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Abstract-The effect of various opiate agonists on the contraction of isolated rat atrium was investigated. Ethyl ketocyclazocine (EKC) (30-100 ,uM) and dynor phin-A(1-13) (10-30 uM), which are ic-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 (fl-type), [Met5] and [Leu5]-enkephalin (O'-type) did not affect both the developed tension and frequency of contractions. These results indicate that the positive inotropic action is specific for r,-type opiate agonists.

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