Temperature‐Dependent Sensitivity of Adrenoreceptors in the Toad's Heart

A decrease of the experimental temperature from 25° to 12° resulted in a 10.2 × increase in the sensitivity of the toad's heart to the positive chronotropic effect of adrenaline, measured as ED50 ratios, and in a 4.9 × increase of the sensitivity to the α‐adrenergic drug, phenylephrine. In contrast, the sensitivity to the β‐adrenergic drug, isoprenaline, did not change significantly with temperature alteration. Acclimatization of the animals to 25° increased the sensitivity of the heart to adrenaline by 2.2 × compared wih the 10° maintained toads. The blocking capacity of the β‐adrenergic blocking agent, propranolol, was decreased with cold. The most pronounced decrease was seen in the blocking capacity to the effect of isoprenaline. On the other hand, the blocking capacity of the α‐adrenergic blocking agent, phenoxybenzamine, was decreased with warmth. In this case the most marked decrease was observable during antagonism between phenoxybenzamine and phenylephrine.