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α-Excitatory and β-inhibitory activities of dopamine and noradrenaline were compared on the rabbit aorta in vitro. Dopamine showed about 1/50 the excitatory activity of noradrenaline. The pA 10 value of phentolamine against dopamine was 6.5 ± 0.03 as compared with the value of 6.2 ± 0.04 against noradrenaline. pA 10 values of haloperidol against dopamine and noradrenaline were 6.9 ± 0.06 and 7.1 ± 0.14 respectively. The inhibitory activity of dopamine was less than 1/10 that of noradrenaline and markedly less than 1/100 that of isoprenaline. When propranolol was used to block the inhibitory action of the catecholamines, original excitatory action was unexpectedly observed, even though the strips had been treated with phenoxybenzamine 3 × 10 −7 g/ml for 5 min.
J D Kohli (Sat,) studied this question.
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