Modulation of noradrenaline release in the conscious rabbit through alpha-adrenoceptors

The effects of selective alpha 1- and alpha 2-adrenoceptor antagonists and agonists on the noradrenaline release rate and plasma catecholamine levels were studied in the conscious rabbit. The selective alpha 2-adrenoceptor blocking drugs yohimbine and rauwolscine (1 mg/kg i.v.) increased the rate of noradrenaline release into the plasma and the plasma noradrenaline and adrenaline levels. This was associated with a rise in blood pressure. The selective alpha 1-blocking drug corynanthine (1 mg/kg i.v.) had no effect. Intravenous infusions of the selective alpha 2-adrenoceptor agonist alpha-methylnoradrenaline (2 micrograms/kg per min) and the alpha 1-agonist phenylephrine (6 micrograms/kg per min) produced equipressor responses. However, only alpha-methylnoradrenaline decreased the noradrenaline release rate and the plasma noradrenaline and adrenaline levels, effects which were blocked by yohimbine. The results are compatible with, but of course do not prove the hypothesis that in the conscious rabbit, noradrenaline release from sympathetic nerves is modulated through presynaptic alpha 2-adrenoceptors. Adrenaline release from the adrenal medulla may also be subject to alpha 2-adrenergic modulation.