THE EFFECTS OF THREE β‐ADRENOCEPTOR BLOCKING DRUGS ON ISOLATED PREPARATIONS OF SKELETAL AND CARDIAC MUSCLE

Abstract 1 The effects of propranolol, oxprenolol and practolol on the isometric twitch responses to electrical stimulation of isolated diaphragm muscles from the rat and of isolated papillary muscles from the rabbit are described. 2 Depression of the twitch responses of the diaphragm muscle was produced by propranolol (20 μg/ml), by oxprenolol (100 μg/ml) and by practolol (500 μg/ml). 3 Depression of the twitch responses of the papillary muscles was produced by propranolol (20 μg/ml) by oxprenolol (100 μg/ml) and by practolol (200 μg/ml). 4 No increase of twitch tension was produced by oxprenolol or practolol on either tissue. 5 It is concluded that propranolol, oxprenolol and practolol produce negative inotropic actions on isolated cardiac muscle by a mechanism unrelated to blockade of β-adrenoceptors and which occurs at doses which are well in excess of those doses required to produce β-blockade.