Presynaptic imidazoline receptors and alpha 2-adrenoceptors in the human heart: discrimination by clonidine and moxonidine.

The involvement of presynaptic alpha 2-autoreceptors and imidazoline receptors in the modulation of noradrenaline release was investigated in strips from human atrial appendages preincubated with [3H]noradrenaline and superfused with medium containing desipramine and corticosterone. Electrical impulses were applied transmurally at 2 Hz. The imidazoline derivatives moxonidine and clonidine reduced to evoked tritium overflow in a concentration-dependent manner. Moxonidine was 18-fold more potent than clonidine. The concentration-response curve for moxonidine, but not for clonidine was shifted to the right by the alpha 2-adrenoceptor antagonist rauwolscine. The apparent pA2 value of rauwolscine against moxonidine was 8.41. An inhibitory effect was also observed for the imidazoline derivative BDF 6143 (4-chloro-2-(2-imidazolin-2-ylamino)-isoindoline), a mixed alpha 2-adrenoceptor antagonist/imidazoline receptor agonist; BDF 6143 was about 2-fold more potent than clonidine. Rauwolscine (1 microM) did not substantially shift the concentration-response curve of BDF 6143. It is concluded that noradrenaline release in the human atrium is inhibited not only via presynaptic alpha 2-autoreceptors but also via presynaptic non-I1, non-I2 imidazoline receptors. The failure of rauwolscine to antagonize the inhibitory effect of clonidine suggests that clonidine preferentially stimulates the presynaptic imidazoline receptors; the alpha 2-adrenoceptor component of its action is probably suppressed by an inhibitory interaction between the two receptors. In contrast, moxonidine acts via presynaptic alpha 2-autoreceptors, leaving the presynaptic imidazoline receptor unaffected.