Raised tone reveals purinergic-mediated responses to sympathetic nerve stimulation in the rat perfused mesenteric vascular bed
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.
Section snippets
Mesenteric vascular bed preparations
Male Wistar rats (225–250 g) (Charles River Laboratory; Kent, UK) were killed by CO2 overdose followed by exsanguination. The abdominal cavity was opened and the superior mesenteric artery was identified, cleaned of connective tissue and cannulated with a blunted hypodermic needle (No. 21). The superior mesenteric vein was cut and the preparation flushed with 0.5 ml Krebs’ solution. The mesenteric vascular bed was separated from the gut by carefully cutting close to the intestinal wall. The
Responses to nerve stimulation in the rat perfused mesenteric vascular bed under basal and raised tone conditions
Under basal tone conditions (baseline pressure = 22 ± 1.7 mmHg, n = 18), electrical field stimulation produced an increase in perfusion pressure that was frequency-dependent (Fig. 1). Pre-treatment with endothelin-1 (1.5–2 nM) significantly increased the perfusion pressure of preparations from 23 ± 1.3 mmHg to 51 ± 3.9 mmHg (P < 0.05, Student's paired t-test) (n = 19). Under these conditions, the response to electrical field stimulation was biphasic reflecting an initial pressor followed by a depressor
Discussion
The main findings of the present study are that inducing tone with either U46619 or endothelin-1, uncovered a purinergic response to nerve stimulation that was absent under basal conditions in the rat perfused mesenteric vascular bed. Further investigations showed that endothelin-1 enhanced postjunctional activation of P2X receptors but reduced the amplitude of excitatory junction potentials produced as a consequence of the release of ATP from sympathetic nerve varicosities. These observations
Acknowledgements
This work was supported by a grant from the British Heart Foundation (PG/03/116/16045); PP holds a scholarship from the Royal Thai Government.
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