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The brief stimulatory effect of capsaicin (2 and 10 μg/ml) on the rate and isometric tension by isolated rat right and left atria respectively was slightly affected by a calcium antagonist, verapamil (0.05 μg/ml), whether both drugs were added simultaneously or capsaicin added 15 min after verapamil. However, in the presence of capsaicin, the negative chronotropism and inotropism induced by verapamil were markedly attenuated. Reserpine pretreatment did not abolish this antagonistic activity of capsaicin, indicating that endogenous catecholamines were not involved. Procainamide (10 μg/ml) which produced insignificant atrial stimulation was also found to mitigate the verapamil-induced atrial depression. The possible mechanism by which capsaicin and procainamide antagonize verapamil effect on isolated rat atria is discussed.
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