Sensitivity to dynorphin-(1-13) of the presynaptic inhibitory opiate receptors of the guinea-pig heart.

Dynorphin-(1-13) at the concentrations of 1-10 microM, produced a dose-dependent inhibition of the cardiac response to field stimulation of the adrenergic nerve terminals in guinea-pig atria pretreated with peptidase inhibitors. This inhibitory effect was competitively antagonized in a dose-dependent manner by 5 and 10 microM naloxone. Since dynorphin-(1-13) did not modify the dose-inotropic effect curve of exogenous noradrenaline, it was concluded that the depressant effect of the opioid agonist was due to the stimulation of the presynaptic inhibitory opiate receptors belonging to the kappa subtype.