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CARDIAC ADRENOCEPTORS AT LOW TEMPERATURE AND THE ADRENOCEPTOR INTERCONVERSION HYPOTHESIS

1 The hypothesis that low temperature converts inotropic β‐adrenoceptors to α‐adrenoceptors has been tested in isolated heart preparations of the frog and rat. 2 The results do not support the adrenoceptor interconversion hypothesis. In the frog ventricle strip lowering the temperature from 24°C to 14°C did not significantly alter the inotropic potency of the sympathomimetic drugs isoprenaline, adrenaline and phenylephrine and did not reduce the potency of the β‐adrenoceptor blocking drug propranolol as an isoprenaline antagonist. In the isolated left atrium of the rat lowering the temperature from 31°C and 24°C to 17–19°C did not significantly alter the inotropic potency of isoprenaline, noradrenaline and phenylephrine, did not diminish the potency of the β‐adrenoceptor blocking drug propranolol and did not increase the potency of the α‐adrenoceptor blocking drug phentolamine. 3 Phenylephrine acted on α‐ and β‐adrenoceptors in rat left atrium at 31°C and 24°C, but only on β‐adrenoceptors at 17–19°C.