The detection of differential β‐adrenoceptor blockade in the cat

Abstract

Tachycardia and bronchodilatation were produced by intravenous dimethyl‐phenylpiperazinium iodide (DMPP) in the spinal or pithed cat. Responses other than those involving β‐adrenoceptors were minimized by appropriate blocking agents. With intact adrenals, both the tachycardia and bronchodilatation with 10 μg/kg DMPP were equal to those with 1 μg/kg adrenaline. After adrenalectomy, tachycardia with 100 μg/kg DMPP was equal to that with 1·5 μg/kg adrenaline, but the bronchodilatation was equal to that with only 0·3 μg/kg. This difference may reflect the relative activity of the sympathetic supply to the organs. After adrenalectomy, propranolol was 3 times as effective against bronchodilatation due to DMPP as against tachycardia. Practolol was inactive against bronchodilatation. With intact adrenals, propranolol reduced both tachycardia and bronchodilatation due to DMPP equally. Propranolol also antagonized equally both effects of adrenaline in the pithed cat after adrenalectomy. In the spinal cat, propranolol caused bradycardia and bronchoconstriction that persisted after adrenalectomy or pithing. It was reduced by pempidine and guanethidine and is attributed to spontaneous adrenergic neuronal activity. The Appendix describes a device for cycle‐by‐cycle spirometry and correction for zero drift of a micromanometer, used in this study for the continuous recording of bronchial resistance. 1973 British Pharmacological Society