Serum factors that inhibit the binding of (125)I-labeled alpha-bungarotoxin to the acetylcholine receptor extracted in detergent from denervated rat muscle were detected by a sensitive assay. The serum of at least 5 and possibly 11 out of 15 patients with myasthenia gravis showed inhibitory activity that was localized to the globulin fraction. No controls showed inhibitory activity. The demonstration of inhibitory globulins may help explain the involvement of the immune system in the pathophysiology of the neuromuscular junction in patients with myasthenia gravis.

Pulsing electromagnetic fields of low frequency and strength have been inductively coupled across skin, directly to bone, to enhance the repair of canine osteotomies. The induced voltage field in bone appears to increase the organization and strength of the repair process at 28 days after "fracture."

The equilibrium binding of [125I]-a-bungarotoxin to muscle fractions derived from normal and denervated, slow and fast muscle was examined. The toxin bound to normal slow and fast muscle with an affinity constant (A") in the order of 10s l./mol. The number of receptor sites (TV) was approximately 8.5 pmol/g of muscle. The toxin bound to both types of denervated muscle with an affinity constant of 109 l./mol. The number of receptor sites (TV) was more than tenfold higher than the number of sites in the corresponding normal muscle. Binding of the toxin to the receptor derived from both normal and denervated muscle closely approximated the isotherm predicted from the mass law equation for the interaction of 1 mol of homogeneous ligand with 1 mol of identical and independent sites. Further experiments suggested that the observed difference between normal and denervated muscle in the free energy of binding may be attributed to factors other than the prir-p 1. he normal and denervated states of slow and fast mammalian skeletal muscle provide a unique system for studying the acetylcholine receptor. In fast muscle, the iontophoretic application of acetylcholine (ACH)* 1 *to the end-plate results in a change in membrane sodium and potassium conductance which is not noted following application of ACH to non-end-plate regions of the membrane. In slow muscle ACH sensitivity is noted not only at the end-plate but also at the muscle-tendon junction and to a very limited extent over the entire cell surface. In contrast, the application of ACH to any point on the surface of denervated slow or fast muscle results in ion conductance changes almost comparable to the response observed at the end-plate region of normal muscle (Albuquerque and Thesleff, 1968;Albuquerque and Mclsaac, 1970).Recent studies have employed the elapid neurotoxins abungarotoxin and cobra -toxin to investigate the changes in bioelectric responsiveness induced by denervation. These homologous toxins are potent cholinergic antagonists which specifically block the ACH receptor site (Chang and Lee, 1964;Lester, 1970). Following denervation a dramatic increase in -bungarotoxin binding to rat diaphragm preparations has been observed Fambrough, 1970; Berg et al1972). In accord with the physiological data, the increased toxin binding has been localized