Some effects of noradrenaline on electrical activity of guinea-pig ureteric smooth muscle

Summary  THE double sucrose-gap was used to investigate the electrophysiological basis for the positive inotropic effect of noradrenaline on the isolated guinea-pig ureter. In unmodified Krebs solution noradrenaline (1×10−5 g/ml) increased the duration of the action potentnal and increased the number, but decreased the amplitude, of the superimposed spikes. This change in action potential configuration was paralleled by an increase in the force and duration of the mechanical response. Ion substitution experiments suggest that the most important action of noradrenaline is the enhancement of the slow inward current carried by sodium ions. No evidence was found for a chloride or potassium contribution to the noradrenaline effect. Within 10–12 min of losing electrical and mechanical activity in calcium-free solution, or in the presence of manganese (2 mM), noradrenaline caused return of both the action potential and the contraction. The experimental findings suggest that the latter action of noradrenaline is a consequence of the release of calcium from a loosely bound store of the activator; probably the surface membrane. Such an effect of noradrenaline may contribute to, or cause, the action of the drug in unmodified Krebs solution.