Effects of DPI (2-(3,4-dihydroxyphenylimino)-imidazolidine) on prejunctional alpha-adrenoceptors and prejunctional dopamine receptors in the rabbit ear artery.

The effects of DPI on transmitter noradrenaline release were examined in rabbit isolated ear arteries previously incubated in [ 3H ]noradrenaline. DPI (0.1 mumol/l) reduced transmitter release with periarterial electrical stimulation at 1 Hz (1 ms pulses, supramaximal voltage for 30 s), which is consistent with an agonistic effect of DPI on prejunctional alpha-adrenoceptors. At stimulation frequencies of 2 or 5 Hz DPI alone did not affect release, however in the presence of phentolamine (3 mumol/l, which increased release per se) DPI again significantly reduced release; this effect was blocked by metoclopramide (1 mumol/l) indicating an agonistic effect of DPI on prejunctional dopamine receptors.