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COMPARISON OF PREJUNCTIONALα‐ADRENOCEPTORS AT THE NEUROMUSCULAR JUNCTION WITH VASCULAR POSTJUNCTIONALα‐RECEPTORS IN CAT SKELETAL MUSCLE
Author(s) -
MALTA E.,
McPHERSON G.A.,
RAPER C.
Publication year - 1979
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1979.tb07825.x
Subject(s) - oxymetazoline , clonidine , phenylephrine , endocrinology , medicine , phentolamine , neuromuscular junction , chemistry , methoxamine , agonist , receptor , biology , neuroscience , blood pressure
1 Activation of pre‐junctional α‐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 α‐adrenoceptors at motor nerve terminals have been compared with post‐junctional (vascular) α‐adrenoceptors*in the cat hind limb. 2 Reversal of partial (‐I‐)‐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 α‐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‐junctionalα‐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‐junctionalα‐receptors at the skeletal neuromuscular junction and at autonomic nerve endings. The pre‐ and post‐junctionalα‐receptors in skeletal muscle show less divergence.