metabolic syndrome (MetS) is associated with elevations in blood pressure (BP); however, the relationships between key variables remain murky. First of all, defining and modeling MetS is difficult at best. In its simplest terms, MetS may be described as a “disease state” or set of conditions resulting from an excess of stored and/or circulating energy. Associated conditions include visceral adiposity, hyperinsulinemia (due to insulin resistance of major metabolic tissues), dyslipidemia, and hypertension. A current estimate by the American Heart Association is that one in three United States adults has MetS (1). How (or whether) the various metabolic features of MetS lead to hypertension is an area of intense scrutiny, not only due to the large numbers of persons afflicted but also due to the high lifetime cardiovascular risk of chronically elevated BP.
Using a number of elegant approaches, Nizar et al. (8) recently examined the role of the epithelial Na+ channel (ENaC) in the Na+ retention and hypertension associated with MetS. The investigators initially screened two different mouse models of MetS, i.e., chronically high-fat-diet fed and high-fructose-diet fed male C57Bl/6 mice. After several weeks of treatment, only mice on the high-fat regimen (60% high-fat diet for 12 wk) developed symptoms of MetS, including weight gain, hyperinsulinemia, hypercholesterolemia, glucose intolerance, and modestly elevated BP (∼3 mmHg, by radiotelemetry). Therefore, they used this model to perform additional experiments. One stumbling block in modeling hypertension associated with MetS is that many rodent models do not develop significant hypertension when obese. The obese Zucker rat, e.g., is markedly obese, insulin resistant, and hypertriglyceridemic; however, they show only a modest elevations in BP except when infused with aldosterone (9). Fructose-fed rats develop insulin resistance, hypertriglyceridemia, and hypertension but are not necessarily obese. Moreover, the strain of mice selected in the Nizar et al. study (8), i.e., C57Bl/6, have inconsistent changes in BP with high-fat diets (5). Nonetheless, in agreement, not all human subjects with MetS have hypertension.
Nizar et al. (8) demonstrated that high-fat diet-fed mice had relatively impaired natriuresis when switched from a normal diet to a high-NaCl diet, and this impairment coincided with a significantly greater salt-sensitive rise in BP. It is assumed they chose to focus on ENaC because it is a prime candidate with regard to linking the metabolic factors associated with MetS to hypertension (Fig. 1). ENaC is expressed in the collecting duct, the final regulatory step in Na+ handling, where fine tuning of Na+ reabsorption occurs. Mutations in the β- or γ-subunit of ENaC (Liddle's syndrome) are associated with constitutively active ENaC and a 15- to 30-mmHg rise in systolic BP (7). Moreover, there are numerous circulating, paracrine, and basic physiological conditions, e.g., acid-base homeostasis, that may be altered in MetS and have been found to increase ENaC activity (4). Others include, but are not limited to, ANG II, aldosterone, superoxide, and insulin. ENaC has been demonstrated to be activated by insulin; however, the ex vivo doses needed to show activation (1–100 nM) are often much higher than the physiological range for insulin (∼0.02–2 nM or 14–290 μIU/ml). Frindt et al. (3) did not find activation of ENaC by insulin in split-open rat cortical collecting ducts using 2 nM insulin. In contrast, we found that administration of insulin to achieve a circulating level of no greater than 0.3 nM resulted in antinatriuresis. This antinatriuresis was partially attenuated by benzamil (10) or in collecting duct principal cell insulin receptor knockout mice (6). Acute insulin administration was also associated with greater ENaC localization of the apical membrane of the collecting duct (10). Additional studies will be needed to clarify the actions of insulin on ENaC.
Nevertheless, in the Nizar et al. study (8), ENaC activity was found not to be different between high-fat diet-fed mice and control mice, as measured in vivo by benzamil (ENaC antagonist) sensitivity and using isolated perfused cortical collecting ducts. Antagonization of ENaC with benzamil in a more chronic fashion also did not affect the elevation in BP. Moreover, the investigators also found no differences in aldosterone excretion between the groups. Therefore, they concluded that MetS, at least in this particular mouse model, was associated with Na+ retention and an elevation in BP independent from elevated ENaC and likely due to activation of more upstream Na+-retentive mechanisms.
This group of investigators did not set out to test the role of elevated insulin or any other specific metabolic factor in their analyses. More than likely, they were aiming to develop a meaningful model of human MetS, with all its complexity. The mice were hyperinsulinemic, and insulin has been demonstrated to be antinatriuretic at a number of sites along the renal tubule in addition to the collecting duct, including the proximal tubule, thick ascending limb, and distal convoluted tubule. Therefore, there are a number of potential alternative Na+-retentive pathways that could be activated by insulin or another factor.
Insulin may explain Na+ retention, but its direct effects on BP are less certain. There are other systems that manage BP as their principal action, e.g., the renin-angiotensin-aldosterone system, autonomic nervous system, and sympathetic nervous system, as well as local mediators, e.g., nitric oxide and endothelin (2). Morever, insulin has been demonstrated to have two distinct opposing actions on BP, i.e., Na+ retention and nitric oxide release. The balance of these countering actions may underlie some of the inconsistencies observed. Finally, the high-fat-diet fed mice in the present study were found not only to be hyperinsulinemic but also insulin resistant (using the homeostatic model assessment of insulin resistance assay). Insulin receptor resistance can diminish but also alter intracellular signaling, so that the end-point ramifications are markedly different. Whether or not the kidney insulin receptor becomes resistant in MetS is uncertain.
Thus, it is likely that a combination of factors or a distinct “metabolic milieu” is required in order for MetS to result in hypertension. In addition to insulin, important factors likely include the renin-angiotensin-aldosterone system, inflammatory mediators, free radicals, and adipokines. The authors provide keen evidence of a renal role in this hypertension with Na+ retention distinct from ENaC as a characteristic feature in C57Bl/6 mice fed a high-fat diet.
No conflicts of interest, financial or otherwise, are declared by the author(s).
C.M.E. prepared figures; C.M.E. drafted manuscript; C.M.E. edited and revised manuscript; C.M.E. approved final version of manuscript.
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