α1-ADRENOCEPTOR ACTIVATION CAN INCREASE HEART RATE DIRECTLY OR DECREASE IT INDIRECTLY THROUGH PARASYMPATHETIC ACTIVATION

Open Access
Publisher Website
Google Scholar
Abstract
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. α1-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 α1-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 α1-adrenoceptor antagonists. This may indicate α1-adrenoceptors on preganglionic parasympathetic nerves, stimulation of these receptors causing release of acetylcholine. 5 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.

This publication has 24 references indexed in Scilit: