Larval muscle fibers of a beetle, Xylotrupes dichotomus, produce calcium spikes that are maintained when the fibers are bathed in saline solutions containing manganese, cadmium, zinc, or beryllium instead of calcium. This indicates that these cations permeate the calcium channels of the muscle fiber. By contrast, cobalt, nickel, and magnesium are nonpermeating and behave as competitive inhibitors of the permeation of the other divalent cations. Some of the permeating cations suppress delayed rectification.
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