THE SELECTIVITY OF β‐ADRENOCEPTOR ANTAGONISTS ON CARDIOVASCULAR AND BRONCHODILATOR RESPONSES TO ISOPRENALINE IN THE ANAESTHETIZED DOG

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Abstract
1 The actions of five β-adrenoceptor antagonists, chosen because of reported differences in their selectivities, were compared using the positive chronotropic, vasodepressor and bronchodilator responses to isoprenaline in anaesthetized dogs. 2 Propranolol was a potent antagonist of the isoprenaline responses in all three systems. 3 Practolol and acebutolol (M & B 17,803) blocked the positive chronotropic responses to isoprenaline to a greater extent than the vasodepressor or bronchodilator responses. 4 Butoxamine and α-methyl dichloroisoprenaline showed the opposite selectivity, blocking the vasodepressor and bronchodilator responses to isoprenaline to a greater extent than positive chronotropic responses. However, both drugs were considerably less potent than the other antagonists studied and their selectivities were less clear-cut than those of practolol or acebutolol. 5 All the antagonists lowered the resting heart rate and to a lesser extent the diastolic blood pressure. The effects of propranolol, practolol and acebutolol on heart rate probably result from cardiac β-adrenoceptor blockade. With butoxamine and α-methyl dichloroisoprenaline, however, the effects on heart rate probably result from a direct cardiodepressant action. 6 The relevance of the results to the problem of the sub-classification of β-adrenoceptors is discussed.