The Role of the Sympathetic Nervous System in the Perivascular Adipose Tissue Anti-Contractile Effect

Objective: Healthy perivascular adipose tissue (PVAT) exerts an anti-contractile effect on blood vessels, and therefore plays an important role in maintaining vascular tone (1).However the mechanisms behind this effect are not well understood. The vasculature is predominantly innervated by sympathetic nerve fibres (2), and sympathetic nerves have been shown to be present in PVAT (3). Therefore we propose that firing of sympathetic nerves within PVAT triggers the release of anti-contractile factors.

Methods & Results: The frequency stimulation profiles (0.1-30Hz, 20V, 0.2ms pulse duration, 4s train duration) of mouse mesenteric arteries (<200μm, C57BL6/J), +/- PVAT were characterised using wire myography (4). PVAT elicited a significant endothelium-independent anti-contractile effect (n=8, P<0.0001), which could be replicated using exogenous application of PVAT onto –PVAT vessels (n=8, P<0.0001). Solution transfer studies from electrically-stimulated exogenous PVAT, onto –PVAT vessels, demonstrated that the anti-contractile effect is due to a transferable factor released from PVAT (n=8, P<0.001). Tetrodotoxin (1μM) and 6-hydroxydopamine (2μM) were used to sympathetically denervate exogenous PVAT, and successfully abolished the anti-contractile effect (both n=8, TTX P<0.0001, 6-hydroxydopamine P<0.001), confirming that the effect is due to sympathetic nerve firing. Noradrenaline re-uptake inhibitor nisoxetine (1μM) also inhibited the anti-contractile effect of exogenous PVAT (n=8, P<0.001), demonstrating that sympathetic nerves within PVAT release noradrenaline to exert an anti-contractile effect. Specific adrenoceptor stimulation of PVAT was investigated using β₃-adrenoceptor agonist CL-316,243 (10μM), and antagonist SR59230A (100nM). CL-316,243 significantly enhanced the anti-contractile effect (n=8, P<0.01), and SR59230A significantly reduced the anti-contractile effect (n=4, P<0.01). This suggests that noradrenaline released from sympathetic nerve fibres stimulates β₃-adrenoceptors within the PVAT, to trigger the release of an anti-contractile factor. Nitric oxide synthase inhibitor L-NMMA (100μM) was used to investigate the role of nitric oxide (NO), and was found to inhibit the exogenous PVAT anti-contractile effect (n=8, P<0.0001). Iberiotoxin (100nM), the large-conductance calcium-activated potassium channel (BK_Ca) toxin, also significantly reduced the anti-contractile effect (n=8, P<0.001). These results suggest that downstream of β₃-adrenoceptors, NO and BK_Ca channels play a part in inducing the anti-contractile effect.

Conclusion: These results clearly demonstrate that stimulation of sympathetic nerves within PVAT can provoke the release of anti-contractile factors via a mechanism involving β₃-adrenoceptors, NO and BK_Ca channels. The anti-contractile function of PVAT is lost in obesity (5); therefore further studies will be carried out in an obese model to determine how over-activity of sympathetic nerves in obesity (6) may lead to de-sensitisation, and a loss of PVAT function.

References:

1 – Soltis EE and Cassis LA (1991). ClinExpHypertens 12: 277-296.

2 –Bevan JA and Su C (1973). Annu Rev Pharmacol 13: 269-85.

3 – Giordano A et al. (2006). Am J PhyiolRegulIntegr Comp Physiol 291: R1243-1255.

4 – Mulvany MJ and Halpern W (1977). Circ Res 41: 19-26.

5 – Greenstein AS et al., (2009).Circulation 119: 1661-1670.

6 – Smith MM and Minson CT (2012). J Physiol 590: 1787-1801.