Abstract
The effects of bromide, nitrate, iodide, and thiocyanate ions on the neurally evoked fast and slow contractions of a crab muscle were investigated. Both types of contraction were depressed in bromide and nitrate. In iodide and thiocyanate, the slow contraction was often depressed but the fast contraction was potentiated. The foreign anions increased muscle fiber membrane resistance and the amplitudes of both fast and slow postsynaptic potentials. Records of tension development in single muscle fibers showed that more stimulating current was required to produce a given tension in nitrate than in the standard bathing fluid; this change was related to hyperpolarization of the muscle fiber membrane in nitrate. Potassium contractures also were inhibited by nitrate, because of the less effective depolarization of the cell membrane by potassium ion in the presence of nitrate. No marked shift in the membrane potential threshold for contraction occurred after treatment with the foreign anions.
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