Effects of electromagnetic stimuli on bone and bone cells in vitro: Inhibition of responses to parathyroid hormone by low-energy low-frequency fields

Abstract

Low-energy electromagnetic fields pulsed at frequencies of 10-90 Hz significantly increase healing ofchronic fracture nonunions in man. These fields are effective at tissue current levels several orders of magnitude lower than those required for transmembrane depolarization ofnormal cells. We have examined the effects of two clinically used pulsed electromagnetic fields on cultures of the osteoblast-like mouse bone cell line MMB-1. Both fields significantly reduced cellular production of cAMP in response to parathyroid hormone and osteoclast activating factor. Neither basal nor fluoride-activated levels of adenylate cyclase were altered in membranes from cells cultured in the fields; however, the same membrane -preparations exhibited markedly inhibited responses to parathyroid hormone. The fields blocked the inhibitory effects of the hormone on collagen synthesis by MMB-1 cells. However, there was no effect on the inhibition of collagen synthesis by 1,25-dihydroxyvitamin D3, which is believed to act primarily by a nuclear, rather than by a membrane-dependent, mechanism. No significant differences were noted between effects of the two fields, one generating continuous pulse trains (72 Hz) and the other generating recurrent bursts (15 Hz) of shorter pulses. We hypothesize that these field effects are mediated primarily at the plasma membrane of osteoblasts, either by interference with hormone-receptor interactions or by blocking of receptor-cyclase coupling in the membrane. These responses occurred with induced extracellular fields of 1 mV/cm or less, even though transmembrane potential gradients are typically 105 V/cm.Clinical studies (1-5) have demonstrated the usefulness ofelectromagnetic fields in stimulatinghealing ofchronically ununited fractures in humans. Devices generating such fields have been approved for clinical use. However, the mechanisms of action of these fields are not clear. Oscillating electromagnetic fields proven effective in clinical use generate electrochemical gradients in the tissue fluid surrounding cells (6), but these gradients are considerably weaker than the levels required to depolarize cell membranes. Typically, the devices in question impose -20-G pulsed magnetic fields, which induce current densities of -1 uA/cm2 and associated electric gradients of 1-10 mV/cm in extracellular fluids (2). Because of the high resistance ofcell membranes, any transmembrane electrical components ofimposed fields would be lower than the extracellular gradients by two to three orders of magnitude (6) and thus as much as six orders of magnitude less than the typical excitatory threshold currents of 1 mA/cm2 observed for axonal depolarization (7).It would therefore appear that the effectiveness ofsuch weak stimuli in generating cellular processes must depend on a series of amplification mechanisms, either before or during the transmembrane coupling of the initial stimulus (8). Likely loci for amplification and extension of weak electrochemical triggering events at the cell membrane may involve ...

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