Regulation of subcutaneous adipose tissue blood flow is related to measures of vascular and autonomic function

100th anniversary of the discovery of insulin perspective: insulin and adipose tissue fatty acid metabolism

Insulin inhibits systemic nonesterified fatty acid (NEFA) flux to a greater degree than glucose or any other metabolite. This remarkable effect is mainly due to insulin-mediated inhibition of intracellular triglyceride (TG) lipolysis in adipose tissues and is essential to prevent diabetic ketoacidosis, but also to limit the potential lipotoxic effects of NEFA in lean tissues that contribute to the development of diabetes complications. Insulin also regulates adipose tissue fatty acid esterification, glycerol and TG synthesis, lipogenesis, and possibly oxidation, contributing to the trapping of dietary fatty acids in the postprandial state. Excess NEFA flux at a given insulin level has been used to define in vivo adipose tissue insulin resistance. Adipose tissue insulin resistance defined in this fashion has been associated with several dysmetabolic features and complications of diabetes, but the mechanistic significance of this concept is not fully understood. This review focusses on the in vivo regulation of adipose tissue fatty acid metabolism by insulin and the mechanistic significance of the current definition of adipose tissue insulin resistance. One hundred years after the discovery of insulin and despite decades of investigations, much is still to be understood about the multifaceted in vivo actions of this hormone on adipose tissue fatty acid metabolism.

Sex differences in healthy human heart rate variability: A meta-analysis

The present meta-analysis aimed to quantify current evidence on sex differences in the autonomic control of the heart, indexed by measures of heart rate variability (HRV) in healthy human subjects. An extensive search of the literature yielded 2020 titles and abstracts, of which 172 provided sufficient reporting of sex difference in HRV. Data from 63,612 participants (31,970 females) were available for analysis. Meta-analysis yielded a total of 1154 effect size estimates (k) across 50 different measures of HRV in a cumulated total of 296,247 participants. Females showed a significantly lower mean RR interval and standard deviation of RR intervals (SDNN). The power spectral density of HRV in females is characterized by significantly less total power that contains significantly greater high- (HF) and less low-frequency (LF) power. This is further reflected by a lower LF/HF ratio. Meta-regression revealed significant effects of age, respiration control and the length of recording available for analysis. Although women showed greater mean heart rate, they showed greater vagal activity indexed by HF power of HRV. Underlying mechanisms of these findings are discussed.

Regulation of blood flow in adipose tissue: involvement of the cholinergic system

Acetylcholine (Ach) has vasodilatory actions. However, data are conflicting about the role of Ach in regulating blood flow in subcutaneous adipose tissue (ATBF). This may be related to inaccurate ATBF recording or to the responder/nonresponder (R/NR) phenomenon. We showed previously that healthy individuals are R (ATBF increases postprandially by >50% of baseline BF) or NR (ATBF increases ≤50% postprandially). Our objective was to assess the role of the cholinergic system on ATBF in R and NR subjects. ATBF was manipulated by in situ microinfusion of vasoactive agents (VA) in AT and monitored by the (133)Xenon washout technique (both recognized methods) at the VA site and at the control site. We tested incrementally increasing doses of Ach (10(-5), 10(-3), and 10(-1) mol/l; n = 15) and Ach receptor antagonists (Ra) before and after oral administration of 75-g glucose using atropine (muscarinic Ra; 10(-4) mol/l, n = 13; 10(-5) mol/l, n = 22) and mecamylamine (nicotinic Ra; 10(-3) mol/l, n = 15; 10(-4) mol/l, n = 10). Compared with baseline [2.41 (1.36-2.83) ml·100 g(-1)·min(-1)], Ach increased ATBF dose dependently [3.32 (2.80-5.09), 6.46 (4.36-9.51), and 10.31 (7.98-11.52), P < 0.0001], with no difference between R and NR. Compared with control side, atropine (both concentrations) had no effect on fasting ATBF; only atropine 10(-4) mol/l decreased post-glucose ATBF [iAUC: 1.25 (0.32-2.91) vs. 1.98 (0.64-2.94); P = 0.04]. This effect was further apparent in R. Mecamylamine had no impact on fasting and postglucose ATBF in R and NR. Our results suggest that the cholinergic system is implicated in ATBF regulation, although it has no role in the blunting of ATBF response in NR.

Moderate Obesity and Endothelial Dysfunction in Humans: Influence of Gender and Systemic Inflammation

Our objective was to determine whether moderate obesity (Body Mass Index [BMI] ≥ 30 kg/m²) is associated with impaired conduit and microvascular endothelial function, and whether men or women are more susceptible to impairment of endothelial function related to moderate obesity. Forty-one middle aged, nondiabetic moderately obese (BMI 34.7 ± 4.0 kg/m²) and nonobese (BMI 24.3 ± 2.6 kg/m²) subjects of both sexes underwent noninvasive studies of endothelial function (brachial reactivity) and measurements of endothelial-dependent vasodilation of gluteal subcutaneous arterioles to acetylcholine (Ach). Endothelium-dependent vasodilation to Ach was decreased in the moderately obese compared with the nonobese (P < 0.001). Stratified analysis based on sex showed impairment of arteriolar endothelial function in women BMI ≥ 30 kg/m² (P = 0.02), but not men. There was no difference between in vivo endothelial function flow-mediated dilation (FMD%) by BMI category. Sex-specific analysis showed FMD% was lower in women with BMI ≥ 30 kg/m² compared to those with BMI < 30 kg/m² (P = 0.02). No differences were seen in men based on BMI category (P = 0.18). In women, high sensitivity C-reactive protein (hsCRP) correlated with BMI (ρ = 0.68, P = 0.006). Moderate obesity is associated with impaired resistance arteriolar endothelial function. This is more prominent in women than men and is associated with systemic inflammation.

Mechanisms and metabolic implications of regional differences among fat depots

Fat distribution is closely linked to metabolic disease risk. Distribution varies with sex, genetic background, disease state, certain drugs and hormones, development, and aging. Preadipocyte replication and differentiation, developmental gene expression, susceptibility to apoptosis and cellular senescence, vascularity, inflammatory cell infiltration, and adipokine secretion vary among depots, as do fatty-acid handling and mechanisms of enlargement with positive-energy and loss with negative-energy balance. How interdepot differences in these molecular, cellular, and pathophysiological properties are related is incompletely understood. Whether fat redistribution causes metabolic disease or whether it is a marker of underlying processes that are primarily responsible is an open question.

Update on adipose tissue blood flow regulation

According to Fick's principle, any metabolic or hormonal exchange through a given tissue depends on the product of the blood flow to that tissue and the arteriovenous difference. The proper function of adipose tissue relies on adequate adipose tissue blood flow (ATBF), which determines the influx and efflux of metabolites as well as regulatory endocrine signals. Adequate functioning of adipose tissue in intermediary metabolism requires finely tuned perfusion. Because metabolic and vascular processes are so tightly interconnected, any disruption in one will necessarily impact the other. Although altered ATBF is one consequence of expanding fat tissue, it may also aggravate the negative impacts of obesity on the body's metabolic milieu. This review attempts to summarize the current state of knowledge on adipose tissue vascular bed behavior under physiological conditions and the various factors that contribute to its regulation as well as the possible participation of altered ATBF in the pathophysiology of metabolic syndrome.