Differential effects of potassium on the potency of different opioids in the mouse vas deferens.

The effect of potassium on the potency of different opioids to inhibit electrically evoked twitches of the mouse vas deferens has been studied. In general, opioids lose activity as the concentration of potassium increases. Distinct differences, however, were observed between specific opiate receptor agonists. Least sensitive to the effect of potassium proved mu- and delta-opiate receptor agonists, showing only a 2-fold increase in the concentration required to cause a half-maximal inhibition at 14 mM K+. In contrast, the supposed dynorphin receptor/kappa-receptor agonists exhibited at this concentration up to an 8-fold loss of potency. Dynorphin1-13 was found to be intermediate with a 5-fold loss of inhibitory activity. These data suggest a modulatory action of potassium for dynorphin- and kappa-receptors.