THE BETA-ADRENERGIC BLOCKING AND PRESSOR ACTION OF ISOPRENALINE IN THE CAT.

In the anaesthetized cat a single, large, intravenous dose of (-)-isoprenaline reversibly blocked the depressor properties of isoprenaline. Further large doses then caused monophasic rises in blood pressure, which were similar in rate of onset and duration to those produced by adrenaline and noradrenaline. This pressor effect was not due to an increase in cardiac output, but the main cause was vasoconstriction in skeletal muscle. The responses to isoprenaline, adrenaline and noradrenaline were compared, both before and after the blocking dose of isoprenaline. Isoprenaline prevented or antagonized the potentiation of the pressor effects of adrenaline and noradrenaline by cocaine. The pressor effect was blocked but not reversed by ergotamine and tolazine, which block sympathetic alpha-receptors. It is suggested that isoprenaline in large doses has two actions on the vascular system of the cat: a blockade of the sympathetic beta-receptors and an excitation of alpha-receptors.