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The action of isoprenaline on the perfused vessels of the rabbit ear

In rabbit isolated perfused ear preparations isoprenaline in doses of 1 to 5 μg caused a vasodilatation which was specifically blocked by propranolol. In a minority of experiments larger doses of isoprenaline (5 to 10 μg) caused vasoconstriction. The vasodilatations to isoprenaline were increased by raising vascular tone with barium chloride whereas ergotamine and sympathetic stimulation were relatively ineffective. In the isolated ear central artery preparation, isoprenaline caused a vasoconstriction due to α‐receptor stimulation. A vasodilator response due to β‐receptor stimulation was revealed when perfusion pressure was raised with barium chloride. It is concluded that α‐ and β‐adrenergic receptors are present in both preparations but a higher proportion of β‐receptors are present in the whole ear.