Contribution of imidazoline receptors and alpha2-adrenoceptors in the rostral ventrolateral medulla to sympathetic baroreflex inhibition by systemic rilmenidine.
OBJECTIVE
To determine whether the hypotensive and sympathetic baroreflex inhibition by rilmenidine administered systemically are mediated via imidazoline receptors in the rostral ventrolateral medulla (RVLM).


METHODS
Initial dose-response curves to rilmenidine were determined in urethane anaesthetized rabbits. Effects of a single intravenous dose of rilmenidine (445 microg/kg) on the renal sympathetic nerve activity (RSNA) baroreflex were examined before and after microinjection into the RVLM of the mixed imidazoline/alpha2-adrenoceptor antagonist idazoxan and the alpha2-adrenoceptor antagonist 2-methoxyidazoxan (2-MI).


RESULTS
Intravenous administration of rilmenidine lowered mean arterial pressure and RSNA, inhibited the RSNA baroreflex range by 33% and shifted the baroreflex curve to the left. Idazoxan injected into the RVLM reversed the hypotension and completely restored the baroreflex curve at doses that did not affect the hypotension produced by the selective alpha2-adrenoceptor agonist alpha-methylnoradrenaline. The alpha2-adrenoceptor antagonist, 2-MI also reversed the rilmenidine sympatho-inhibition suggesting that alpha2-adrenoceptors are activated as well.


CONCLUSIONS
The results of the present study show that the hypotensive and sympatho-inhibitory actions of systemic rilmenidine are primarily mediated via imidazoline receptors in the RVLM. However, alpha2-adrenoceptors are also involved, probably as a direct result of the imidazoline receptor action.
Chan
Candy K S
CK
Neuropharmacology Laboratory, Baker Heart Research Institute, PO Box 6492, St Kilda Road Central, Melbourne, Victoria 8008, Australia.
Burke
Sandra L
SL
Head
Geoffrey A
GA
D058647:Adrenergic alpha-2 Receptor Agonists; D000316:Adrenergic alpha-Agonists / Q000008:administration & dosage / Q000494:pharmacology*; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D000959:Antihypertensive Agents / Q000008:administration & dosage / Q000494:pharmacology*; D017704:Baroreflex / Q000187:drug effects*; D001794:Blood Pressure / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D019329:Idazoxan / Q000031:analogs & derivatives / Q000494:pharmacology; D054755:Imidazoline Receptors; D007275:Injections, Intravenous; D007668:Kidney / Q000294:innervation; D008526:Medulla Oblongata / Q000187:drug effects* / Q000378:metabolism; D009433:Neural Inhibition / Q000187:drug effects; D010080:Oxazoles / Q000008:administration & dosage / Q000494:pharmacology*; D011817:Rabbits; D018341:Receptors, Adrenergic, alpha-2 / Q000378:metabolism; D011955:Receptors, Drug / Q000819:agonists* / Q000378:metabolism; D000077769:Rilmenidine; D013564:Sympathetic Nervous System / Q000187:drug effects* / Q000378:metabolism
D058647:Adrenergic alpha-2 Receptor Agonists; D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D000959:Antihypertensive Agents; D054755:Imidazoline Receptors; D010080:Oxazoles; D018341:Receptors, Adrenergic, alpha-2; D011955:Receptors, Drug; C410887:imidazoline I1 receptors; C058525:2-methoxyidazoxan; D000077769:Rilmenidine; D019329:Idazoxan