title: alpha2-Adrenergic stimulation enhances growth hormone secretion in the dog: a presynaptic mechanism?
abstract: Intravenous administration of clonidine (CLO), (2,4 and 8/micrograms/Kg), a predominantly alpha 2-adrenergic receptor agonist, induced in unanesthetized dogs clear-cut and dose-related rises in plasma GH (cGH) levels. Pretreatment with the selective antagonist of alpha 1-adrenergic receptors prazosin (0.1 mg/Kg iv) left unaltered the cGH rise induced by 4/micrograms/Kg of CLO whilst blockade of alpha 2-adrenergic receptors by yohimbine (2.5 mg/Kg iv) completely prevented it. In dogs treated 24 h previously, with reserpine (0.5 mg/Kg iv), a depletor of brain catecholamine stores, CLO was ineffective to stimulate cGH release. These data indicate that in the dog the GH-releasing effect of CLO occurs via stimulation of alpha 2-adrenergic receptors and suggest that the latter are located presynaptically in relation to norepinephrine neurons.
mesh_terms: D000818:Animals; D003000:Clonidine / Q000494:pharmacology*; D004285:Dogs; D005260:Female; D013006:Growth Hormone / Q000097:blood / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D011224:Prazosin / Q000494:pharmacology*; D011799:Quinazolines / Q000494:pharmacology*; D011941:Receptors, Adrenergic / Q000187:drug effects*; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects* / Q000502:physiology; D012110:Reserpine / Q000494:pharmacology*; D013569:Synapses / Q000502:physiology*; D015016:Yohimbine / Q000494:pharmacology*
chemical_list: D011799:Quinazolines; D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; D012110:Reserpine; D013006:Growth Hormone; D003000:Clonidine; D011224:Prazosin