α 1‐ADRENOCEPTOR ACTIVATION CAN INCREASE HEART RATE DIRECTLY OR DECREASE IT INDIRECTLY THROUGH PARASYMPATHETIC ACTIVATION

N. A. FLAVAHAN, J. C. McGRATH

Research output: Contribution to journalArticle

Abstract

The chronotropic effects of α‐ and β‐adrenoceptor agonists were investigated in the pithed rat. The β‐adrenoceptor agonist, isoprenaline, produced only a positive chronotropic response. α1‐Adrenoceptor agonists, phenylephrine and amidephrine, produced positive and negative chronotropic effects. Part of the response to phenylephrine was β‐mediated. A positive chronotropic response to amidephrine and phenylephrine was mediated directly through cardiac α1‐adrenoceptors and had a different time course from β‐adrenoceptor‐mediated responses. A negative chronotropic response to α‐agonists was potentiated by neostigmine and blocked by atropine, tetrodotoxin or hexamethonium as well as by α1‐adrenoceptor antagonists. This may indicate α1‐adrenoceptors on preganglionic parasympathetic nerves, stimulation of these receptors causing release of acetylcholine. The α2‐adrenoceptor agonist, xylazine, produced a direct negative chronotropic effect on the heart, independent of α‐adrenoceptors. No evidence was found for functional post‐junctional α2‐adrenoceptors. At high doses xylazine stimulated cardiac α1‐adrenoceptors. 1982 British Pharmacological Society

Original languageEnglish (US)
Pages (from-to)319-328
Number of pages10
JournalBritish Journal of Pharmacology
Volume77
Issue number2
DOIs
StatePublished - Oct 1982
Externally publishedYes

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ASJC Scopus subject areas

  • Pharmacology

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