Sympathetic Neurovascular Transmission In The Mouse Tail Artery

Claire Stevenson1, Craig Daly1, Elspeth McLachlan2, John Christie McGrath1. 1School of Life Science, MVLS, University of Glasgow, Glasgow, UK, 2Neuroscience Research Australia, Randwick, NSW, Australia

In order to analyse the postjunctional mechanisms involved in sympathetic neurovascular transmission, we are developing transgenic mice, particularly α1-adrenoceptor (AR) knockouts. The first step, however, is to define neurovascular transmission in the tail artery of normal mice using pharmacological antagonists, as has been done for the rat. Perivascular stimulation of rat tail artery evokes depolarisations mediated by ATP and noradrenaline (Sneddon and Burnstock, 1984) but contraction is mediated largely by α1- and α2-ARs, probably acting synergistically (Brock et al., 1997; Yeoh et al., 2004). The α2-AR-mediated effects are more readily shown distally (Medgett, 1985). Further, activation of capsaicin-sensitive peptidergic afferent nerves, present in small numbers around the rat tail artery (Li and Duckles, 1993), can elicit relaxation. Here we report a pharmacological analysis of neurovascular transmission in proximal and distal mouse tail artery.

Male C57Bl mice (4-6 months) were killed with CO2. Ring segments of the tail artery 2mm long were prepared from proximal (2cm) and distal (5cm) sites. Vessels were mounted on a wire myograph in oxygenated physiological saline at 37oC. Vessels were incubated for 30 minutes in 1 µM capsaicin to block release of calcitonin gene-related peptide from afferent nerves. Frequency response curves (FRC) to maximal stimuli were constructed in the presence of capsaicin(0.5Hz-8Hz; 20 pulses; 0.3ms pulse width; 20V) and in the presence of various combinations of 100nM prazosin (α1-AR antagonist), 100nM rauwolscine (α2-AR antagonist), and 1mM suramin (P2X receptor antagonist). Paired and unpaired t-tests were carried out as appropriate, with P<0.05 taken as significant. IGOR Pro was used to display averaged traces from different groups of animals.

In proximal and distal segments, prazosin reduced responses to 0.5 Hz by 34.3 ± 7.8% and 31.4 ± 5.1% respectively (n=7). The proximal response to 8 Hz was reduced by 51.3 ± 4.1% and the distal response by 41.1 ± 5.7% (n=7). Rauwolscine reduced the response to 0.5 Hz by 82.0 ± 4.3% and 75.1 ± 5.1% in proximal and distal segments respectively. The proximal response to 8 Hz was reduced by rauwolscine by 33.2 ± 5.3% and the distal response by 29.0 ± 3.6%. The effect of rauwolscine was significantly greater than that of prazosin in proximal and distal segments at 0.5 Hz (P<0.005) and proximal segments at 8 Hz (P<0.05; n=7). Suramin had variable effects with potentiation of responses in 4 out of 7 proximal segments and 2 out of 7 distal segments.

While both post-junctional α1- and α2-ARs are involved in nerve evoked contraction of the proximal mouse tail artery, contractile responses mediated by α2-ARs dominate at low stimulation frequencies, especially in the distal part. Unlike the rat tail artery, the contribution of P2X receptors to nerve evoked contraction is substantial and will be clarified in mice that lack α1-ARs.

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Brock et al. (1997) Br J Pharmacol. 106: 1513-1521

Yeoh et al. (2004) J Phsyiol. 561: 583-596

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Li and Duckles. (1993) J Pharmacol Exp Ther. 264: 1305-1310