CARDIAC ADRENOCEPTORS AT LOW TEMPERATURE AND THE ADRENOCEPTOR INTERCONVERSION HYPOTHESIS

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Abstract
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.