Effects of ephedrine on postsynaptic alpha-adrenoceptors and presynaptic beta-adrenoceptors in isolated guinea pig portal veins

The effects of ephedrine (Eph) were compared with those of tyramine (Tyr) and phenylephrine (Phe) in ring segments of guinea pig portal vein in vitro. Eph (3-1000 mumol/L), Tyr (10-1000 mumol/L) and Phe (1-1000 mumol/L) all produced concentration-dependent contractile responses, which were exceedingly depressed by alpha-adrenoceptor blocker phentolamine (31 mumol/L). Pretreatment with reserpine 1 mg/(kg.d) x 2 d markedly diminished the effect of Tyr, but greatly potentiated the effects of Eph and Phe. Both Eph (1-30 mumol/L) and Tyr (10-100 mumol/L), but not Phe, significantly increased the electrical field stimulation (duration 2 ms, 3 Hz, 10 s, 50 V, 10 min intervals) evoked contractions of the portal veins. beta-Adrenoceptor blocker propranolol (0.5 mumol/L) greatly inhibited this effect of Eph, without affecting that of Tyr. It is suggested that the effect of Tyr is mainly due to its release of endogenous norepinephrine (NE) from the nerve terminals; conversely, Eph mainly bcts on postsynaptic alpha-adrenoceptors directly with some NE-releasing action which may involve the activation of presynaptic beta-adrenoceptors.