With the use of a point voltage-clamp technique, the effects of Zn2+, UO22+, tetraethylammonium, and several other homologous quaternary ammonium ions on the electrical properties of the frog sartorius muscle and its mechanical threshold were studied. None of the agents separated the voltage thresholds for mechanical activation and delayed rectification. However, Zn2+, UO22+, and TEA, which are known to potentiate the twitch, caused some inhibition of the normal increase in potassium conductance during delayed rectification. Zn2+ and UO22+ also slowed the rate of development of the outward current. A strength-duration relation was studied for depolarization pulses capable of initiating contraction. With a depolarizing pulse of 2.5 msec the mechanical threshold is about -13 mv at about 20°C. UO22+, 0.5 μM, which markedly reduced the outward current produced by such a short pulse, did not raise the mechanical threshold. All findings indicate that there is no direct causal relation between delayed rectification and mechanical activation.
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