Summary
The pre- and post-junctional effects of five α-adrenoceptor agonist drugs were examined in the pithed rat. On pre-junctional receptors on the cardioac-celerator nerves, oxymetazoline, guanabenz and xylazine had effects which were qualitatively similar to the established effects of clonidine; phenylephrine had not. These pre-junctional effects could be antagonised by yohimbine. Post-junctional effects of the agonists, assessed as the diastolic arterial pressor response or contraction of the anococcygeus, showed an unexpected pattern. The effects of phenylephrine were almost abolished by prazosin (1 mg/kg) whereas the effects of the other four agonists, clonidine, oxymetazoline, guanabenz and xylazine were only moderately antagonised, guanabenz and xylazine being particularly resistant; this resulted in an order of agonist potency for the “prazosin-resistant” responses which was similar to that for pre-junctional α-adrenoceptor agonism on the cardiac or vas deferens sympathetic nerves. Yohimbine (1 mg/kg) produced greater antagonism of the pressor effects of guanabenz and xylazine than did prazosin. It is postulated that at least two types of post-junctional α-adrenoceptors exist in rat tissues, one of which is similar to α1-adrenoceptors previously demonstrated and another which has similarities to the prejunctional α2-adrenoceptors. Noradrenaline released from vasopressor nerves or administered intravenously could be shown to produce post-junctional effects which were partly resistant to prazosin, suggesting a physiological role for each of these α-adrenoceptors.
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Docherty, J.R., McGrath, J.C. A comparison of pre- and post-junctional potencies of several alpha-adrenoceptor agonists in the cardiovascular system and anococcygeus muscle of the rat. Naunyn-Schmiedeberg's Arch. Pharmacol. 312, 107–116 (1980). https://doi.org/10.1007/BF00569718
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DOI: https://doi.org/10.1007/BF00569718