Capsaicin-induced Relaxation in Rabbit Coronary Artery

Abstract

In the present study mechanism of inhibitory effects of capsaicin on the contractility of rabbit coronary artery were studied by measurement of isometric tension and intracellular Ca²⁺ concentration. Capsaicin (1 μM to 30 μM) relaxed the coronary artery pre-contracted with prostaglandin (PG) F_2α (1 μM) in a concentration-dependent manner. The PGF_{2 α}-induced increase in intracellular Ca²⁺ concentration was also inhibited. The effects of capsaicin were readily reversed by washing capsaicin from the bath. Capsaicin-induced relaxation was not attenuated by pretreatment with capsazepine (1 μM), a blocker of vanilloid receptor or ruthenium red (1 μM), a blocker of non-selective cation channel. Previous exposure to a high concentration of capsaicin (100 μM) or repeated application of capsaicin did not eliminate the relaxation response to subsequent application of capsaicin. Increasing the external K⁺ concentration to 80 mM significantly attenuated the capsaicin-induced relaxation with simultaneous change in intracellular Ca²⁺ concentration. Pretreatment with iberiotoxin (100 nM), a blocker of Ca²⁺-activated K⁺ channel, only partially inhibited the capsaicin-induced relaxation. However, application of 4-aminopyridine (4-AP, 1 mM), a blocker of delayed rectifier K⁺ current significantly inhibited the capsaicin-induced relaxation with concomitant attenuation of the effect on intracellular Ca²⁺ concentration. These results indicate that capsaicin may have a direct relaxing effect on the smooth muscle contractility, and relaxation may be due to activation of the 4-AP-sensitive, delayed rectifier K⁺ channels in the rabbit coronary artery.