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1 Effects of the a2-adrenoceptor agonists, UK14304 and clonidine, the 5-HT1 receptor agonist, sumatriptan and the K-opioid receptor agonist, GR103545, on sensory neurotransmission in histaminecontracted guinea-pig isolated pulmonary artery (GPPA) have been studied. 2 Electrical field stimulation (EFS) induced frequency-dependent relaxations of histamine-contracted GPPA, which were attenuated by tetrodotoxin and capsaicin pretreatment but not by the nitric oxide synthase inhibitor, Nw-nitro-L-arginine methyl ester (L-NAME).3 Substance P (0.3 tM) induced relaxations which were subject to rapid tachyphylaxis. Neither the NK, receptor antagonist, ( ± )-CP 96,345, nor desensitization to substance P had any effect of EFS-induced relaxations of histamine-contracted GPPA. 5 Sumatriptan (0.1 and 1 jtM) and the selective x-opioid receptor agonist, GR103545 (10 and 100 nM) had no effect on EFS-induced relaxations of histamine-contracted GPPA. In contrast, the M2-adrenoceptor agonists UK14304 (1-I00 nM) and clonidine (300 nM) attenuated responses to EFS, the attenuation of UK14304 (100 nM) being reversed by yohimbine (300 nM). 6 It is concluded that in GPPA, where a presynaptic inhibition of sensory neurotransmission by M2-adrenoceptor activation could be shown, there was no evidence for such modulation by either sumatriptan-sensitive 5-HT, receptors or x-opioid receptors.

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