Unravelling the role of α2-adrenoceptors and P2X purinoceptors in vascular sympathetic neurotransmission using a mouse lacking α1-adrenoceptors

Stevenson, Claire (2015) Unravelling the role of α2-adrenoceptors and P2X purinoceptors in vascular sympathetic neurotransmission using a mouse lacking α1-adrenoceptors. PhD thesis, University of Glasgow.

Abstract

The experiments presented in this thesis describe the roles of the post-junctional α1, α2-adrenoceptors (AR) and P2X purinoceptors in response to sympathetic nerve stimulation in mouse mesenteric and tail arteries. Such roles were determined by combining wire myography techniques and nerve stimulation alongside various selective antagonists. The influence of each receptor on the response to nerve stimulation was first defined in wild type (WT) mice before analysis of α1-AR knock out (KO) mice. Therefore the effect that genetic removal of the α1-ARs had on vascular response could be investigated. The response is the force generated in milligrams/tension when the nerves are stimulated and correspond to activation of post-junctional receptors and subsequent smooth muscle cell contraction.

Systolic Blood Pressure and the α1-ARs
The aim of the first study (Chapter Three) was to determine whether systolic blood pressure (BP) was different in the KO mice compared to WT controls. Tail cuff measurements of systolic BP were recorded in WT and AR KO mice. It was found that BP was not affected by loss of the α1A- and α1D¬-AR subtypes (ADKO) or by loss of all three α1-AR subtypes (α1-null). Therefore, the α1-AR role in maintaining BP may not be as crucial as previously understood or, the remaining receptors may have compensated for the loss.

Calcitonin Gene Related Peptide (CGRP) in Mouse mesenteric and tail arteries
The second study (Chapter Four) examined whether, under the stimulation parameters used, the potent vasodilator CGRP masked the vasoconstrictor response to nerve stimulation in mouse mesenteric and tail arteries. The potent neurotoxin capsaicin depletes the sensory nerves of CGRP. Responses prior to capsaicin incubation were compared with those following capsaicin treatment in WT, ADKO and α1-null mice. In mesenteric artery from each mouse strain, capsaicin incubation had no significant effect on the peak response to nerve stimulation. Furthermore, in the tail artery, capsaicin treatment had no significant effects on the responses in WT and α1-null mice. However, in the ADKO mouse tail artery preparations, capsaicin treatment significantly increased the peak response at low frequency stimulation. These findings may indicate an interaction between the α1-AR subtypes and the release of CGRP whereby the presence of all or none of the subtypes (WT and α1-null) has no effect on CGRP release but loss of the α1A- and α1D-AR subtypes alters the balance and reveals a CGRP induced effect.

Response to perivascular nerve stimulation in Mouse mesenteric and tail arteries from Wild Type mice
The aim of Chapter Five was to determine which receptors were involved in the response to nerve stimulation in mouse mesenteric and tail arteries. These results would then act as a comparison with the ADKO and α1-null responses in the later chapters. A contractile response to nerve stimulation was recorded in each of the studied vessels and abolished by combined blocked with antagonists for the α1-, α2¬-ARs and P2X receptors. Individual receptor blockade and component analysis then revealed the roles of the individual receptors. In the mesenteric arteries, the α1-ARs were the main contributors to the response followed by the P2X receptors and finally the α2-ARs which displayed both pre- and post-junctional effects. No interactions were discovered between the α1-ARs and the other receptors. The P2X receptors initiated the contraction and prolonged the response at the low frequency and contributed to the contractile response at the high frequency. In the tail artery, the α2-ARs were the dominant receptors but required the presence of the α1-ARs, and to some extent the P2X receptors in order to produce a full contractile response upon activation. The P2X receptors alone initiated the response at low frequency stimulation with both α1-ARs and P2X receptors involved in initiation of the contraction at the high frequency.

Response to perivascular nerve stimulation in mesenteric and tail arteries from mice lacking α1A- and α1D-AR subtypes (ADKO)
Chapter Six determined the role of the α1B-ARs in the response to nerve stimulation and therefore examined whether loss of the α1A- and α1D-AR subtypes (ADKO) affected the response. In the mesenteric arteries, at the low frequency, there was a potential interaction between the receptors. However, no single receptor was responsible for the contraction although there was a trend for the P2X receptors to be active at the beginning of the response and the α2-ARs to be active at the latter stage of the response. This was also true at the higher frequency. The ADKO vessels displayed no evidence of pre-junctional α2-ARs. In the mesenteric arteries, there is little role for the α1B-ARs. The overall contribution in the tail artery was reduced in the ADKO, particularly at the higher frequency. Similar to the WT mice, the α2-ARs were the main contributor to the response. The P2X receptors initiated the contraction but required the α2-ARs to be active at the low frequency. The α1B-ARs also required an interaction with the other receptors at low frequency stimulation in order to contribute to the initiation of the response. The α1B-ARs also contributed to the initiation of the response at the high frequency without requiring the presence of the other receptors.

Response to perivascular nerve stimulation in mesenteric and tail arteries from mice lacking α1A-, α1B- and α1D-AR subtypes (α1-null)
In the final study (Chapter Seven), α1-null mice were utilised in order to determine whether loss of the α1-ARs had a significant effect on the response to nerve stimulation in mouse mesenteric and tail arteries. Furthermore, the influence of the α2-ARs and P2X receptors were compared with the response in the WT vessels to determine whether they compensate for the loss of the α1-ARs. The removal of the α1-ARs dramatically reduced the response to nerve stimulation in the mesenteric arteries, with no apparent compensation from the α2-ARs or P2X receptors. There was no evidence of pre-junctional α2-ARs. The small response recorded at the high frequency was initiated by the P2X receptors and maintained by the α2-ARs and P2X receptors. The tail artery response was smaller in the α1-null mice compared with WT, particularly at the higher frequency. However, the response was still mediated largely by the α2-ARs with an interaction with the P2X receptors likely. A potential compensatory mechanism was recorded at the lower frequency as the response to P2X receptor antagonism was greater in the α1-null than in the WT mice. As was shown in the WT, activation of the P2X receptors initiated the contraction, particularly at the high frequency.

Findings and Results
Collectively, the findings of the studies presented in this thesis demonstrate that the α1-, α2-ARs and P2X receptors are involved in the response to nerve stimulation in the mouse mesenteric and tail arteries. Genetic removal of the α1-ARs in the ADKO and α1-null is most effective in the mesenteric arteries with little evidence of any compensatory mechanisms. In the mouse tail artery, the α2-ARs are the main contributors to the response and so loss of the α1-ARs has less of an effect. Interactions between the receptors were most clearly shown in the tail artery with little interaction between the α1-ARs and the other receptors demonstrated in the mesentery. Furthermore, it has been demonstrated here that systolic BP is unaffected in the KO mice and there is little input from the CGRP nerves using the parameters tested. From these results, the importance of α1-AR activation in nerve mediated responses was determined. The absence of a compensatory mechanism to match the response lost in the ADKO and α1-null mice, and the presentation of a normal BP alongside the survival of these transgenic mice challenges the importance of the α1-ARs being crucial in the maintenance of vascular tone. This therefore complements the knowledge that treatment of primary hypertension with α1-AR antagonists is largely unsuccessful. The contractile response mediated by α2-ARs and P2X receptors may be used as potential therapeutic targets in controlling hypertension.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: adrenoceptors, purinoceptors, sympathetic neurotransmission, myography, mouse mesenteric artery, mouse tail artery, blood pressure, CGRP,
• Subjects: Q Science > Q Science (General); Q Science > QP Physiology; R Medicine > RM Therapeutics. Pharmacology
• Colleges/Schools: College of Medical Veterinary and Life Sciences
• Supervisor's Name: Daly, Dr. Craig and McGrath, Professor Ian
• Date of Award: 2015
• Depositing User: Dr Claire Stevenson
• Unique ID: glathesis:2015-6431
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 15 Jun 2015 11:53
• Last Modified: 18 Jun 2015 14:55
• URI: https://theses.gla.ac.uk/id/eprint/6431