Temperature dependence of phenoxybenzamine effects and the adrenoceptor transformation hypothesis

Abstract

ADRENOCEPTORS can be classified into two types, α or β, according to their sensitivity to agonists and antagonists. It has been claimed that the β-adrenoceptors mediating the effect of adrenaline on the contractility of frog isolated heart are changed to α-adrenoceptors by lowering the temperature from 24 to 14 °C (ref. 1). The evidence for this suggestion was that a high concentration of the α-adrenoceptor-blocking drug phenoxybenzamine inhibits the effect of adrenaline at the low temperature but not at the higher temperature, and that the frog heart binds almost twice as much phenoxybenzamine at the low temperature than at the higher temperature. Phenoxybenzamine, particularly in high concentration, does not, however, specifically block α-adrenoceptors, and the purpose of the present experiments was to test the hypothesis of the receptor transformation by procedures not dependent on the use of this drug.