Is there a peripheral site of action contributing to the voiding effects of α₂-adrenoceptor agonists and antagonists?

Purpose: Since it has not been established whether there is an effect on voiding exerted by direct stimulation or blockade of α2-adrenoceptors in the bladder and urethra, MK-467, a peripherally acting α2-adrenoceptor antagonist not penetrating into the CNS, was used to test whether part of the voiding effects of systemically given α2-adrenoceptor agonists is peripheral.

Methods: Urodynamic recordings from 27 conscious male adult C57/Bl J-strain mice were performed. After vehicle (saline) administration, two groups of animals were treated first with the selective α2-adrenoceptor agonist dexmedetomidine (Dex) and then with the selective α2-adrenoceptor antagonists atipamezole (Ati) or MK-467. Two other groups were first treated with Ati or MK-467 and then with Dex.

Results: Treatment with vehicle or α2-adrenoceptor antagonists alone did not affect micturition parameters. All animals treated first with Dex-developed overflow incontinence. Treatment with Ati after Dex reversed almost totally the effects of Dex on all voiding parameters, but treatment with MK-467 after Dex showed no detectable improvement. Treatment with Dex after Ati had no effect on any voiding parameter except maximal pressure. When mice were treated with Dex after MK-467, overflow incontinence was produced in seven of eight animals studied.

Conclusions: The absence of functionally relevant peripheral effects on voiding mediated via α2-adrenoceptors is supported by the finding that neither Ati nor MK-467 alone had any effect on micturition parameters and by the inability of MK-467 to inhibit the effects of Dex, suggesting that the relevant Dex effects were exerted within the CNS.