Investigating the Role of α2-adrenoceptors in Tail Artery of Wild Type Mice and Mice Lacking α1-adrenoceptors (α1-null)

In rat tail artery, perivascular nerve stimulation leads to vessel contraction through stimulation of α1-, α2-adrenoceptors (AR) and P2X receptors. The ensuing contraction has 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). Generation of mice lacking all three α1-AR subtypes has allowed the α2-ARs to be fully investigated. Male Wild Type (C57Bl) and α1-null mice (aged 4-6 months) were killed by carbon dioxide asphyxiation. 2mm ring segments were prepared free of fat from the tail artery and mounted on a wire myograph suspended in freshly gassed PSS at 37oC. In both strains of mice, responses to 1µM 5-HT were recorded and frequency response curves (FRCs) were constructed in the absence and presence of 100nM rauwolscine (α2-AR antagonist). Vessels were stimulated with the following parameters: 0.5Hz-8Hz; 0.3ms pulse width; 20pulses; 20v. The response to 5-HT (1µM) was significantly smaller in the α1-null mice (0.51g ± 0.20) than in the WT (0.69g ± 0.24) (P<0.05; unpaired t test; WT – n=13, α1-null – n=8). Nerve stimulation revealed no significant difference in size of contraction between the strains at 0.5Hz and 1Hz. However at 2Hz, 4Hz and 8Hz the WT response was significantly greater than in the α1-null strain (P<0.05; unpaired t test; WT – n=12, α1-null – n=6). For example, at 8Hz the WT response was 0.53g (± 0.21) and the α1-null response was 0.30g (± 0.16). In the presence of rauwolscine, both strains produced responses at all frequencies that were significantly lower than control responses (P<0.05; unpaired t test; WT – n=6, α1-null – n=5) with a greater percentage reduction at the lower frequencies. In the WT at 0.5Hz and 1Hz there was an 87±9% and 83±5% reduction respectively. As the frequency increases the percentage reduction in response decreases so that at 8Hz there was a 56±5% decrease. In the α1-null mice the percentage decrease in response after rauwolscine incubation was similar across the range of frequencies (68%-74%). In conclusion, the α1-null vessels showed a modest reduction in contractility to the test agonist 5-HT but demonstrated adrenergic nerve responses equivalent to those in WT mice at low frequencies. At higher frequencies the WT had greater responses that can be attributed to α1-ARs, which were lost in the α1-null mice and survived α2-AR blockade by rauwolscine; however they had a substantial rauwolscine-sensitive, α2-AR-mediated component. In the α1-null vessels throughout the frequency range there was a substantial reduction in response by rauwolscine, indicating a dominant α2-AR response to nerve activation that can be studied without the complication of drug action overlapping with α1-ARs