Increased noradrenaline release in rat portal vein during sympathetic nerve stimulation due to activation of presynaptic beta-adrenoceptors by noradrenaline and adrenaline.

With the aim of investigating whether exogenous noradrenaline (NA) and adrenaline (A) can modulate transmitter release via the stimulation of presynaptic beta-adrenoceptors, 3H-release from isolated portal veins was studied after pretreatment with 3H-1-NA, phenoxybenzamine, desipramine and normetanephrine. NA (10 muM) and A (0.05 muM) increased the fractional 3H-release elicited by sympathetic nerve stimulation by 30%. This effect could be blocked by d, 1-propranolol which per se reduced the release by 10%. It is concluded that NA can facilitate its own release via a presynaptic beta-adrenoceptor-mediated positive feed-back mechanism and that adrenaline can stimulate this beta-adrenoceptor-mediated mechanism.