Renal sympathetic activity affects blood pressure in part by increasing renovascular resistance via release of norepinephrine (NE) from sympathetic nerves onto renal arteries. Here we test the idea that adipose tissue adjacent to renal blood vessels, i.e. renal perivascular adipose tissue (RPVAT), contains a pool of NE which can be released to alter renal vascular function. RPVAT was obtained from around the main renal artery/vein of the male Sprague Dawley rats. Thoracic aortic PVAT and mesenteric PVAT also were studied as brown-like and white fat comparators respectively. RPVAT was identified as a mix of white and brown adipocytes, because of expression of both brown-like (e.g. uncoupling protein 1) and white adipogenic genes. All PVATs contained NE (ng/g tissue, RPVAT:524 ± 68, TAPVAT:740 ± 16, MPVAT:96 ± 24). NE was visualized specifically in RPVAT adipocytes by immunohistochemistry. The presence of RPVAT (+RPVAT) did not alter the response of isolated renal arteries to NE compared to responses of arteries without RPVAT (-RPVAT). By contrast, the maximum contraction to the sympathomimetic tyramine was ~2× greater in the renal artery +PVAT versus -PVAT. Tyramine-induced contraction in +RPVAT renal arteries was reduced by the α-adrenoceptor antagonist prazosin and the NE transporter inhibitor nisoxetine. These results suggest that tyramine caused release of NE from RPVAT. Renal denervation significantly (>50%) reduced NE content of RPVAT but did not modify tyramine-induced contraction of +RPVAT renal arteries. Collectively, these data support the existence of a releasable pool of NE in RPVAT that is independent of renal sympathetic innervation and has the potential to change renal arterial function.

Rat perivascular adipose tissue (PVAT) stores, takes up, and releases norepinephrine (NE; Ayala-Lopez et al 2014). We hypothesized that 3T3-L1 adipocytes would exhibit similar behaviors and, thus, could serve as a model for PVAT adipocytes. However, basal levels of NE were not detected in 3T3-L1 adipocytes. While incubation of 3T3-L1 adipocytes with exogenous NE increased their cellular NE content, the expression of mRNAs of several NE transporters [e.g. norepinephrine transporter (NET)] were not detected in these cells. Similarly, we observed expression of the vesicular monoamine transporter 1 (VMAT1) in 3T3-L1 adipocytes by qRT-PCR and immunostaining, but stimulation of the cells with tyramine (100 µM) did not cause a significant release of NE. These studies support that 3T3-L1 adipocytes are not an adequate model of perivascular adipocytes for studying NE handling.

Perivascular adipose tissue (PVAT) is gaining importance as its ability to secrete and take up substances that modify vascular tone, and thus blood pressure. The kidney is inarguably important to blood pressure regulation, but little is known about renal arterial PVAT (RP) relative to its adipogenic profile or its function to modify arterial contraction as specifically conducted by a resident adrenergic system. We hypothesized that NE was present in RP, that RP had a mixture of both brown and white adipocyte gene signatures and that the NE in RP could promote renal artery contraction. Our model was the isolated renal artery +/- PVAT of the male Sprague Dawley rat, with thoracic aortic PVAT (TAP) and mesenteric PVAT (MP) as brown-like and white fat comparators, respectively. HPLC and immunohistochemistry (IHC) were used to detect NE; RT-PCR to construct an adipogenic/adrenergic profile; and myography to measure contraction. All PVATs contained NE (ng/g tissue, n=4-6): RP:524±68, TAP:740±16, MP:96±24. In RP, NE, visualized by IHC, was clearly present in adipose tissue resembling both brown and white fat. The identification of brown fat was validated by positive uncoupling protein-1 (UCP-1) staining in the RP; this made up some but not all of the RP. RT-PCR measures (2-ddCt values, n=4) supported expression of both brown-like (UCP-1: RP: 1.0±0.1, MP 3.2e-05±2.94e-05, TAP: 1.3±0.1; Cidea: RP: 1.0±0.1, MP: 0.01±0.002, TAP: 0.9±0.1) and white adipogenic genes (Tcf21: RP: 1.0±0.2, MP: 5.9±1.0, TAP: 0.3±0.1). The existence of a functional adrenergic system in the RP was supported by two findings. First, the efficacy of the indirect sympathomimetic tyramine to cause contraction was greater in isolated renal artery (n=11) +PVAT (14.4+1.8mN) vs -PVAT (7.6+1.2mN). Second, tyramine-induced contraction in +PVAT tissues was reduced by the α1-adrenoceptor antagonist prazosin (100 nM; 90+5%, n=6) and NE transporter inhibitor nisoxetine (1 uM; -logEC50 [M]- Veh: -5.0+0.4; nisoxetine = 3.7+0.2 n=4). Collectively, these data support the existence of a functional adrenergic system in a PVAT that has a profound potential to change renal arterial function. Knowledge about this fat depot may increase our ability to modify renal function in the treatment of hypertension.