Thesis (M.Sc.)--Chulalongkorn University, 1984

Capsaicin at three doses, 0.2, 2 and 10 ug/ml, was found to produce brief stimulation of the rate and isometric tension by isolated rat right and left atria respectively. With high dose (10 ug/ml) this initial stimulation was followed by depression of both the rate and contractile force. The capsaicin-induced cardiac stimulation was not significantly affected by propranolol, propranolol plus methysergide, and reserpine pretreatment. of the three cardiac drugs studied viz., propranolol, verapamil and procainamide, striking drug interaction was observed between capsaicin and verapamil. The negative chronotropy and inotropy mediated by verapamil ( 0.05ug/ml) was reduced by capsaicin at dose as low as 0.2 ug/ml. Capsaicin still retained this antagonizing activity when verapamil was added either after most of the capsaicin-evoked cardiac stimulation had declined, or 5 min before capsaicin. Reserpine pretreatment did not prevent the capacity of capsaicin to reverse verapamil action. Procainamide (5 and 10 ug/ml) also significantly mitigated verapamil action on the rate and isometric force. It is concluded that a) capsaicin stimulates the rate and contractile force by mechanism not involving catecholamines and serotonin release from intra-cardiac stores; b) capsaicin and procainamide presumably attenuate verapamil effect by interfering with and/or competing for the hydrophobic verapamil binding site; and c) untoward drug interaction may occur when verapamil-treated cardiac patients are on diet containing large quantity of hot pepper.