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α 1 ‐ADRENOCEPTOR ACTIVATION CAN INCREASE HEART RATE DIRECTLY OR DECREASE IT INDIRECTLY THROUGH PARASYMPATHETIC ACTIVATION
Author(s) -
FLAVAHAN N.A.,
McGRATH J.C.
Publication year - 1982
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.1476-5381.1982.tb09301.x
Subject(s) - chronotropic , phenylephrine , isoprenaline , medicine , endocrinology , hexamethonium , agonist , atropine , stimulation , practolol , phentolamine , chemistry , heart rate , propranolol , receptor , blood pressure
1 The chronotropic effects of α‐ and β‐adrenoceptor agonists were investigated in the pithed rat. 2 The β‐adrenoceptor agonist, isoprenaline, produced only a positive chronotropic response. α 1 ‐Adrenoceptor agonists, phenylephrine and amidephrine, produced positive and negative chronotropic effects. Part of the response to phenylephrine was β‐mediated. 3 A positive chronotropic response to amidephrine and phenylephrine was mediated directly through cardiac α 1 ‐adrenoceptors and had a different time course from β‐adrenoceptor‐mediated responses. 4 A negative chronotropic response to α‐agonists was potentiated by neostigmine and blocked by atropine, tetrodotoxin or hexamethonium as well as by α 1 ‐adrenoceptor antagonists. This may indicate α 1 ‐adrenoceptors on preganglionic parasympathetic nerves, stimulation of these receptors causing release of acetylcholine. 5 The α 2 ‐adrenoceptor agonist, xylazine, produced a direct negative chronotropic effect on the heart, independent of α‐adrenoceptors. No evidence was found for functional post‐junctional α 2 ‐adrenoceptors. At high doses xylazine stimulated cardiac α 1 ‐adrenoceptors.