Calcium‐antagonistic and Anti‐arrhythmic Effects of Nifedipine on the Isolated Rat Atrium

Increasing concentrations of calcium in the Ringer solution, [Ca ++ ]o, increased the rate and amplitude of contractions of spontaneously beating, isolated rat atria. Nifedipine 100 μg/l had negative chronotropic and inotropic effects when [Ca ++ ] o was 2.0 and 5.0 meq./l, and atrial arrest occurred in some experiments at these calcium concentrations. The sinus node recovery time decreased with increasing [Ca ++ ] o , but was increased by nifedipine 100 μg/l. All the effects of nifedipine were most pronounced at the lowest [Ca ++ ] o studied (2.0 meq./l), while these effects were negligible at the highest [Ca ++ ] o used (8.0 meq./l). Nifedipine 100 μg/l did not influence the threshold for electricall) induced atrial fibrillation. However, pretreatment with the drug completely protected the atria against calcium‐induced arrhythmias. It is concluded that the anti‐arrhythmic activity of nifedipine is due to its calcium‐antagonistic effect.