1. We examined effects of capsaicin (10(-9)-10(-5) M), a pungent agent extracted from red pepper, on the contractile tension of rat ventricular papillary muscles stimulated at various cycle lengths (0.2, 0.5, 1, 2 and 5 sec).
2. Capsaicin produced a marked concentration dependent decrease in the amplitude, the rate of rise (dp/dt) and the rate of relaxation (dr/dt) of the tension.
3. However, the half relaxation time and the time to peak tension of the tension were slightly affected by the agent.
4. The negative inotropic effect of capsaicin was stimulus cycle length (CL) dependent. In particular, IC50 (50% inhibitive concentration) of the agent in the amplitude of the tension was stimulus CL dependent. That is, the values of IC50 were around 10(-7) M at longer CLs (1, 2 and 5 sec), and the value of IC50 at CL 0.2 sec was 4 X 10(-6) M.
5. These capsaicin induced negative inotropic effects were reversible. Other studies from our laboratory show that the negative inotropic effects may be largely due to a decrease in Ca2+ current.
