Pharmacological characterization of alpha 2-adrenoceptors in isolated jejunum of rabbits.
In the isolated jejunum of rabbits, norepinephrine (NE) lowered the muscle tone in a dose-dependent manner which was potentiated by yohimbine, an antagonist of alpha 2-adrenoceptors. Guanethidine and/or bretylium, the adrenergic neuron blockers, attenuated this action of yohimbine, indicating the participation of neuronal alpha 2-adrenoceptors. In the presence of guanethidine and atropine, clonidine and guanabenz reduced the relative responses to forskolin, but they did not modify the responses to dibutyryl cAMP. The inhibition mediated by these alpha 2-adrenergic agonists was abolished by pertussis toxin, an inhibitor of Gi protein. The actions of clonidine and guanabenz were also blocked by yohimbine and/or rauwolscine. Thus, the Gi protein mediated inhibition of adenylate cyclase by postsynaptic alpha 2-adrenoceptors in muscle cells can be considered. The obtained data indicate that alpha 2-adrenoceptors are presented in adrenergic nerve terminals and smooth muscle of jejunum of rabbits to function as the feed-back autoreceptors in autonomic neurotransmission.
Cheng
J T
JT
Department of Pharmacology, College of Medicine, National Cheng Kung University, Taipei City, Taiwan, R.O.C.
Tsai
L H
LH
D000818:Animals; D003000:Clonidine / Q000494:pharmacology; D005576:Colforsin / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000502:physiology; D006143:Guanabenz / Q000494:pharmacology; D066298:In Vitro Techniques; D007583:Jejunum / Q000187:drug effects* / Q000502:physiology; D009119:Muscle Contraction / Q000187:drug effects; D009638:Norepinephrine / Q000494:pharmacology; D011817:Rabbits; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects*
D011942:Receptors, Adrenergic, alpha; D005576:Colforsin; D019204:GTP-Binding Proteins; D006143:Guanabenz; D003000:Clonidine; D009638:Norepinephrine