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1. The effects of Na-free and K-free solutions, tetraethyl ammonium (TEA), Mn2+, verapamil and ouabain on the electrophysiological properties of the smooth muscle cells of guinea-pig ureter have been studied, using the double sucrose-gap method. 2. TEA (5 mM) increased the amplitude and duration of both the initial spike component and the subsequent plateau of the action potential. The repetitive spike discharge on the plateau was abolished. The amplitude and duration of the phasic contraction was increased. The threshold for excitation was lowered while the resting potential and membrane resistance were unaffected. 3. In Na-free solution the duration of the action potential decreased mainly due to the suppression of the plateau. A similar effect was produced by exposure to K-free solution and also by ouabain. 4. Mn2+ (2 mM) suppressed the spike component and raised the threshold for excitation. The amplitude of the remaining part of the action potential was markedly increased but the contraction was rapidly abolished. The resting potential and membrane resistance were unchanged. When Mn2+ was added to Na-free solution it produced an increase in the amplitude and duration of the remaining part of the action potential but the phasic contraction was abolished. 5. Verapamil did not specifically block the fast component of the action potential but initially increased the amplitude of the spike and shortened the plateau. Subsequently, both the action potential and the phasic contraction became smaller. 6. The observations indicate that the phasic contractions are triggered by the initial spike component of the action potential, whereas the plateau is associated with the amplitude and particularly the duration of the contraction.

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