Pharmacological Analysis of Sympathetically-Mediated Constriction in Wild Type and α1-AR Knock Out Mouse Tail Artery

We have developed transgenic mice lacking the three α1-adrenoceptor (AR) subtypes. Here we report a pharmacological analysis of neurovascular transmission in segments of proximal and distal tail artery of C57/Bl (Wild type), α1AD-AR knock out (ADKO) and α1ABD-AR knock out (ABDKO) mice. In rat proximal tail artery, perivascular stimulation evokes depolarisations mediated by ATP and noradrenaline (1) but contraction is mediated largely by α1- and α2-ARs (2). α2-AR-mediated effects predominate distally (3). The ADKO enables isolation of the α1B-AR which is believed to play a minor role in the vasculature (4) whereas the ABDKO allows study of the remaining receptors to determine the compensatory mechanisms. Male Wild Type (WT), ADKO and ABDKO mice (4-6 months) were killed by CO2 asphyxiation. Ring segments (2mm) of the tail artery were prepared from proximal (2cm) and distal (5cm) sites. Vessels were mounted on wire myographs under 200mg tension. Contractions were evoked by supramaximal perivascular stimuli (20 V; 0.3 ms pulse width; 20 pulses at 0.5- 8 Hz). Tension was recorded using ‘Chart’. The effects of 100nM rauwolscine (α2-AR antagonist), 1mM suramin (P2X receptor antagonist) and 100nM prazosin (α1-AR antagonist) were tested. Data are expressed as mean ± S.E.M. Un-paired t-tests were carried out, with P<0.05 taken as significant. Proximally, at 0.5 and 8 Hz, the response to nerve stimulation was lower in KO animals compared to WT (P<0.05; WT-n=41; ADKO-n=8; ABDKO-n=14). Distally, the response to nerve stimulation was similar across the strains; except at 8 Hz where the ABDKO response was smaller than the WT response (P<0.05; WT-n=41; ADKO-n=10; ABDKO-n=11). The inhibition of response following rauwolscine incubation at 0.5 Hz was similar in each strain and location (70 % ± 10). However, proximally, at 8 Hz, the reduction was greater in the ABDKO strain compared with the WT and ADKO strain (P<0.05; WT-n=14; ADKO-n=6; ABDKO-n=6). Distally, the reduction was greater in both KO strains compared with the WT response at 8 Hz (P<0.05; WT-n=7; ADKO-n=7; ABDKO-n=6). At both locations following rauwolscine and suramin incubations, a response was detected in the WT and ADKO strains. This response was removed following further incubation with prazosin. The removal of all α1-ARs or isolation of the α1B-AR has a greater effect proximally. In the WT vessels at 0.5 Hz, the receptor mainly responsible for producing the nerve-mediated response appears to be the α2-AR and this role switches to the α1-ARs at 8 Hz. The residual response following incubation with α2-AR and P2X receptor antagonists in the ADKO indicates a role for the α1B-AR. Even with loss of all of the α1-AR subtypes, the α2-ARs and P2X receptors can support the contraction at both frequencies