Alpha2-adrenoceptor subtypes involved in the regulation of catecholamine release from the adrenal medulla of mice.
OBJECTIVE
This study was carried out to elucidate which alpha(2)-adrenoceptor subtypes mediated the inhibition of noradrenaline and adrenaline release from the adrenal medulla of mice.


METHODS
Isolated adrenal medullae from wild-type and alpha(2A), alpha(2B) and alpha(2C)-adrenoceptor knockout (KO) mice were placed in superfusion chambers. Catecholamine overflow was evoked by 1,1-dimethyl-4-phenylpiperazinium (500 microM) in absence or in presence of the alpha(2)-adrenoceptor agonist medetomidine. The effect of medetomidine was tested in presence of the alpha-adrenoceptor antagonists rauwolscine, WB 4101, spiroxatrine, phentolamine and prazosin.


RESULTS
In wild-type mice, medetomidine reduced noradrenaline and adrenaline overflow in a concentration-dependent manner (EC(50) in nM: 1.54 and 1.92; E(max) in % of inhibition: 91 and 94, for noradrenaline and adrenaline, respectively). The pK (D) values of the antagonists for noradrenaline overflow did not correlate with pK(D) values at alpha(2A), alpha(2B), or alpha(2C) binding sites. The pK (D) values of the antagonists for adrenaline overflow correlated positively with pK(D) values at alpha(2C) binding sites (opossum kidney cells). The effect of medetomidine (100 nM) on noradrenaline overflow was significantly reduced in all three alpha(2)KO mice (57, 54, 44 % inhibition, for alpha(2A), alpha(2B), and alpha(2C), respectively), whereas the effect of medetomidine on adrenaline overflow was greatly reduced in alpha(2C)KO mice (14 % inhibition).


CONCLUSIONS
In the adrenal medulla of mice, all three alpha(2)-adrenoceptor subtypes (alpha(2A), alpha(2B), and alpha(2C)) play an equal role in the inhibition of noradrenaline overflow, whereas the alpha(2C)-adrenoceptor is the predominant alpha(2)-adrenoceptor subtype involved in the inhibitory mechanism controlling adrenaline overflow.
Moura
E
E
Institute of Pharmacology and Therapeutics, Faculty of Medicine, University of Porto, Alameda Prof. Hernani Monteiro, Porto, Portugal.
Afonso
J
J
Hein
L
L
Vieira-Coelho
M A
MA
D000313:Adrenal Medulla / Q000378:metabolism*; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000818:Animals; D002395:Catecholamines / Q000378:metabolism*; D020927:Dexmedetomidine / Q000494:pharmacology; D004295:Dihydroxyphenylalanine / Q000494:pharmacology; D004246:Dimethylphenylpiperazinium Iodide / Q000494:pharmacology; D004298:Dopamine / Q000378:metabolism; D015259:Dopamine Agents / Q000494:pharmacology; D004837:Epinephrine / Q000378:metabolism; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D018722:Nicotinic Agonists / Q000494:pharmacology; D009638:Norepinephrine / Q000378:metabolism; D018341:Receptors, Adrenergic, alpha-2 / Q000187:drug effects* / Q000235:genetics*; D014446:Tyrosine 3-Monooxygenase / Q000378:metabolism
D000316:Adrenergic alpha-Agonists; D002395:Catecholamines; D015259:Dopamine Agents; D018722:Nicotinic Agonists; D018341:Receptors, Adrenergic, alpha-2; D004246:Dimethylphenylpiperazinium Iodide; D004295:Dihydroxyphenylalanine; D020927:Dexmedetomidine; D014446:Tyrosine 3-Monooxygenase; D004298:Dopamine; D009638:Norepinephrine; D004837:Epinephrine