The Effect of Nifedipine on the Effective Refractory Period and Excitability of the Isolated Rat Atrium at Different Calcium Levels and Frequencies of Stimulation

The effect of calcium and a calcium–antagonistic drug, nifedipine, on the effective refractory period (E. R. P.) of electrically stimulated, isolated rat left atria was studied and compared with the effect on the duration of the intracellularly recorded action potential. To determine E. R. P., two different methods were used. When E. R. P. was determined from the maximum following frequency (m. f. f.), neither varying concentrations of calcium in the Ringer solution, [Ca ++ ] o , nor nifedipine 100 μg/1 influenced E. R. P. significantly. However, when E. R. P. was measured by using programmed electrical stimulation, E. R. P. was shorter than when determined from m. f. f., increasing [Ca ++ ] o decreased E. R. P. and nifedipine 100 μg/1 shortened E. R. P. at low [Ca ++ ] o (2.0 meq./l). Increasing atrial rates also produced a decrease in E. R. P., and the shortening effect of nifedipine was enhanced by increasing the stimulus frequency. The electrical threshold for stimulation increased both with increasing [Ca ++ ] o and increasing stimulus frequencies. However, nifedipine 100 μg/1 had no significant effect on the excitability. It is concluded that programmed electrical stimulation is a more accurate method than m. f. f. for determining E. R. P., and also that a correlation exists between the effect of calcium on E. R. P. and on the action potential duration.