Abstract

Perivascular adipose tissue (PVAT) exerts an anti-contractile effect which is vital in regulating blood pressure. Evidence suggests that the sympathetic nervous stimulation of PVAT triggers the release of anti-contractile factors via activation of beta₃-adrenoceptors. There is considerable evidence of sympathetic over-activity in obesity, which could result in the loss of PVAT function, and subsequent hypertension. Therefore it was decided to examine beta₃-adrenoceptor function in obesity.

Electrical field stimulation (EFS) profiles of healthy and obese mouse mesenteric arteries (<200 um, +/-PVAT) were characterised using wire myography (0.1–30 Hz, 20V, 0.2 ms pulse duration, 4s train duration). To demonstrate the release of an anti-contractile factor in health, the solution surrounding stimulated exogenous PVAT was transferred to a PVAT denuded vessel. Beta_3-adrenoceptor function was investigated using the agonist CL-316,243 (10uM) and antagonist SR59203A (100nM). The role of the vasodilator nitric oxide (NO) was studied using nitric oxide synthase (NOS) inhibitor L-NMMA (100uM), and NOS activator histamine (100uM).

During EFS healthy PVAT elicits an anti-contractile effect (n = 8, P < 0.001); however the anti-contractile function of obese PVAT is lost (n = 8, P = 0.35). Inhibition of beta_3-adrenoceptors in healthy PVAT using SR59230A significantly reduced the anti-contractile effect (n = 8, P < 0.01), whereas activation of beta_3-adrenoceptors in obese PVAT using CL-316,243 did not restore function (n = 7, P = 0.77). Solution transfer from stimulated healthy exogenous PVAT to a -PVAT vessel significantly reduced contraction (n = 8, P < 0.01), confirming that stimulated PVAT releases a transferable anti-contractile factor. The release of this factor could be inhibited using SR59230A (n = 7, P = 0.47). Solution transfer from obese PVAT had no effect on contraction (n = 6, P = 0.41), and again could not be restored using CL-316,243 (n = 6, P = 0.14). In healthy PVAT, inhibition of NOS using L-NMMA abolished the anti-contractile effect (n = 8, P < 0.01). In obese PVAT, activation of NOS using histamine was able to restore the anti-contractile function (n = 4, P < 0.05).

These results demonstrate that in health PVAT releases an anti-contractile factor via activation of beta₃-adrenoreceptors, which downstream trigger the release of NO. In obesity, the anti-contractile effect is lost and cannot be restored by beta₃-adrenoceptor activation, but is restored by activation of NOS. This suggests that in obesity beta₃-adrenoreceptors must be downregulated or desensitised, leading to a loss of anti-contractile function, which may contribute to the development of hypertension.