Effects of nicardipine on the spontaneous beat of isolated atria of guinea pigs

Nicardipine was found to produce a concentration-dependent depression of the isometric contraction of the isolated, spontaneously beating atria of the guinea-pig. It also depressed the atrial rate of the isolated, spontaneously beating atria of the guinea-pig. The effect of the increasing concentrations of nicardipine on the heart rate was negligibly weaker than its effect on the isometric contraction. A time-dependent depression of the isometric contraction and the atrial rate after the addition of a single dose of nicardipine was also found up to the 10th min. Calcium almost completely, isoprenaline completely and aminophylline partially antagonized the depressive action of nicardipine on the isometric contractility of the atria. Only isoprenaline antagonized the negative chronotropic action of higher doses of nicardipine. It is possible that these substances restore the contractility by compensating the calcium balance, previously changed by nicardipine, or by producing an increase in the intracellular cAMP content (isoprenaline and aminophylline).