title: Comparison of pre-junctional alpha-adrenoceptors at the neuromuscular junction with vascular post-junctional alpha-receptors in cat skeletal muscle.
abstract: 1 Activation of pre-junctional alpha-adrenoceptors at the skeletal neuromuscular junction enhances acetylcholine release whereas activation of such receptors at autonomic nerve endings inhibits transmitter output. In the present study the characteristics of pre-junctional alpha-adrenoceptors at motor nerve terminals have been compared with post-junctional (vascular) alpha-adrenoceptors in the cat hind limb.2 Reversal of partial (+)-tubocurarine blockade of contractions of the tibialis anterior muscle was used to monitor pre-junctional activity and increases in hindlimb vascular resistance to assess post-junctional actions at alpha-adrenoceptors.3 Responses to intra-arterial injections of noradrenaline, adrenaline, phenylephrine, oxymetazoline, methoxamine and clonidine were monitored. Dose-response lines for all the compounds except clonidine were parallel. The latter agent produced only weak and inconsistent effects.4 Ratios of the doses of the agents required to produce pre- and post-junctional effects indicated that oxymetazoline and adrenaline possessed some preferential activity at post-junctional sites, whereas the remaining agents were non-selective in their actions. If dose-ratios with respect to noradrenaline were compared at the two sites none of the compounds possessed a marked degree of selectivity.5 In the presence of phentolamine or tolazoline, dose-response curves to the pre- and post-junctional effects of phenylephrine were shifted to a similar extent. Thymoxamine showed preferential activity as a pre-junctional alpha-receptor antagonist.6 In comparing the results of this study with those of other authors, it is apparent that there are marked differences in the characteristics of pre-junctional alpha-receptors at the skeletal neuromuscular junction and at autonomic nerve endings. The pre- and post-junctional alpha-receptors in skeletal muscle show less divergence.
mesh_terms: D000818:Animals; D002415:Cats; D004347:Drug Interactions; D005260:Female; D006614:Hindlimb / Q000098:blood supply; D066298:In Vitro Techniques; D008297:Male; D013948:Moxisylyte / Q000494:pharmacology; D009132:Muscles / Q000098:blood supply*; D009469:Neuromuscular Junction / Q000187:drug effects*; D010646:Phentolamine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D011941:Receptors, Adrenergic / Q000187:drug effects*; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects*; D012039:Regional Blood Flow / Q000187:drug effects; D014043:Tolazoline / Q000494:pharmacology; D014655:Vascular Resistance / Q000187:drug effects
chemical_list: D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; D010656:Phenylephrine; D014043:Tolazoline; D013948:Moxisylyte; D010646:Phentolamine