DIRECT CARDIAC ACTIONS OF PHENYLEPHRINE WHEN USED IN ASSESSMENT OF BAROREFLEX FUNCTION IN THE RAT

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Abstract
1 In pentobarbitone-anaesthesized wistar rats, the baroreflex bradycardia in response to the vasopressor agent phenylephrine was converted to a trachycardia by propranolol. 2 In pithed wistar rats, phenylephrine produced a marked tachycardia which was mediated largely by .beta.-adrenoreceptors since propranolol markedly antagonized the response. A component of the tachycardia was mediated by .alpha.2-adrenoreceptors since the tachycardia to phenylephrine in the presence of propranaolol was antagonized by prazosin and mimicked by the .alpha.1-adrenoreceptor agonist amidephrine. 3 It is concluded that phenylephrine is not a useful agent for assessment of baroreflex function in the rat due to its marked cardiac stimulant actions.