Neurogenic double‐peaked vasoconstriction of human gastroepiploic artery is mediated by both α 1 ‐ and α 2 ‐adrenoceptors

1 The contribution of postjunctional P2X receptors and subtypes of α ‐adrenoceptors to vasoconstrictor responses following periarterial electrical nerve stimulation (PNS, 30 s trains of pulses at a frequency of 2, 4 or 8 Hz) was investigated in human gastroepiploic arteries. 2 The vasoconstrictor response to PNS at a stimulation of 4 or 8 Hz was a two‐peaked response, whereas at a frequency of 2 Hz it appeared only as a late peak. All vasoconstrictions evoked by PNS were abolished by phentolamine, a nonselective α ‐adrenoceptor inhibitor, but not by α , β ‐methylene ATP, a P2X receptor‐desensitizing agent. 3 The early peak to PNS at 4 or 8 Hz was abolished by prazosin, an α 1 ‐adrenoceptor antagonist, while the late one still remained, although it was markedly inhibited. The responses remaining after prazosin were blocked by rauwolscine. The vasoconstrictor response to PNS at 2 Hz was not affected by prazosin (0.1  μ M ), but was abolished by rauwolscine (0.1  μ M ), an α 2 ‐adrenoceptor antagonist. 4 OPC‐28326(10  μ M ), a newly developed vasodilator, which preferentially exerts its antagonistic actions on the α 2B ‐ and α 2C ‐adrenoceptors, significantly reduced the noradrenaline‐induced vasoconstriction in the absence or presence of prazosin. OPC‐28326 had a greater inhibitory effect on the late peak evoked by PNS than the early one. The neurogenic responses remaining after OPC‐28326 were abolished by prazosin. 5 The present results suggest that sympathetic vasoconstriction of the human gastroepiploic artery is mediated by both α 1 ‐ and α 2 ‐adrenoceptors postjunctionally, but not by P2X receptors. The α 2 ‐adrenoceptors may be preferentially activated at a low frequency of stimulation, which induces a constriction more slowly than that by α 1 ‐adrenoceptors. The existence of α 2 ‐adrenoceptors may cause an enhancement of α 1 ‐adrenoceptor‐induced responses.British Journal of Pharmacology (2005) 144 , 737–742. doi: 10.1038/sj.bjp.0705975