Positive inotropic action of kappa opiate agonists, ethylketocyclazocine and dynorphin-A(1-13), in isolated rat atrium.

The effect of various opiate agonists on the contraction of isolated rat atrium was investigated. Ethylketocyclazocine (EKC) (30-100 microM) and dynorphin-A(1-13) (10-30 microM), which are kappa-type agonists, caused positive inotropic effects on electrically stimulated left atrium in a dose-dependent manner. In addition, EKC decreased the frequency of spontaneous beating in the right atrium. Morphine (mu-type), [Met5]- and [Leu5]-enkephalin (delta-type) did not affect both the developed tension and frequency of contractions. These results indicate that the positive inotropic action is specific for kappa-type opiate agonists.