1 The modulatory effects of agonists and antagonists of prejunctional alpha (2)-adrenoceptors on the electrical field stimulation (EFS, 0.3 ms, 12 V)-induced release of endogenous noradrenaline (NA) and the cotransmitter adenosine 5' triphosphate (ATP) were measured in endothelium-denuded segments of; canine inferior mesenteric artery and compared with effects in mesenteric vein. The overflow of NA and ATP was evoked by long-duration (2 min) EFS at low frequency (4 Hz) and high frequency (16 Hz) of stimulation and was analysed using HPLC techniques with electrochemical detection and fluorescence detection, respectively. 2 The EFS-evoked overflow of both NA and ATP was significantly reduced by tetrodotoxin (1 muM) and guanethidine (10 muM) in the artery and vein. Desipramine (10 muM), a blocker of neuronal uptake of NA, increased the EFS (4 and 16 Hz)-evoked overflow of NA in both artery and vein. EFS-evoked overflow of NA in vein exceeded the NA overflow in artery at both 4 and 16 Hz in control preparations as well as in the presence of desipramine. However, the EFS-evoked overflow of ATP was equal in the artery and vein. 3 Stimulation of alpha (2)-adrenoceptors with clonidine (0.1 muM) and oxymethazoline (0.3 muM) reduced the EFS evoked overflow of NA in both artery and vein at 4 Hz, whereas the NA overflow at 16 Hz remained unchanged in both blood vessels. The overflow of ATP as well as of ADP land hence ATP:ADP ratio) was unaffected by the az-adrenoceptor agonists in the artery and vein. 4 In artery, blockade of alpha (2)-adrenoceptors with yohimbine at a concentration of 0.1 muM caused no effect on the NA overflow neither at 4 Hz nor at 16 Hz of EFS. Yohimbine at a concentration of 1 muM increased the overflow of NA at 4 Hz but not 16 Hz of EFS. In vein, however, yohimbine (0.1 and 1 muM) increased NA overflow at both 4 and 16 Hz of stimulation. Idazoxan (1 muM) increased the :NA overflow in artery only at 4 Hz, whereas in vein idazoxan increased the NA overflow at both 4 and 16 Hz. No changes of EFS-evoked ATP overflow were observed in the presence of 0.1 muM yohimbine in both artery and vein. Greater concentration of yohimbine (i.e. 1 muM) increased the overflow of ATP in both the artery and vein only at 4 Hz EFS. Idazoxan (1 muM) enhanced the ATP overflow only at 16 Hz in vein. The overflow of ADP was affected by both yohimbine and idazoxan in a similar manner to the ATP overflow so that the ATP:ADP ratios were not changed. 5 In conclusion, sympathetic nerves in both mesenteric arteries and veins appear to release ATP along with NA. Release of NA in veins exceeds release of NA in arteries, whereas both the canine artery and vein release equal amount of ATP. At long-duration nerve stimulation las might occur during stress) the alpha (2)-adrenoceptors appear to rather modulate release of NA than release of the cotransmitter ATP. The prejunctional autoinhibition of NA release is more effective at lower frequencies of nerve stimulation. The alpha (2)-adrenoceptor-mediated neuromodulation plays a greater role in veins than arteries. Quantitative differences in alpha (2)-adrenoceptor-mediated neuromodulation in the arteries and veins may participate to differing contributions of mesenteric blood vessels to the control of blood now and volume distribution in splanchnic circulation.
