Ca2+ uptake and an up-regulation of Ca²⁺ release, induced by exposing SR to a 0.4 mT MF for 30min.