1. The effects of tetraethylammonium (TEA) ion on the action potential of isolated longitudinal muscle of the dog antrum were used to gain some insight into the mechanism of generation of the plateau potential of the action potential complex. The double sucrose gap was used. 2. In concentrations of TEA up to 5 mM, the amplitude of the upstroke potential was increased. In 10mM‐TEA there was also an increase in the amplitude of the plateau potential and in the maximum rate of rise of the upstroke potential. 3. Concentrations of TEA (3 mM and greater) increased the duration of the action potential. Five mM‐TEA produced spike potentials which occurred only during the plateau potential of the action potential. Each spike caused a contraction. 4. The steady‐state voltage‐current relation was studied in normal Krebs solution and in TEA containing Krebs solution. In normal Krebs solution the voltage response was not a linear function of the applied current when outward current pulses were used. In TEA solution the voltage response was a linear function of the entire range of applied depolarizing current. 5. In low concentrations of TEA (2‐4 mM), when the steady‐state voltage‐current relation was linear, constant current pulses were applied between action potentials and during the plateau potential to determine if there were a decrease in membrane slope resistance during the plateau. It was found that the amplitude of the electrotonic potential recorded during the plateau was significantly less than the amplitude of the electrotonic potential recorded between action potentials. 6. The rate of repolarization of the plateau potential was studied in normal Krebs solution and in 2 mM‐TEA Krebs solution. The rate of repolarization of the plateau potential was slowed in TEA Krebs solution. 7. It is concluded that there is an increase in the membrane conductance during the plateau potential. The repolarization following the plateau potential is due to a TEA‐sensitive outward current.
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