Raised tone reveals purinergic-mediated responses to sympathetic nerve stimulation in the rat perfused mesenteric vascular bed

Abstract

Noradrenaline and ATP are sympathetic co-transmitters. In rat isolated mesenteric small arteries, activation of sympathetic nerves can produce a vasoconstrictor response mediated by ATP. In contrast, the rat perfused mesenteric bed displays vasoconstrictor responses that are blocked solely by α₁-adrenoceptor antagonists. This study assessed the effect of raising tone with a vasoconstrictor on purinergic and noradrenergic responses to sympathetic nerve stimulation in the rat perfused mesentery. Rat mesenteric vascular beds were perfused with physiological salt solution and responses to nerve stimulation, or P2X-receptor agonists, were determined under basal conditions and after raising tone with endothelin-1. The contribution of noradrenaline and ATP to sympathetic nerve-mediated responses was assessed using the α₁-adrenoceptor antagonist, prazosin and the P2X-receptor desensitizing agent, α,β-methyleneATP. The effect of endothelin-1 on excitatory junction potentials generated in response to nerve stimulation in isolated mesenteric arteries was also assessed. Under baseline conditions, responses to nerve stimulation were mediated solely by activation of α₁-adrenoceptors. After raising perfusion pressure with endothelin-1 or the thromboxane mimetic 9,11-dideoxy-11α,9α-epoxymethanoprostaglandin F_2α (U44619), sympathetic nerve-mediated responses were larger than under basal conditions and the response was partly sensitive to P2X-receptor desensitization. Responses to exogenous P2X-receptor agonists were enhanced after treatment with endothelin-1, while endothelin-1 decreased the amplitude of excitatory junction potentials. These results indicate that ATP acts as an important, functional, sympathetic neurotransmitter in the perfused mesentery under raised tone conditions, where the perfusion pressure is closer to that found in vivo. This effect is due to a postjunctional enhancement of purinergic function.

Introduction

ATP and noradrenaline are co-stored and co-released from sympathetic neurons and can cause vasoconstriction by activating vascular P2X receptors and α₁-adrenoceptors respectively (Burnstock and Kennedy, 1985, Brock and Cunnane, 1999). There is now a substantial body of evidence indicating that both noradrenaline and ATP act as functional co-transmitters in many isolated large arteries including rabbit isolated mesenteric arteries (von Kugelgen and Starke, 1985), rat tail artery (Sneddon and Burnstock, 1984) and rabbit proximal saphenous artery (Burnstock and Warland, 1987).

The potential role for ATP as a functional sympathetic neurotransmitter in the resistance vasculature appears to depend upon the experimental methodology employed to study nerve-mediated responses, or the prevailing experimental conditions. In isometrically-mounted rat small mesenteric arteries, a small purinergic response to nerve stimulation was first suggested by Angus et al. (1988), although the major contribution (> 90%) was provided by noradrenaline, acting via postjunctional α₁-adrenoceptors. Subsequently, Sjoblom-Widfeldt et al. (1990) showed that purinergic responses were more evident at low frequencies of nerve stimulation. More recently, Gitterman and Evans (2001) have provided evidence for ATP involvement in sympathetic nerve-mediated responses in Mg²⁺-free modified physiological buffer. They suggested an increased role for activation of P2X receptors as the size of the mesenteric arteries decreased. Similarly, Luo et al. (2003) have shown that ATP is the principal sympathetic neurotransmitter in superfused small mesenteric arteries (< 200 μm) in which changes in diameter in response to nerve stimulation were monitored using a video-tracking system.

While it is of interest to determine the involvement of ATP as a sympathetic neurotransmitter in isolated small arteries, it is also important to determine which sympathetic neurotransmitters are responsible for controlling vascular resistance in the whole organ. Most studies that have examined responses to sympathetic nerve stimulation in the isolated, perfused, mesenteric vascular bed have shown that the response is almost completely sensitive to α-adrenoceptor antagonists (Eikenburg, 1984, Kong et al., 1994, Williams and Clarke, 1995). Donoso et al. (1997) proposed a modest role for ATP as a sympathetic neurotransmitter in this vascular bed, while Yamamoto et al. (1992) showed that cooling could uncover a purinergic response to periarterial nerve stimulation. Given the available evidence indicating a role for P2X-receptor activation after nerve stimulation in rat isolated small mesenteric arteries, it is surprising that it has been relatively difficult to demonstrate a purinergic response in the perfused mesenteric vascular bed, which contains both small and large arteries.

In the present study, we have assessed the potential role of ATP as a sympathetic neurotransmitter in the rat mesenteric vascular bed under conditions where the baseline perfusion pressure was increased to a more physiological level using either endothelin-1 or the thromboxane mimetic, U46619.