The mechanism of the excitatory action of catecholamines and histamine on the smooth muscle of guinea‐pig ureter.

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Abstract
The ionic mechanism of the excitatory action of catecholamines and histamine on the smooth muscle cells of guinea-pig ureter was studied with the double sucrose-gap method. In normal conditions adrenaline and noradrenaline in a concentration of 10-5g/ml, and histamine in a concentration of 10-6 g/ml prolonged the duration of the plateau of the action potential and increased the amplitude and duration of the phasic contraction. Sometimes these changes were accompanied by a slight depolarization of the muscle membrane and by a small increase (with noradrenaline) or decrease (with histamine) of the membrane resistance. The amplitude and duration of the fast spike component of the action potential were not changed. Isoprenaline in a concentration of 10-5 g/ml either caused no change or it decreased the duration of the plateau, reduced the amplitude of contractions and reduced excitability. Tetraethyl ammonium (TEA; 5 mM), which blocks the delayed outward K current, did not prevent the increase in the duration of the plateau nor the increase of the amplitude and duration of the contractions by noradrenaline and histamine. In Na-free or in K-free solution or in the presence of ouabain, i.e., in conditions in which the Na-gradient across the membrane was reduced, noradrenaline and histamine were unable to increase the duration of the plateau and the amplitude and duration of the contraction. In the presence of Mn2+ (2 mM) which suppressed the spike component of the action potential and the phasic contraction, the effects of noradrenaline and histamine were almost abolished.