Hyperinsulinemia: a link between obesity and hypertension?

Obesity, kidney dysfunction, and inflammation: interactions in hypertension

Obesity contributes 65-75% of the risk for human primary (essential) hypertension (HT) which is a major driver of cardiovascular and kidney diseases. Kidney dysfunction, associated with increased renal sodium reabsorption and compensatory glomerular hyperfiltration, plays a key role in initiating obesity-HT and target organ injury. Mediators of kidney dysfunction and increased blood pressure include (i) elevated renal sympathetic nerve activity (RSNA); (ii) increased antinatriuretic hormones such as angiotensin II and aldosterone; (iii) relative deficiency of natriuretic hormones; (iv) renal compression by fat in and around the kidneys; and (v) activation of innate and adaptive immune cells that invade tissues throughout the body, producing inflammatory cytokines/chemokines that contribute to vascular and target organ injury, and exacerbate HT. These neurohormonal, renal, and inflammatory mechanisms of obesity-HT are interdependent. For example, excess adiposity increases the adipocyte-derived cytokine leptin which increases RSNA by stimulating the central nervous system proopiomelanocortin-melanocortin 4 receptor pathway. Excess visceral, perirenal and renal sinus fat compress the kidneys which, along with increased RSNA, contribute to renin-angiotensin-aldosterone system activation, although obesity may also activate mineralocorticoid receptors independent of aldosterone. Prolonged obesity, HT, metabolic abnormalities, and inflammation cause progressive renal injury, making HT more resistant to therapy and often requiring multiple antihypertensive drugs and concurrent treatment of dyslipidaemia, insulin resistance, diabetes, and inflammation. More effective anti-obesity drugs are needed to prevent the cascade of cardiorenal, metabolic, and immune disorders that threaten to overwhelm health care systems as obesity prevalence continues to increase.

Obesity, kidney dysfunction and hypertension: mechanistic links

Excessive adiposity raises blood pressure and accounts for 65-75% of primary hypertension, which is a major driver of cardiovascular and kidney diseases. In obesity, abnormal kidney function and associated increases in tubular sodium reabsorption initiate hypertension, which is often mild before the development of target organ injury. Factors that contribute to increased sodium reabsorption in obesity include kidney compression by visceral, perirenal and renal sinus fat; increased renal sympathetic nerve activity (RSNA); increased levels of anti-natriuretic hormones, such as angiotensin II and aldosterone; and adipokines, particularly leptin. The renal and neurohormonal pathways of obesity and hypertension are intertwined. For example, leptin increases RSNA by stimulating the central nervous system proopiomelanocortin-melanocortin 4 receptor pathway, and kidney compression and RSNA contribute to renin-angiotensin-aldosterone system activation. Glucocorticoids and/or oxidative stress may also contribute to mineralocorticoid receptor activation in obesity. Prolonged obesity and progressive renal injury often lead to the development of treatment-resistant hypertension. Patient management therefore often requires multiple antihypertensive drugs and concurrent treatment of dyslipidaemia, insulin resistance, diabetes and inflammation. If more effective strategies for the prevention and control of obesity are not developed, cardiorenal, metabolic and other obesity-associated diseases could overwhelm health-care systems in the future.

Role of Hyperinsulinemia and Insulin Resistance in Hypertension: Metabolic Syndrome Revisited

Hyperinsulinemia and insulin resistance were proposed more than 30 years ago to be important contributors to elevated blood pressure (BP) associated with obesity and the metabolic syndrome, also called syndrome X. Support for this concept initially came from clinical and population studies showing correlations among hyperinsulinemia, insulin resistance, and elevated BP in individuals with metabolic syndrome. Short-term studies in experimental animals and in humans provided additional evidence that hyperinsulinemia may evoke increases in sympathetic nervous system (SNS) activity and renal sodium retention that, if sustained, could increase BP. Although insulin infusions may increase SNS activity and modestly raise BP in rodents, chronic insulin administration does not significantly increase BP in lean or obese insulin-resistant rabbits, dogs, horses, or humans. Multiple studies in humans and experimental animals have also shown that severe insulin resistance and hyperinsulinemia may occur in the absence of elevated BP. These observations question whether insulin resistance and hyperinsulinemia are major factors linking obesity/metabolic syndrome with hypertension. Other mechanisms, such as physical compression of the kidneys, activation of the renin-angiotensin-aldosterone system, hyperleptinemia, stimulation of the brain melanocortin system, and SNS activation, appear to play a more critical role in initiating hypertension in obese subjects with metabolic syndrome. However, the metabolic effects of insulin resistance, including hyperglycemia and dyslipidemia, appear to interact synergistically with increased BP to cause vascular and kidney injury that can exacerbate the hypertension and associated injury to the kidneys and cardiovascular system.

Loss of renal SNX5 results in impaired IDE activity and insulin resistance in mice

AIMS/HYPOTHESIS

We hypothesised that renal sorting nexin 5 (SNX5) regulates the insulin-degrading enzyme (IDE) and, thus, circulating insulin levels. We therefore studied the dynamic interaction between SNX5 and IDE in human renal proximal tubule cells (hRPTCs), as well as in rat and mouse kidneys.

METHODS

The regulation of IDE by SNX5 expressed in the kidney was studied in vitro and in vivo. Snx5 or mock siRNA was added to immortalised hRPTCs (passage <20) in culture or selectively infused, via osmotic mini-pump, into the remnant kidney of uninephrectomised mice and rats.

RESULTS

SNX5 co-localised with IDE at the plasma membrane and perinuclear area of hRPTCs and in the brush border membrane of proximal tubules of human, rat, and mouse kidneys. Insulin increased the co-localisation and co-immunoprecipitation of SNX5 and IDE in hRPTCs. Silencing SNX5 in hRPTCs decreased IDE expression and activity. Renal-selective silencing of Snx5 (SNX5 protein: 100 ± 25 vs 29 ± 10, p < 0.05 [% of control]) in C57Bl/6J mice decreased IDE protein (100 ± 13 vs 57 ± 6, p < 0.05 [% of control]) and urinary insulin excretion, impaired the responses to insulin and glucose, and increased blood insulin and glucose levels. Spontaneously hypertensive rats (SHRs) had increased blood insulin and glucose levels and decreased renal SNX5 (100 ± 27 vs 29 ± 6, p < 0.05 [% of control]) and IDE (100 ± 5 vs 75 ± 4, p < 0.05 [% of control]) proteins, compared with normotensive Wistar-Kyoto (WKY) rats. Kidney Snx5-depleted WKY rats also had increased blood insulin and glucose levels. The expression of SNX5 and IDE was decreased in RPTCs from SHRs and hypertensive humans compared with cells from normotensive volunteers, indicating a common cause for hyperinsulinaemia and hypertension.

CONCLUSIONS/INTERPRETATION

Renal SNX5 positively regulates IDE expression and function. This study is the first to demonstrate the novel and crucial role of renal SNX5 in insulin and glucose metabolism.

CNS Targets of Adipokines

Our understanding of adipose tissue as an endocrine organ has been transformed over the last 20 years. During this time, a number of adipocyte-derived factors or adipokines have been identified. This article will review evidence for how adipokines acting via the central nervous system (CNS) regulate normal physiology and disease pathology. The reported CNS-mediated effects of adipokines are varied and include the regulation of energy homeostasis, autonomic nervous system activity, the reproductive axis, neurodevelopment, cardiovascular function, and cognition. Due to the wealth of information available and the diversity of their known functions, the archetypal adipokines leptin and adiponectin will be focused on extensively. Other adipokines with established CNS actions will also be discussed. Due to the difficulties associated with studying CNS function on a molecular level in humans, the majority of our knowledge, and as such the studies described in this paper, comes from work in experimental animal models; however, where possible the relevant data from human studies are also highlighted. © 2017 American Physiological Society. Compr Physiol 7:1359-1406, 2017.

Roles of renal proximal tubule transport in acid/base balance and blood pressure regulation

Sodium-coupled bicarbonate absorption from renal proximal tubules (PTs) plays a pivotal role in the maintenance of systemic acid/base balance. Indeed, mutations in the Na⁺-HCO₃⁻ cotransporter NBCe1, which mediates a majority of bicarbonate exit from PTs, cause severe proximal renal tubular acidosis associated with ocular and other extrarenal abnormalities. Sodium transport in PTs also plays an important role in the regulation of blood pressure. For example, PT transport stimulation by insulin may be involved in the pathogenesis of hypertension associated with insulin resistance. Type 1 angiotensin (Ang) II receptors in PT are critical for blood pressure homeostasis. Paradoxically, the effects of Ang II on PT transport are known to be biphasic. Unlike in other species, however, Ang II is recently shown to dose-dependently stimulate human PT transport via nitric oxide/cGMP/ERK pathway, which may represent a novel therapeutic target in human hypertension. In this paper, we will review the physiological and pathophysiological roles of PT transport.

Pathophysiology of the diabetic kidney

Diabetes mellitus contributes greatly to morbidity, mortality, and overall health care costs. In major part, these outcomes derive from the high incidence of progressive kidney dysfunction in patients with diabetes making diabetic nephropathy a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved and of the early dysfunctions observed in the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. Here we review the pathophysiological changes that occur in the kidney in response to hyperglycemia, including the cellular responses to high glucose and the responses in vascular, glomerular, podocyte, and tubular function. The molecular basis, characteristics, and consequences of the unique growth phenotypes observed in the diabetic kidney, including glomerular structures and tubular segments, are outlined. We delineate mechanisms of early diabetic glomerular hyperfiltration including primary vascular events as well as the primary role of tubular growth, hyperreabsorption, and tubuloglomerular communication as part of a "tubulocentric" concept of early diabetic kidney function. The latter also explains the "salt paradox" of the early diabetic kidney, that is, a unique and inverse relationship between glomerular filtration rate and dietary salt intake. The mechanisms and consequences of the intrarenal activation of the renin-angiotensin system and of diabetes-induced tubular glycogen accumulation are discussed. Moreover, we aim to link the changes that occur early in the diabetic kidney including the growth phenotype, oxidative stress, hypoxia, and formation of advanced glycation end products to mechanisms involved in progressive kidney disease.

Insulin resistance and the relationship between urinary Na(+)/K(+) and ambulatory blood pressure in a community of African ancestry

BACKGROUND

Although groups of African descent are particularly sensitive to blood pressure (BP) effects of salt intake, the role of obesity and insulin resistance in mediating this effect is uncertain. We determined whether obesity or insulin resistance is independently associated with urinary Na(+)/K(+)-BP relationships in a community sample of African ancestry.

METHODS

We measured 24-hour urinary Na(+)/K(+), homeostasis model assessment of insulin resistance (HOMA-IR), and nurse-derived conventional and 24-hour ambulatory BP in 331 participants from a South African community sample of black African descent not receiving treatment for hypertension.

RESULTS

With adjustments for diabetes mellitus and the individual terms, an interaction between waist circumference and urinary Na(+)/K(+) was associated with day diastolic BP (P < 0.05) and an interaction between log HOMA-IR and urinary Na(+)/K(+) was associated with 24-hour and day systolic (P < 0.05) and 24-hour, day, and night diastolic (P < 0.002; P < 0.001) BP. The multivariable-adjusted relationship between urinary Na(+)/K(+) and night diastolic BP increased across tertiles of HOMA-IR (tertile 1: β-coefficient = -0.79 ± 0.47; tertile 2: β-coefficient = 0.65 ± 0.35; tertile 3: β-coefficient = 1.03 ± 0.46; P < 0.05 tertiles 3 and 2 vs. 1). The partial correlation coefficients for relationships between urinary Na(+)/K(+) and 24-hour (partial r = 0.19; P < 0.02), day (partial r = 0.17; P < 0.05), and night (partial r = 0.18; P < 0.02) diastolic BP in participants with log HOMA-IR greater than or equal to the median were greater than those for relationships between urinary Na(+)/K(+) and 24-hour (partial r = -0.08; P = 0.29), day (partial r = -0.10; P < 0.22), and night (partial r = -0.06; P = 0.40) diastolic BP in participants with log HOMA-IR less than the median (comparisons of r values: P < 0.05).

CONCLUSIONS

Insulin resistance may modify the relationship between salt intake, indexed by urinary Na(+)/K(+), and ambulatory BP in groups of African descent.

Insulin resistance, obesity, hypertension, and renal sodium transport

Sodium transport through various nephron segments is quite important in regulating sodium reabsorption and blood pressure. Among several regulators of this process, insulin acts on almost all the nephron segments and is a strong enhancer of sodium reabsorption. Sodium-proton exchanger type 3 (NHE3) is a main regulator of sodium reabsorption in the luminal side of proximal tubule. In the basolateral side of the proximal tubule, sodium-bicarbonate cotransporter (NBCe1) mediates sodium and bicarbonate exit from tubular cells. In the distal nephron and the connecting tubule, epithelial sodium channel (ENaC) is of great importance to sodium reabsorption. NHE3, NBCe1, and ENaC are all regulated by insulin. Recently with-no-lysine (WNK) kinases, responsible for familial hypertension, stimulating sodium reabsorption in the distal nephron, have been found to be also regulated by insulin. We will discuss the regulation of renal sodium transport by insulin and its roles in the pathogenesis of hypertension in insulin resistance.

Mechanisms of obesity-induced hypertension

The relationship between obesity and hypertension is well established both in children and adults. The mechanisms through which obesity directly causes hypertension are still an area of research. Activation of the sympathetic nervous system has been considered to have an important function in the pathogenesis of obesity-related hypertension. The arterial-pressure control mechanism of diuresis and natriuresis, according to the principle of infinite feedback gain, seems to be shifted toward higher blood-pressure levels in obese individuals. During the early phases of obesity, primary sodium retention exists as a result of increase in renal tubular reabsorption. Extracellular-fluid volume is expanded and the kidney-fluid apparatus is resetted to a hypertensive level, consistent with a model of hypertension because of volume overload. Plasma renin activity, angiotensinogen, angiotensin II and aldosterone values display significant increase during obesity. Insulin resistance and inflammation may promote an altered profile of vascular function and consequently hypertension. Leptin and other neuropeptides are possible links between obesity and the development of hypertension. Obesity should be considered as a chronic medical condition, which is likely to require long-term treatment. Understanding of the mechanisms associated with obesity-related hypertension is essential for successful treatment strategies.

Aldosterone contributes to blood pressure variance and to likelihood of hypertension in normal-weight and overweight African Americans

BACKGROUND

Hypertension and obesity are highly prevalent among African Americans (AAs). We have previously reported that both plasma aldosterone (PA) and body mass index (BMI) are higher in hypertensive than in normotensive AAs. This study evaluates the relative contributions of adiposity and PA to hypertension in AAs.

METHODS

A total of 466 AAs (50% hypertensive, 51% women) were evaluated in a Clinical Research Center by stratifying them into three subgroups based on BMI (normal weight, overweight, and obese). Anthropometric measurements, ambulatory blood pressure (BP), fasting glucose, insulin, 24-h urine sodium and potassium, creatinine clearance, standing PA and plasma renin activity (PRA) were measured. Insulin resistance was estimated by the homeostasis model assessment.

RESULTS

Compared to normotensives, hypertensives had higher BMI, waist circumference (WC), and were more insulin resistant (P < or = 0.01). When stratified by BMI, hypertensives in each BMI strata had higher PA (P < or = 0.05) and lower PRA (P < or = 0.01) compared to normotensives. Compared to normotensives, WC was greater in overweight and obese hypertensives, but not in normal-weight hypertensives. In the overall sample, age, WC, PA, and PRA were the major contributors to BP variance and to hypertension. Among normal-weight subjects, PA and PRA significantly predicted BP and the odds ratio for hypertension, whereas WC had no predictive value.

CONCLUSIONS

PA, but not WC, is associated with BP and likelihood of hypertension in normal-weight AAs, whereas both WC and PA are predictive of hypertension in overweight and obese individuals. This suggests that aldosterone antagonists may be useful for the treatment of hypertension among AAs, regardless of BMI.

Renal Effects

Amylin bound to kidney cortex in a distinctive pattern. Binding appeared specific in that it was displaceable with amylin antagonists. It was associated with activation of cyclic AMP (cAMP), and was thereby likely to represent receptor binding and activation. Amylin's principal effects at the kidney included a stimulation of plasma renin activity, reflected in aldosterone increases at quasi-physiological amylin concentrations. It was unclear whether this was a local or a systemic effect. Other renal effects in rats included a diuretic effect and a natriuretic effect. The latter was mainly driven by the diuresis, since urinary sodium concentration did not change. Amylin had a transient effect to lower plasma potassium concentration. This effect was likely to be a consequence of activation of Na+/K+-ATPase, an action shared with insulin and catecholamines. Amylin lowered plasma calcium, particularly ionized calcium, likely due to an antiresorptive effect at osteoclasts. Immunoreactive amylin was detected in the developing kidney. It appeared to have a trophic effect in kidney, and its absence resulted in renal dysgenesis. Neurons in the subfornical organ (SFO), which has a role in fluid/electrolyte homeostasis, were potently activated by amylin. The dipsogenic and renal effects of amylin may be related to effects at the SFO.

Is obesity a major cause of chronic kidney disease?

Excess weight gain is a major risk factor for essential hypertension and for end-stage renal disease (ESRD). Obesity raises blood pressure by increasing renal tubular sodium reabsorption, impairing pressure natriuresis, and causing volume expansion because of activation of the sympathetic nervous system and renin-angiotensin system and by physical compression of the kidneys, especially when visceral obesity is present. Obesity also causes renal vasodilation and glomerular hyperfiltration that initially serve as compensatory mechanisms to maintain sodium balance in the face of increased tubular reabsorption. In the long-term, however, these changes, along with the increased systemic arterial pressure, create a hemodynamic burden on the kidneys that causes glomerular injury. With prolonged obesity, there is increasing urinary protein excretion and gradual loss of nephron function that worsens with time and exacerbates hypertension. With the worsening of metabolic disturbances and the development of type II diabetes in some obese patients, kidney disease progresses much more rapidly. Weight reduction is an essential first step in the management of obesity, hypertension, and kidney disease. Special considerations for the obese patient, in addition to adequately controlling the blood pressure, include correction of the metabolic abnormalities and protection of the kidneys from further injury.

Obesity and hypertension

This article has discussed some of the mechanisms involved in the causal relation between obesity and hypertension. Obesity causes a constellation of maladaptive disorders that individually and synergistically contribute to hypertension, among other cardiovascular morbidities. Well-designed population-based studies are needed to assess the individual contribution of each of these disorders to the development of hypertension. In addition, because the control of obesity may eliminate 48% of the hypertension in whites and 28% in blacks, this article has offered an up-to-date on the management of this problem. It is hoped that this article will help scientists formulate a thorough understanding of obesity hypertension and form the basis for more research in this field, which has a huge impact on human life.

Factors that influence the risk of hypertension in obese individuals

PURPOSE OF REVIEW

Obesity, which has reached epidemic prevalence, is now recognized as an independent risk factor for increasing blood pressure. The complex mechanisms of obesity-related hypertension are unclear, but several studies have provided evidence of a hypertensive shift in pressure natriuresis. Excess sympathetic outflow to the kidneys and changes in renal structure and function may both affect the renal pressure relationship. Other factors that may contribute to altered natriuresis include hyperinsulinemia, hyperleptinemia and activation of the renin-angiotensin system. Disruption of the renal alpha2 adrenoceptors or leptin receptor implicated in natriuresis control may also be an additive risk for the increase in tubular reabsorption in obesity hypertension.

RECENT FINDINGS

Recent advances have highlighted the importance of two adipocyte-derived hormones - leptin and angiotensinogen - in obesity hypertension. Leptin has direct central effects that increase sympathetic outflow to the kidney and the new concept of selective leptin resistance, suggests the maintenance of leptin-induced sympathetic activation in obesity, despite resistance to leptin metabolic effects. On the other hand, a recent study showed that angiotensinogen produced in the adipocyte is also relevant to blood pressure control.

SUMMARY

The article reviews the factors implicated in the disruption of blood pressure control in obesity. Further investigation on the time course of the disease would reveal the relative importance of each of the factors that influence the risk of hypertension in obese individuals.

The kidney, hypertension, and obesity

This paper provides a personal perspective of the role of abnormal renal-pressure natriuresis in the pathogenesis of hypertension. Direct support for a major role of renal-pressure natriuresis in long-term control of arterial pressure and sodium balance comes from studies demonstrating that (1) pressure natriuresis is impaired in all forms of chronic hypertension and (2) prevention of pressure natriuresis from operating, by servo-control of renal perfusion pressure, also prevents the maintenance of sodium balance hypertension. Although the precise mechanisms of impaired pressure natriuresis in essential hypertension have remained elusive, recent evidence suggests that obesity and overweight may play a major role. Obesity increases renal sodium reabsorption and impairs pressure natriuresis by activation of the renin-angiotensin and sympathetic nervous systems and by altered intrarenal physical forces. Chronic obesity also causes marked structural changes in the kidneys that eventually lead to a loss of nephron function, further increases in arterial pressure, and severe renal injury in some cases. Although there are many unanswered questions about the mechanisms of obesity hypertension and renal disease, this is one of the most promising areas for future research, especially in view of the growing, worldwide "epidemic" of obesity.

Impact of visceral fat on blood pressure and insulin sensitivity in hypertensive obese women

OBJECTIVE

The relationship among body fat distribution, blood pressure, serum leptin levels, and insulin resistance was investigated in hypertensive obese women with central distribution of fat.

RESEARCH METHODS AND PROCEDURES

We studied 74 hypertensive women (age, 49.8 +/- 7.5 years; body mass index, 39.1 +/- 5.5 kg/m(2); waist-to-hip ratio, 0.96 +/- 0.08). All patients were submitted to 24-hour blood pressure ambulatory monitoring (24h-ABPM). Abdominal ultrasonography was used to estimate the amount of visceral fat (VF). Fasting blood samples were obtained for serum leptin and insulin determinations. Insulin resistance was estimated by homeostasis model assessment insulin resistance index (HOMA-r index).

RESULTS

Sixty-four percent of the women were postmenopausal, and all patients showed central distribution of fat (waist-to-hip ratio > 0.85). The VF correlated with systolic 24h-ABPM values (r = 0.28, p = 0.01) and with HOMA-r index (r = 0.27; p = 0.01). VF measurement (7.5 +/- 2.3 vs. 5.9 +/- 2.2 cm, p < 0.001) and the systolic 24h-ABPM (133 +/- 14.5 vs. 126 +/- 9.8 mm Hg, p = 0.04), but not HOMA-r index, were significantly higher in the postmenopausal group (n = 48) than in the premenopausal group (n = 26). No correlations were observed between blood pressure levels and HOMA-r index, leptin, or insulin levels. In the multiple regression analysis, visceral fat, but not age, body fat mass, or HOMA-r index, correlated with the 24h-ABPM (p = 0.003).

DISCUSSION

In centrally obese hypertensive women, the accumulation of VF, more often after menopause, is associated with higher levels of blood pressure and insulin resistance. The mechanism through which VF contributes to higher blood pressure levels seems to be independent of leptin or insulin levels.

Insulin resistance: cellular and clinical concepts

Insulin resistance is defined as a clinical state in which a normal or elevated insulin level produces an attenuated biologic response. Specifically, the biologic response most studied is insulin-stimulated glucose disposal, yet the precise cellular mechanism responsible is not yet known. However, the presence of insulin resistance is observed many years before the onset of clinical hyperglycemia and the diagnosis of Type 2 diabetes. Insulin resistance at this stage appears to be significantly associated with a clustering of cardiovascular risk factors predisposing the individual to accelerated cardiovascular disease. An overview of insulin resistance and the associated clinical insulin resistant state will be discussed.

Insulin resistance and cardiovascular risk in the pediatric patient

The insulin resistance syndrome, a cluster of potent risk factors for atherosclerotic cardiovascular disease and type 2 diabetes in adults, is composed of hyerinsulinemia, obesity, hypertension and hyperlipidemia. In addition, left ventricular hypertrophy and its precursor increased left ventricular mass, is known to be a powerful predictor of adverse cardiovascular events, both as an independent risk factor and by association with the insulin resistance syndrome. Obesity appears to have a major role in the relations between the components of the insulin resistance syndrome, and their association with increased heart mass. Of significant impact in the adult population, atherosclerotic cardiovascular disease and death are rarely seen in the young, but the pathologic processes and risk factors associated with its development have been shown to begin during childhood. Recent studies revealed the presence of components of the insulin resistance syndrome also in children and adolescents, however, their associations are not well understood. A direct link between obesity and insulin resistance has also been reported in the young, as has the link between insulin resistance and abnormal lipid profile. There is an increasing amount of data to show that being overweight during childhood and adolescence is significantly associated with insulin resistance, abnormal lipids and elevated blood pressure in young adulthood. Weight loss in these situations results in a decrease in insulin concentration and an increase in insulin sensitivity toward normalcy. Moreover, it has been determined that increased left ventricular mass is present in childhood, and is related to other risk factors, namely obesity and insulin resistance. Based on current knowledge, it is reasonable to suggest that weight control, and lifestyle modification, could alter the incidence of the syndrome of insulin resistance, and improve the risk profiles for cardiovascular disease as children make the transition toward adolescence and young adulthood.

Obesity modifies the relationship between ambulatory blood pressure and natriuresis in children

BACKGROUND

The objective in the present study was to evaluate if obesity beginning in the first two decades of life influences the relationship between ambulatory blood pressure and urinary sodium excretion.

DESIGN AND METHODS

Eighty-five obese and 88 non-obese children aged 3-19 years were included in the study. For each subject, a 24h ambulatory blood pressure monitoring and a complete urine collection were simultaneously performed according to the protocols designed. The averages of ambulatory blood pressure and of the urinary excretion rates for sodium, potassium and creatinine were calculated separately for 24-h, awake and sleep periods as defined by a mini-diary.

RESULTS

Weight and sodium excretion are directly associated with systolic blood pressure; however, the relationship between blood pressure and sodium excretion seems to be modified in obese children as compared to controls. The interaction between sodium excretion and weight was negative indicating that the rate of change of systolic blood pressure by sodium unit is smaller for the obese than for the non-obese, even though at the same urinary sodium excretion level the obese children had higher ambulatory systolic blood pressure.

CONCLUSIONS

Obesity during the first two decades of life seems to restrict sodium excretion, leading to higher blood pressure values. The capacity to excrete sodium seems to be heterogeneous, the lowest capacity being at the highest blood pressure values. Subsets of the obese, those with the lowest ability to excrete sodium, may be further protected by low sodium intake in order to prevent a rise in blood pressure.

Blood pressure changes in relation to sodium and calcium status in induced hyperinsulinemia

Insulin increases renal sodium reabsorption which may contribute to hypertension. However, acute insulin administration may result in vasodilation. The aim of the present study was to investigate effects on blood pressure and alterations in ion status during hyperinsulinemia. Blood pressure and serum sodium and ionized calcium concentrations were measured before and at the end of euglycemic hyperinsulinemic clamp tests performed in 45 patients with essential hypertension. Both the systolic and the diastolic blood pressure decreased, by 4% (p < 0.05) and 3% (p < 0.05), respectively. Circulating ionized calcium concentration increased by 2% (p < 0.001), and the ratio between circulating sodium and ionized calcium concentrations decreased. The changes in circulating sodium concentration correlated to changes in systolic blood pressure (SBP; r = 0.36, p = 0.05). Both ionized calcium concentrations and the ratio between circulating sodium and ionized calcium concentrations correlated to changes in SBP during hyperinsulinemia (r = -0.41, p = 0.03, r = 0.56, p < 0.01, respectively). The changes in ion status were not significantly correlated to age, body mass index or insulin sensitivity. In conclusion, a more pronounced increase in circulating ionized calcium concentration and reduction in the ratio between sodium and ionized calcium concentrations was associated with a greater blood pressure decline during the hyperinsulinemic clamp test when performed in hypertensive patients.

Role of sympathetic nervous system and neuropeptides in obesity hypertension

Obesity is the most common cause of human essential hypertension in most industrialized countries. Although the precise mechanisms of obesity hypertension are not fully understood, considerable evidence suggests that excess renal sodium reabsorption and a hypertensive shift of pressure natriuresis play a major role. Sympathetic activation appears to mediate at least part of the obesity-induced sodium retention and hypertension since adrenergic blockade or renal denervation markedly attenuates these changes. Recent observations suggest that leptin and its multiple interactions with neuropeptides in the hypothalamus may link excess weight gain with increased sympathetic activity. Leptin is produced mainly in adipocytes and is believed to regulate energy balance by acting on the hypothalamus to reduce food intake and to increase energy expenditure via sympathetic activation. Short-term administration of leptin into the cerebral ventricles increases renal sympathetic activity, and long-term leptin infusion at rates that mimic plasma concentrations found in obesity raises arterial pressure and heart rate via adrenergic activation in non-obese rodents. Transgenic mice overexpressing leptin also develop hypertension. Acute studies suggest that the renal sympathetic effects of leptin may depend on interactions with other neurochemical pathways in the hypothalamus, including the melanocortin-4 receptor (MC4-R). However, the role of this pathway in mediating the long-term effects of leptin on blood pressure is unclear. Also, it is uncertain whether there is resistance to the chronic renal sympathetic and blood pressure effects of leptin in obese subjects. In addition, leptin also has other cardiovascular and renal actions, such as stimulation of nitric oxide formation and improvement of insulin sensitivity, which may tend to reduce blood pressure in some conditions. Although the role of these mechanisms in human obesity has not been elucidated, this remains a fruitful area for further investigation, especially in view of the current "epidemic" of obesity in most industrialized countries.

Pathophysiology of obesity hypertension

Excess weight gain is a major cause of essential hypertension, and abnormal kidney function appears to be a cause as well as a consequence of obesity hypertension. Excess renal sodium reabsorption and a hypertensive shift of pressure natriuresis play a major role in mediating increased blood pressure associated with weight gain. Activation of the renin-angiotensin and sympathetic nervous systems and physical compression of the kidneys appear to contribute to obesity-induced increases in sodium reabsorption and hypertension.

Mechanisms of hypertension and kidney disease in obesity

Abnormal kidney function is an important cause as well as a consequence of obesity. Excess renal sodium reabsorption, probably in the loop of Henle, and a hypertensive shift of pressure natriuresis play a major role in initiating increased blood pressure associated with weight gain. The mechanisms responsible for increased sodium reabsorption and altered pressure natriuresis in obesity include activation of the renin-angiotension and sympathetic nervous systems, and physical compression of the kidneys due to accumulation of intrarenal fat and extracellular matrix. Sympathetic activation may be mediated, in part, by elevated circulating leptin and interactions with neuropeptides in the hypothalamus. Renal remodeling and extracellular matrix proliferation likely involve complex interactions between intrarenal physical forces, neurohumoral factors, and local growth factors and cytokines. Although glomerular hyperfiltration and increased arterial pressure help to compensate for increased renal tubular reabsorption in the early phases of obesity, these changes also increase glomerular capillary wall stress which, along with activation of neurohumoral systems and increased lipids and glucose intolerance, cause glomerular cell proliferation, matrix accumulation, and eventually glomerulosclerosis and loss of nephron function in the early phases of obesity. This creates a slowly developing vicious cycle that requires additional increases in arterial pressure to maintain sodium balance and therefore makes effective antihypertensive therapy more difficult. Because obesity is the main cause of Type 2 diabetes and an important cause of human essential hypertension, it seems likely that obesity is also one of the most important risk factors for end-stage renal disease.

Exercise reverses peripheral insulin resistance in trained L-NAME-hypertensive rats

Several studies have demonstrated an increase in peripheral resistance to insulin associated with hypertension. To assess the hemodynamic and metabolic effects of exercise training, normotensive and N(omega)-nitro-L-arginine methyl ester (L-NAME)-hypertensive male Wistar rats were submitted to low-intensity treadmill exercise training for 10 weeks and compared with their sedentary controls. Blood pressure signals were obtained and processed with a data acquisition system (CODAS, 1 kHz) to evaluate mean arterial pressure, heart rate, autonomic control of heart rate, and baroreflex sensitivity. Exercise training induced a nonsignificant 6.5-mm Hg decrease in mean arterial pressure in trained hypertensive rats (163+/-9 mm Hg) compared with sedentary hypertensive rats (169.5+/-5. 5 mm Hg). The hypertensive groups showed impairment of baroreflex function in response to changes in arterial pressure compared with sedentary controls. Furthermore, exercise training improved the tachycardic response to decreasing arterial pressure and reduced intrinsic heart rate in trained control rats compared with all other groups. Sedentary hypertensive rats presented a decrease in body weight compared with normotensive animals. Basal evaluation of the glucose/insulin ratio showed increased insulin resistance in sedentary (28.4+/-3) and trained (23.5+/-2.7) hypertensive rats compared with sedentary control rats (40.5+/-3). However, the glucose/insulin ratio evaluated during the exercise session in trained rats showed an improvement in insulin resistance (54.5+/-5 for control rats and 44+/-9 for hypertensive rats). In conclusion, L-NAME-induced hypertension is accompanied by an increase in insulin resistance in rats. The improvement in peripheral insulin sensitivity during exercise and the body weight gain observed in trained hypertensive rats may support the positive role of physical activity in the management of hypertension.

Effect of insulin resistance on left ventricular hypertrophy and dysfunction in essential hypertension

BACKGROUND

In hypertensive patients, the relationships between glucose tolerance and left ventricular hypertrophy (LVH) and left ventricular diastolic function (LVDF) have been described in several reports.

OBJECTIVE

In this study, we examined the relationships between insulin resistance and LVH and LVDF in hypertensive patients from the therapeutic perspective.

METHODS AND RESULTS

The study participants were essential hypertensive patients with impaired glucose tolerance (IGT-HT, n = 26), hypertensive patients with normal glucose tolerance (NGT-HT, n = 39), and normotensive control individuals (n = 18). Insulin resistance was evaluated by the insulin suppression test by use of the steady-state plasma glucose (SSPG) level. Left ventricular mass index (LVMI) and LVDF, which was determined by the E:A ratio, were estimated by echocardiography. Temocapril, an angiotensin-converting enzyme inhibitor, was administered in an open, non-randomized manner with a mean dose of 2.8+/-0.2 mg/ day, and the mean administration period was 18 weeks. The systolic and diastolic blood pressure, the LVMI, and the SSPG level were significantly higher in the hypertensive patients than in the control individuals. The mean systolic and diastolic blood pressures were significantly decreased by treatment with Temocapril. Before treatment, stepwise regression analysis showed that SSPG is an independent predictor for LVMI and LVDF. After treatment, the changes in LVMI (D-LVMI; %) (-15.1+/-1.5), the changes in LVDF (D-E:A; %) (-38.2+/-4.1), and the changes in insulin resistance (D-SSPG; %) (-13.7+/-1.7) were significantly higher in the IGT-HT group than in the NGT-HT group (-11.4+/-1.1, -18.1+/-1.7, -9.4+/-1.4, respectively), and the D-SSPG was an independent predictor for D-LVMI and D-E :A.

CONCLUSIONS

The results of this study indicate that insulin resistance is an important factor affecting LVH and LVDF.

Abnormal kidney function as a cause and a consequence of obesity hypertension

1. Obesity is the most common nutritional disorder in the US and is a major cause of human essential hypertension. Although the precise mechanisms by which obesity raises blood pressure (BP) are not fully understood, there is clear evidence that abnormal kidney function plays a key role in obesity hypertension. 2. Obesity increases tubular reabsorption and this shifts pressure natriuresis towards higher BP. The increased tubular reabsorption is not directly related to hyperinsulinaemia, but is closely linked to activation of the sympathetic and renin-angiotensin systems, and possible changes in intrarenal physical forces caused by medullary compression due to accumulation of adipose tissue around the kidney and increased extracellular matrix within the kidney. 3. Obesity is also associated with marked renal vasodilation and increased glomerular filtration rate, which are compensatory responses that help overcome the increased tubular reabsorption and maintain sodium balance. However, chronic renal vasodilation causes increased hydrostatic pressure and wall stress in the glomeruli which, along with increased lipids and glucose intolerance, may cause glomerulosclerosis and loss of nephron function in obese subjects. Because obesity is a primary cause of essential hypertension as well as type II diabetes, there is good reason to believe that obesity may also be the most frequent cause of end-stage renal disease. 4. Future research is needed to determine the mechanisms by which excess weight gain activates the neurohumoral systems and alters renal structure and function. Because of the high prevalence of obesity in most industrialized countries, unravelling these mechanisms will likely provide a better understanding of the pathophysiology of human essential hypertension and chronic renal failure.

Relation of fasting insulin to blood pressure and lipids in adolescents and parents

This study was intended to clarify the relation between fasting insulin, lipids, and blood pressure in adolescents before the onset of hypertension and to examine the association of these data with similar data obtained in their parents. The participants in this study were 183 adolescents 14 to 18 years old (96 girls) completing a 4-year intervention trial and their parents (164 mothers, 122 fathers). Blood pressure was measured twice on the right arm in a seated position using a random-zero sphygmomanometer. Fasting blood samples were obtained for lipid and insulin analyses. Fasting insulin was significantly correlated with systolic blood pressure in the adolescents and also in the parents before and after adjustment for body mass index. Fasting insulin was correlated significantly with levels of cholesterol, triglycerides, and HDL and LDL cholesterol in the adolescents. It was correlated only with triglycerides and HDL-cholesterol in mothers and fathers. After adjustment for body mass index, the correlations between fasting insulin and lipids in the children were not significant. A significant relation was shown between children's systolic blood pressure and mothers' fasting insulin and systolic blood pressure. Significant correlations were found between the children's and fathers' triglycerides and HDL-cholesterol, whereas significant correlations were found for fasting insulin and all lipids between mothers and children, and these remained significant after adjustment for body mass index. These results show (1) a significant relation between fasting insulin and both lipids and systolic blood pressure in adolescents and (2) a significant relation for these factors between adolescents and their parents. Although weight appears to play an important role in this relation during adolescence, genetic and environmental factors other than those mediated via weight may control insulin metabolism within families. The data support a role for studies during early biological development to address these issues.

Exercise training in aging: hemodynamic, metabolic, and oxidative stress evaluations

The effects of exercise training on hemodynamic and metabolic parameters as well as on responses to oxidative stress in aged individuals are controversial. The aim of the present study was to investigate changes in heart hate, mean arterial pressure, vasoreactivity, and plasma levels of insulin and glucose in male aged Wistar rats submitted to exercise training for 11 weeks (1 h/d; 5 d/wk) in a treadmill. The isolated heart was perfused by H2O2, and oxidative stress was evaluated using thiobarbituric acid reactive substances. Cardiovascular functions were recorded with a data acquisition system (CODAS, 1 kHz). Trained aged rats were bradycardic as compared with sedentary aged rats (298+/-7 versus 336+/-16 bpm) but presented similar mean arterial pressure and vasoreactivity and plasma levels of insulin and of glucose, which were quantified by radioimmunoassay and colorimetric enzymatic test. Plasma levels of insulin and of glucose ratio were increased in trained aged rats (6.9+/-0.7 versus 3.5+/-0.4 in sedentary aged rats), and the response to oxidative stress was decreased (0.4+/-0.1 versus 0.7+/-0.1 nmol/mg protein in sedentary aged rats). These results showed that exercise training produced a lower resting heart rate as well as changes in metabolic and oxidative responses. This suggests a higher myocardium protection of trained than sedentary aged rats.

Lymphocytic Na(+)-H+ exchange increases after an oral glucose challenge

The effects of oral glucose challenge on plasma glucose concentration, plasma insulin concentration, arterial blood pressure, cytosolic pH (pHi), cytosolic free Na+ concentration ([Na+]i), and cellular Na(+)-H+ exchange activity were investigated in 16 healthy subjects. The pHi, [Na+]i, and Na(+)-H+ exchange activity were measured in intact lymphocytes by using the fluorescent dye technique. The oral glucose challenge significantly increased plasma glucose, plasma insulin, and the lymphocytic Na(+)-H+ exchange activity, measured as change of pHi per second (control [0 hours], 5.20 +/- 0.53 x 10(-3) dpHi/s; 1 hour after glucose administration, 8.28 +/- 1.07 x 10(-3) dpHi/s; 2 hours after glucose administration, 8.15 +/- 1.18 x 10(-3) dpHi/s; P = .002). The lymphocytic Na(+)-H+ exchange was significantly correlated with plasma glucose concentration (r = .357, P = .041). During steady state euglycemic hyperinsulinemic clamp, the Na(+)-H+ exchange activity was not significantly changed compared with baseline values. The study shows that changes of blood glucose levels can induce an acute increase in Na(+)-H+ exchange activity. Systolic blood pressure and Na(+)-H+ exchange activity were significantly (P < .001) but weakly correlated during an oral glucose challenge.

Cardiovascular actions of insulin: are they important in long-term blood pressure regulation?

1. In recent years, there has been considerable interest in the possibility that insulin may have important cardiovascular as well as metabolic actions. Perhaps the best documented cardiovascular effect of insulin is to cause peripheral vasodilation, especially in skeletal muscle. Hyperinsulinaemia also stimulates sympathetic activity and causes antinatriuresis, but these effects may be linked, at least in part, to the metabolic actions of insulin that elicit peripheral vasodilation and a tendency toward hypotension. Normal, fasting levels of insulin appear to have very little influence on peripheral vascular resistance, sympathetic activity or renal sodium excretion. 2. Decreased sensitivity of the peripheral tissues to the metabolic effects of insulin and compensatory hyperinsulinaemia have been postulated to play key roles in the pathophysiology of diseases such as hypertension and atherosclerosis. Although impaired insulin action (insulin resistance) and hyperinsulinaemia often accompany essential hypertension, especially when associated with obesity, there is currently little direct evidence for a cause and effect relationship between insulin resistance, hyperinsulinaemia and increased arterial pressure. Chronic increases in plasma insulin levels in dogs and humans have not been shown to cause hypertension, although hyperinsulinaemia raises blood pressure in rats. 3. Further research is needed to determine whether there are pathophysiological conditions or genetic factors that may predispose humans to a hypertensive effect of hyperinsulinaemia and/or insulin resistance.

Amylin stimulates plasma renin concentration in humans

Although insulin resistance and hypertension are commonly associated, the underlying cause for this association remains unknown. Plasma concentrations of the recently described hormone amylin, which is cosecreted with insulin by the pancreatic beta cell, are reported to be elevated in various states of insulin resistance, including hypertension and obesity. Preliminary studies by our group have suggested that there are amylin binding sites in the kidney. In nine healthy humans an infusion of human amylin that resulted in steady state plasma amylin levels in the subnanomolar range led to significant increases in plasma renin and aldosterone concentrations. These changes occurred in the absence of significant changes in plasma electrolytes, catecholamines, vasopressin, total renin, or osmolality. Diastolic pressure at 30 minutes and plasma glucose at 60 minutes rose modestly. Since amylin has both metabolic and renal actions, this peptide may be an important link between hypertension, insulin resistance, and the renin-angiotensin system.

Hemodynamic and renal responses to chronic hyperinsulinemia in obese, insulin-resistant dogs

We previously reported that chronic hyperinsulinemia does not cause hypertension in normal insulin-sensitive dogs. However, resistance to the metabolic and vasodilator effects of insulin may be a prerequisite for hyperinsulinemia to elevate blood pressure. The present study tested this hypothesis by comparing the control of systemic hemodynamics and renal function during chronic hyperinsulinemia in instrumented normal conscious dogs (n = 6) and in dogs made obese and insulin resistant by feeding them a high-fat diet for 6 weeks (n = 6). After 6 weeks of the high-fat diet, body weight increased from 24.0 +/- 1.2 to 40.9 +/- 1.2 kg, arterial pressure rose from 83 +/- 5 to 106 +/- 4 mm Hg, and cardiac output rose from 2.98 +/- 0.29 to 5.27 +/- 0.54 L/min. Insulin sensitivity, assessed by fasting hyperinsulinemia and by the hyperinsulinemic euglycemic clamp technique, was markedly reduced in obese dogs. Insulin infusion (1.0 mU/kg per minute for 7 days) in obese dogs elevated plasma insulin from 42 +/- 12 microU/mL to 95 to 219 microU/mL but failed to increase arterial pressure, which averaged 106 +/- 4 mm Hg during control and 102 +/- 4 mm Hg during 7 days of insulin infusion. Hyperinsulinemia for 7 days in obese dogs elevated heart rate from 116 +/- 8 to 135 +/- 7 beats per minute but caused no significant changes in cardiac output, in contrast to normal dogs (n = 6), in which marked increases in cardiac output (31 +/- 5% after 7 days) and decreases in total peripheral resistance occurred during chronic insulin infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal denervation attenuates the sodium retention and hypertension associated with obesity

Recent studies have indicated that obesity is associated with hypertension, sodium retention, and increased sympathetic nervous system activity. The purpose of this study was to determine the role of renal nerves in mediating the sodium retention and hypertension associated with obesity. We determined the hemodynamic and renal excretory responses to a high-fat diet in control (n = 6) and bilaterally renal-denervated (n = 7) chronically instrumented dogs. After a control period of 8 days, dogs were placed on a high-fat diet for 5 weeks. In response to a high-fat diet, body weight increased from 19.9 +/- 2.2 to 29.9 +/- 2.4 kg in the control group and from 21.1 +/- 2.0 to 32.4 +/- 1.9 kg in the bilaterally renal-denervated group. Heart rate increased from 81 +/- 8 to 113 +/- 7 beats per minute in the control group and from 79 +/- 7 to 103 +/- 8 beats per minute in the bilaterally renal-denervated group. Arterial pressure increased significantly from 95 +/- 2 to 109 +/- 4 mm Hg in the control group. In contrast, 5 weeks of a high-fat diet in the bilaterally renal-denervated group did not significantly increase arterial pressure (which went from 87 +/- 3 to 90 +/- 4 mm Hg). Furthermore, the decrease in sodium excretion in response to the high-fat diet was significantly greater in the control group than in the bilaterally renal-denervated group.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin blunts the natriuretic action of atrial natriuretic peptide in hypertension

Hyperinsulinemia and insulin resistance are implicated in the etiology of hypertension, but the mechanisms involved have not been established. The objectives of this study were to determine whether untreated essential hypertensive patients are more sensitive to the antinatriuretic action of insulin and more resistant to the counteracting natriuretic effect of atrial natriuretic peptide in contrast to age- and sex-matched normotensive control subjects. Urinary sodium excretion was measured at baseline, during hyperinsulinemic euglycemic clamp, and during coadministration of insulin and atrial natriuretic peptide. Baseline urinary sodium excretion was not significantly different in the normotensive subjects (415 +/- 47 mumol/min, n = 12) and hypertensive patients (381 +/- 18 mumol/min, n = 10); with the institution of insulin infusion, there was a similar and significant decline from baseline (P < .001) to 289 +/- 35 mumol/min in normotensive subjects and 235 +/- 17 mumol/min in hypertensive patients. Atrial natriuretic peptide was able to oppose the antinatriuretic action of insulin in normotensive subjects, increasing urinary sodium excretion significantly to a mean level of 352 +/- 31 mumol/min (P < .05), which did not differ significantly from baseline. In the hypertensive group, atrial natriuretic peptide infusion had no effect on urinary sodium excretion (238 +/- 18 mumol/min), and the difference from baseline remained highly significant (P < .001). The hypertensive patients were significantly less insulin sensitive than their normotensive counterparts, as reflected by a lower glucose utilization rate and higher mean baseline plasma insulin level (P < .05 for each).(ABSTRACT TRUNCATED AT 250 WORDS)

Louis K. Dahl Memorial Lecture. Renal and cardiovascular mechanisms of hypertension in obesity

In all forms of hypertension, including human essential hypertension, pressure natriuresis is reset to higher blood pressures. Because human essential hypertension is a heterogeneous disease, it is likely that there are multiple neurohumoral and intrarenal causes of abnormal pressure natriuresis and increased blood pressure. Weight gain is recognized to be an important contributor to essential hypertension, although the mechanisms that link obesity with altered renal function and high blood pressure have not been fully elucidated. In obese dogs and humans, the shift of pressure natriuresis to higher blood pressures appears to be due mainly to increased tubular reabsorption, as glomerular filtration rate and renal plasma flow are increased compared with normal. Multiple causes of increased tubular reabsorption and hypertension in obesity have been postulated, including insulin resistance and hyperinsulinemia, activation of the sympathetic nervous and renin-angiotensin systems, and physical changes within the kidney itself. Support for the insulin resistance-hyperinsulinemia link between obesity and hypertension has been inferred mainly from acute and epidemiologic studies showing a correlation between insulin and blood pressure. Recent studies suggest that chronic hyperinsulinemia, comparable to that found in obesity, cannot account for obesity hypertension in dogs or humans. Activation of the sympathetic nervous system may play a role in obesity-induced hypertension, and there is evidence for a role of altered intrarenal physical forces caused by histological changes within the renal medulla. The quantitative importance of each of these abnormalities in altering renal function and raising blood pressure in obesity remains to be determined but is an important area of research for understanding human essential hypertension.