AN ANALYSIS OF SOME FACTORS INFLUENCING ALPHA-ADRENOCEPTOR FEEDBACK AT THE SYMPATHETIC JUNCTION IN THE RAT-HEART

The effects of the selective prejunctional α‐adrenoceptor antagonist, yohimbine, on the cardio‐acceleration responses to sympathetic stimulation were examined in the pithed rat. Yohimbine reversed the inhibitory effects of the α‐adrenoceptor agonist, clonidine. on the stimulation‐induced tachycardia. Yohimbine failed to potentiate significantly responses to stimulation in the absence of clonidine when stimulation was applied at the optimal level for cardiac responses (C6‐T1). When the pithing rod electrode was moved to T2‐T6, cardioaccelerator responses were smaller and yohimbine produced potentiation at frequencies of ≥ 1 Hz. This potentiation was prejunctional since responses to exogenous noradrenaline (NA) were not increased by yohimbine. In the presence of cocaine, potentiation by yohimbine could be shown at the lower frequency of 0.1 Hz. The possible physiological significance of a negative feed‐back effect of noradrenaline on cardiac sympathetic nerves is discussed. 1979 British Pharmacological Society