In the rat perfused mesenteric arterial bed raising the tone with a contractile agent revealed purinergic-mediated responses to sympathetic nerve stimulation which were not evident at basal tone conditions (Pakdeechote et al. 2007). The present study characterized contractile responses to nerve stimulation under basal and raised tone conditions in the porcine isolated perfused mesenteric vascular bed. The first order branch of the mesenteric arterial arcade was cannulated with a blunted hypodermic needle and perfused at a flow rate of 5ml/min using peristaltic pump. The effect of prazosin (0.1 mM), an a1-adrenoceptor antagonist, alone, or ab-methylene ATP (1mM), a P2X receptor desensitizing agent, on the contractile responses to electrical field stimulation (EFS) (2- 16 Hz, 1 ms, 90 V, 30 s), were investigated under basal tone conditions and after raising the tone with U46619 (10–50nM), a thromboxane A2 receptor agonist. Data were analysed by ANOVA followed by Bonferroni post test. At basal tone prazosin significantly reduced the contractile responses to electrical field stimulation (by 70 ± 8% at 16 Hz, P < 0.001, n=12), while ab-methylene ATP had no significant effect (the response was 92 ± 20% of the control). Raising the tone with U46619 increased the perfusion pressure and the size of the contractile responses to EFS (the response was increased at 16 Hz by 236 ± 75%, P < 0.05, n=8). Under these conditions prazosin reduced the contractile responses (by 60 ± 8% of 16 Hz of control, P < 0.001, n=12), while ab-methylene ATP also significantly attenuated the contractile responses to EFS (by 49 ± 9% of the control response to 16 Hz, P < 0.001, n=5). The present study has shown no significant contribution of ATP to sympathetic neurotransmission in the porcine mesenteric arterial bed under basal tone conditions, but an involvement of ATP at raised tone.

Pakdeechote P. et al. 2007. Eur. J Pharmacol 563, 180–186