Resistin increases islet blood flow and decreases subcutaneous adipose tissue blood flow in anaesthetized rats

Keeping It Local in Metabolic Disease: Adipose Tissue Paracrine Signaling and Insulin Resistance

Alterations in adipose tissue composition and function are associated with obesity and contribute to the development of type 2 diabetes. While the significance of this relationship has been cemented, our understanding of the multifaceted role of adipose tissue in metabolic heath and disease continues to evolve and expand. Heterogenous populations of cells that make up adipose tissue throughout the body generate diverse secretomes containing a mosaic of bioactive compounds with vast structural and signaling capabilities. While there are many reports highlighting the important role of adipose tissue endocrine signaling in insulin resistance and type 2 diabetes, the direct, local, paracrine effect of adipose tissue has received less attention. Recent studies have begun to underscore the importance of considering anatomically discrete adipose depots for their specific impact on local microenvironments and metabolic function in neighboring tissues as well as regulation of whole-body physiology. This article highlights the important role of adipose tissue paracrine signaling on metabolic function and insulin sensitivity in nearby tissues and organs, specifically focusing on visceral, pancreatic, subcutaneous, intermuscular, and perivascular adipose tissue depots.

Pancreatic Blood Flow with Special Emphasis on Blood Perfusion of the Islets of Langerhans

The pancreatic islets are more richly vascularized than the exocrine pancreas, and possess a 5- to 10-fold higher basal and stimulated blood flow, which is separately regulated. This is reflected in the vascular anatomy of the pancreas where islets have separate arterioles. There is also an insulo-acinar portal system, where numerous venules connect each islet to the acinar capillaries. Both islets and acini possess strong metabolic regulation of their blood perfusion. Of particular importance, especially in the islets, is adenosine and ATP/ADP. Basal and stimulated blood flow is modified by local endothelial mediators, the nervous system as well as gastrointestinal hormones. Normally the responses to the nervous system, especially the parasympathetic and sympathetic nerves, are fairly similar in endocrine and exocrine parts. The islets seem to be more sensitive to the effects of endothelial mediators, especially nitric oxide, which is a permissive factor to maintain the high basal islet blood flow. The gastrointestinal hormones with pancreatic effects mainly influence the exocrine pancreatic blood flow, whereas islets are less affected. A notable exception is incretin hormones and adipokines, which preferentially affect islet vasculature. Islet hormones can influence both exocrine and endocrine blood vessels, and these complex effects are discussed. Secondary changes in pancreatic and islet blood flow occur during several conditions. To what extent changes in blood perfusion may affect the pathogenesis of pancreatic diseases is discussed. Both type 2 diabetes mellitus and acute pancreatitis are conditions where we think there is evidence that blood flow may contribute to disease manifestations. © 2019 American Physiological Society. Compr Physiol 9:799-837, 2019.

The control of insulin secretion by adipokines: current evidence for adipocyte-beta cell endocrine signalling in metabolic homeostasis

Metabolic homeostasis is maintained by the coordinated action of multiple organ systems. Insulin secretion is often enhanced during obesity or insulin resistance to maintain glucose and lipid homeostasis, whereas a loss of insulin secretion is associated with type 2 diabetes. Adipocytes secrete hormones known as adipokines which act on multiple cell types to regulate metabolism. Many adipokines have been shown to influence beta cell function by enhancing or inhibiting insulin release or by influencing beta cell survival. Insulin, in turn, regulates lipolysis and promotes glucose uptake and lipid storage in adipocytes. As adipokine secretion and action is strongly influenced by obesity, this provides a potential route by which beta cell function is coordinated with adiposity, independently of alterations in blood glucose or lipid levels. In this review, I assess the evidence for the direct regulation of beta cell function by the adipokines leptin, adiponectin, extracellular nicotinamide phosphoribosyltransferase, apelin, resistin, retinol binding protein 4, fibroblast growth factor 21, nesfatin-1 and fatty acid binding protein 4. I summarise in vitro and in vivo data and discuss the influence of obesity and diabetes on circulating adipokine concentrations, along with the potential for influencing beta cell function in human physiology. Finally, I highlight future research questions that are likely to yield new insights into the exciting field of insulinotropic adipokines.

Dysregulated adipokines in the pathogenesis of type 2 diabetes and vascular disease

Obesity is commonly associated with type 2 diabetes and vascular disease. Changes in body composition in the obese state lead to a dysregulation of secretion of adipocyte-secreted hormones known as adipokines. Adipokines such as leptin and adiponectin are known to be involved in many physiological and pathological processes. Current knowledge suggests that adipokines provide potential therapeutic targets against type 2 diabetes and vascular disease.