Sustained prejunctional facilitation of noradrenergic neurotransmission by adrenaline as a co‐transmitter in the portal vein of freely moving rats

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Abstract
1 The duration of the facilitatory effect of adrenaline on the electrically evoked overflow of noradrenaline was studied in the portal vein of permanently adreno-demedullated freely moving rats. 2 Rats were infused with adrenaline (20 or 100 ng min-1) for 2h. After an interval of 1 h, when plasma adrenaline had returned to undetectable levels, electrical stimulation resulted in an enhanced catecholamine overflow amounting to 219% (noradrenaline) and 241% (noradrenaline plus adrenaline) of control (saline infusion), respectively. 3 When stimulation was applied again, in the same animal, at 24, 48 and 72 h after the first stimulation episode, the evoked noradrenaline overflow was 150, 111 and 102% (after 20 ng ml-1 adrenaline) and 158, 134 and 105% (after 100 ng min-1 adrenaline) of control. 4 The β2-adrenoceptor antagonist, ICI 118,551 (0.3 mg kg-1), blocked the facilitatory effect obtained after the 100 ng min-1 adrenaline infusion on all days. 5 The results show that adrenaline, after being taken up by and released from sympathetic nerve terminals, is able to facilitate the evoked noradrenaline overflow through activation of prejunctional β2-adrenoceptors for at least 48 h after administration.