Effect of harmaline on pacemaker activity of guinea pig sinus node.

Transmembrane potentials of pacemaker fibers of the sinus node were recorded to analyze the effect of harmaline (HME) on cardiac automaticity. A short exposure to HME 2.1 X 10(-5) or 8.3 X 10(-5) M produced a long lasting non-cholinergic depression of the automaticity: the slope of diastolic depolarization of the pacemaker fibers was depressed, but the maximum diastolic potential remained unmodified. The automaticity of the sinus node was not abolished by prolonged exposure to HME 8.3 X 10(-4) M, but a reversible sino-atrial block developed.