Cardiac adrenoceptors at low temperature and the adrenoceptor interconversion hypothesis.

1 The hypothesis that low temperature converts inotropic beta-adrenoceptors to alpha-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 degrees C to 14 degrees C did not significantly alter the inotropic potency of the sympathomimetic drugs isoprenaline, adrenaline and phenylephrine and did not reduce the potency of the beta-adrenoceptor blocking drug propranolol as an isoprenaline antagonist. In the isolated left atrium of the rat lowering the temperature from 31 degrees C and 24 degrees C to 17-19 degrees C did not significantly alter the inotropic potency of isoprenaline, noradrenaline and phenylephrine, did not diminish the potency of the beta-adrenoceptor blocking drug propranolol and did not increase the potency of the alpha-adrenoceptor blocking drug phentolamine. 3 Phenylephrine acted on alpha- and beta-adrenoceptors in rat left atrium at 31 degrees C and 24 degrees C, but only on beta-adrenoceptors at 17-19 degrees C.