Frequency dependent alpha(2)-adrenoceptor mediated modulation of excitatory junction potentials in guinea-pig mesenteric artery.

Excitatory junctional potentials (EJPs) elicited with brief duration (10 s) electrical field stimulation of guinea-pig mesenteric arteries were nearly abolished at all frequencies by pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, 30 microM) but persisted following reserpinization. Suramin (100 microM) enhanced EJPs at 0.2-0.5 Hz responses and reduced them at 2-32 Hz. Phentolamine (1 microM) and yohimbine (0.1 microM) enhanced EJPs at 0.2-8 Hz but not at 16-32 Hz. Oxymetazoline (0.3 microM) reduced EJPs at 0.2-0.5 Hz but not at 1-32 Hz. Following reserpinization, EJPs were enhanced at 0.2-2 Hz but not at 4-32 Hz. Clonidine (0.1 microM) was without effect at all frequencies in control arteries but reduced EJPs at 0.2-2 Hz in reserpine-treated arteries. In conclusion, pre-junctional alpha(2)-adrenoceptors modulate ATP release during low frequency, brief duration sympathetic nerve stimulation.