Frequency dependence of the effect of harmine on the duration of the action potential of guinea pig atrial muscle.

The effect of harmine on the duration of the action potential (APD) of guinea pig atrial muscle is analyzed. Harmine prolongs APD through both a depressant effect on automaticity and a direct effect on atrial repolarization. In atria driven at a constant rate: (a) the direct effect becomes less with a faster drive; (b) an increase in concentration of harmine does not result in a greater prolongation of APD. Increased extracellular Ca2+ concentration annulles the APD-prolonging effect.