Both left and right sartorius muscle preparations with or without sciatic nerve were isolated from the bullfrog, Rana catesbeiana, and were mounted in the chambers. Left side of the preparation was exposed to a static magnetic field, 0.65T, for several hours and the right side preparation was used as a control. The muscle tension was induced by the nerve or muscle stimulation electrically, with the intervais of 40ms; 100 Pulses, every 30min. The latency and the maximal rate-of rise of muscle tension were measured and the obtained values from the exposed muscle were compared with those from the control muscle. The results showed that the latency was shortened approximately 20% in case of muscle stimulation and about 15% in nerve stimulation, within 3 hours when the preparation was exposed to the magnetic field, although the time courses were different between the two. The maximal rate of rise of the tension became faster by about 10 to 15% in case of the muscle stimulation. However, the maximal rate of rise was changed little when the tension was induced by the nerve stimulation within 3-hr exposure. It was discussed that the differences between the results obtained by the muscle stimulation and those obtained by the nerve stimulation would mean that the transmission mechanisms on the neuro-muscular junction were influenced by the magnetic field.
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