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 ...

Osteoclast activating factor (OAF) is a lymphokine which may participate in the pathologic destruction of bone observed in a number of disorders. In the current studies, we investigated the action of OAF on cAMP accumulation by bones and isolated bone cells in culture. OAF was shown to stimulate accumulation of cAMP in mouse cranial bones at doses between 1 and 1000 ng/ml. Stimulation of bone resorption was observed in bones treated with the same doses of OAF. In order to investigate the cell types responsible for cAMP responses to OAF, we isolated bone cells and grew them in monolayer culture. The cells were isolated by a variety of techniques which separate bone cells into two types of parathyroid hormone (PTH)-responsive populations: (a) cells derived from the periosteal regions of the bone, which also respond to calcitonin with increases in cAMP: and (b) cells derived from the matrix, which do not respond to calcitonin. OAF caused elevation of cAMP levels in both the periosteum-derived cells and the matrixderived cells. The magnitudes and time courses of OAF effects in these populations resembled the effects previously reported for PTH in the same populations. OAF stimulated adenyl cyclase in both types of cell populations, but did not produce significant changes in cAMP phosphodiesterase activity. OAF differed from PTH in that its effects on cAMP accumulation decreased sharply at supramaximal doses in both bone and isolated cells, especially in the matrix-derived populations. Bone resorption did not decrease as markedly as did cAMP accumulation at high doses of OAF, suggesting that cAMP accumulation and resorption could be dissociated under some conditions. These results indicate that OAF has effects on cAMP production in the same cell populations as PTH, and suggest that OAF could modify not only resorption but also formation of bone in vivo. OAF may exert its effects on bone Send offprint requests to R.A. Luben at the above address. by means of cAMP-dependent mechanisms, but more data will be necessary to establish this unequivocally. The observed differences between OAF and PTH may be of relevance in the mechanism and treatment of pathologic bone destruction in vivo.