The Role Of Perivascular Adipose Tissue And Insulin In Regulating Microvascular Tone Within Skeletal Muscle Resistance Arteries

Background & Aims: Microvascular tone of resistance arteries controls downstream nutritive blood flow to tissues, including skeletal muscle: the predominant site of insulin-induced glucose uptake. Thus, microvascular tone has implications for insulin-sensitivity and ultimately the development of type II diabetes. Perivascular adipose tissue (PVAT), surrounding virtually all blood vessels, is a vasoactive tissue which exerts a relaxant ‘anti-contractile’ effect upon the vasculature. However, few studies on skeletal muscle arteries have investigated this effect and the role of insulin on vascular tone. Herein, we aimed to establish an in vitro model for investigating the vasoactive effect of PVAT on skeletal muscle resistance arteries, and also determine the role of insulin in regulating vascular tone.

Methods: Wire myography was used to assess vascular contractility of arterial segments, with (+) or without (-) PVAT, from hindlimb skeletal muscle resistance arteries (proximal caudal femoral artery) of healthy male Wistar rats (&tld;250g). Viable vessels achieved tensions ≥0.3mN/mm when challenged with 60mM high-K+ physiological saline solution (KPSS). Arterial segments (length: &tld;3mm; internal diameter: &tld;150µm) were subsequently pre-constricted with noradrenaline (30µM) and the presence of functional endothelium was confirmed by vasodilation ( ≥25% reduction of noradrenaline-induced tension) in response to carbachol (30µM, &tld;10minutes). Vessels were then challenged with either serotonin or noradrenaline (-8 to -4.5 log10M), and the effects of insulin (2nM, 30min pre-incubation) were investigated on the noradrenaline-induced dose-response. Responses were normalised to post-dose-response KPSS-induced constrictions. All drugs were diluted in physiological saline solution. LogEC50 (shown below: mean ±SEM) and Emax values were calculated and compared via unpaired Student’s t-tests.

Results: The presence or absence of endothelium and/or PVAT did not affect the basal vasoreactivity of skeletal muscle resistance arteries, as KPSS-induced constrictions were unchanged. PVAT was found to exert an endothelium-independent anti-contractile effect in response to serotonin (-PVAT, -7.146 ±0.075 M, N=8 vs +PVAT, -6.918 ±0.069 M, N=8; P=0.043), which was potentiated by the presence of endothelium (+Endothelium, -6.623 ±0.072 M, N=3; P=0.041). Contrastingly, endothelium but not PVAT exerted anti-contractility in response to noradrenaline (+PVAT: -Endothelium, -5.881 ±0.053 M, N=8 vs +Endothelium, -5.685 ±0.069 M, N=10; P=0.047). Following insulin pre-incubation, noradrenaline appeared to induce either PVAT-mediated anti-contractility or pro-contractility depending upon the absence (Noradrenaline, 5.881 ±0.053 M, N=8 vs Insulin, 5.900 ±0.066 M, N=4; P=0.831) or presence (Noradrenaline, 5.685 ±0.069 M, N=10 vs Insulin, 5.975 ±0.005 M, N=2; P=0.102) of endothelium.

Conclusions: Serotonergic and adrenergic stimulation elicit anti-contractile responses through PVAT and the endothelium, respectively. Consistent with previous studies, insulin has a vaso-modulatory role, acting as both a vasodilator and vasoconstrictor through different mechanisms not investigated herein. In future, refinement of this model should allow for the vasoactive effects of insulin to be investigated in relation to obesity-induced insulin-resistance and the development of the devastatingly prevalent metabolic syndrome.