Sympathetic Neurotransmission In Tail Artery Taken From Mice Lacking α1-adrenoceptors

Claire Stevenson1, Craig Daly1, Elspeth McLachlan1,2, John Christie McGrath1. 1School of Life Sciences, University of Glasgow, Glasgow, UK, 2Prince of Wales Medical Research Institute, University of South Wales, New South Wales, Australia.

Studies in the rat tail artery indicate a receptor population including α1-, α2-adrenoceptors (AR) and P2X receptors that respond to electrical stimulation to produce an initial fast depolarisation (P2X receptor activation) followed by a sustained response from α1- and α2-AR activation (Sneddon and Burnstock, 1984; Rummery and Brock, 2011). The receptor population in the mouse tail artery has yet to be fully investigated. The generation of mice lacking all three α1-AR subtypes (α1-null) provides a means of establishing the importance of α1-ARs and the compensatory mechanisms that exist within the mouse vasculature.

Male Wild Type (WT) (C57Bl) and α1-null mice (aged 4-6 months) were killed by carbon dioxide asphyxiation. 2mm ring segments where removed from the tail artery and were prepared free of fat. Vessels were mounted on a wire myograph suspended in freshly gassed PSS at 37oC. Vessel diameter was recorded from each strain of mouse. WT and α1-null vessel response to 1µM 5-HT and 62.5mM KCl (test of vessel ability to contract) was recorded in each segment. Arterial rings were incubated with capsaicin (1µM) for 30 minutes to block the sensory nerve relaxatory response. Control frequency response curves (FRC) were constructed in both strains of mice (0.5Hz-8Hz; 0.3ms pulse width; 20 pulses; 20volts). FRCs in the WT mice were constructed in the presence of 100nM rauwolscine (α2-AR antagonist), 100nM prazosin (α1-AR antagonist) and 1mM suramin (P2X receptor antagonist). Vessel response to αβ-meATP (1µM) was also investigated. Unpaired t-tests were carried out on analysis between the strains and paired t-tests were used when analysing data from the same strain in the presence and absence of pharmacological agents.

No significant difference in response to 5-HT was recorded between the WT and α1-null mice (P = 0.06) (WT – n = 13, α1-null – n = 8). Response from the WT control FRC at each frequency was significantly greater than responses recorded from the α1-null mice (WT – n = 7, α1-null – n = 4). The α1-null vessel response at each frequency was 40%-70% lower than the WT vessel responses. In the presence of rauwolscine, both strains produced responses at all frequencies that were significantly lower than control responses (P<0.05). In the presence of rauwolscine and prazosin, the WT vessels displayed reduced responses that were significantly lower than responses collected in presence of rauwolscine (P<0.05). WT vessels incubated with rauwolscine, prazosin and suramin overall reduced the response from control by between 94% and 99% (WT – n = 7, α1-null – n = 6).

In conclusion, we demonstrate that the α1-null mice display a reduced response to nerve stimulation in the tail artery in comparison with the WT vessel response. Vasoconstriction of WT tail artery is due to activation of α1-, α2-ARs and P2X receptors as response is virtually blocked in the collective presence of α1-, α2-, and P2X receptor antagonists. Further investigation is required to determine the receptor population in the α1-null mice.

Rummery and Brock. (2011) Auton Neurosci. 59:45-50

Sneddon and Burnstock. (1984) Eur J Pharmacol. 106:149-152