SUSTAINED PREJUNCTIONAL FACILITATION OF NORADRENERGIC NEUROTRANSMISSION BY ADRENALINE AS A COTRANSMITTER IN THE PORTAL-VEIN OF FREELY MOVING RATS

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 2 h. 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 B-2-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 B-2-adrenoceptors for at least 48 h after administration.